Bob B

Count me in for at least 1, maybe 2 if the price is good! http://www.machinegunbooks.com/forums/invboard1_1_2/upload/html/emoticons/smile.gif

Hope everything goes well for you without damage. You're in our thoughts. Stay safe, and please let us know. Bob

PK, Here's a copy of the original message and a consolidated reduced image of the two huge closeups. Hope this helps. QUOTE JimFromFL Posted: Aug 21 2004, 05:45 PM RKI Member Group: Donation Made Posts: 789 Member No.: 106 Joined: 14-July 03 These images contain 4 blish locks. The left most one is unused. The remaining 3 are each have a few thousand rounds (less than 10,000 each). Although the wear on the 3 may not look like much via the pictures, seeing them in person seems a bit excessive esspecially only after a few thousand rounds. Maybe after 20K rounds or so, it would look normal, but not after just a few thousand rounds. I have run my fingers along the rails in the upper and they are smooth. Is this what your blish lock looks like after a few thousand rounds? http://img40.exs.cx/img40/9572/BlishLockwearsmall.jpg The original post and pictures are here (GREAT detail, but takes forever to load): http://www.machinegunbooks.com/forums/invb...t=ST&f=3&t=3028 Bob

AZDoug, The reference to "titanium aluminum bronze" is quoted directly from the article by Phillip Sharpe published in the 1932-33 journal of the American Institute of Criminal Law. While I don't doubt that some of the TSMG's use of the Blish locking principle was hyped and perhaps even to some degree misrepresented , this early reference leaves no doubt that such an alloy existed (in fact or in theory) at that time. Titanium was discovered in 1791 and isolated as an elemental metal around 1910. Titanium Dioxide was discovered in 1821 and saw limited use as an artist's pigment until it was first mass produced in 1916. It's certainly possible some form of titanium was used as an alloying element before solid shapes were introduced in 1953. That said, the original Blish patents place no reliance on special alloys or the frictional coefficients of different metals. In the patents, pressure/release curves detail the precise relationship between breech pressure and the degree of intercepted angle - the presumption being that all parts are of hardened steel. Different breech pressures in different guns are accommodated by variations in lock angle and nothing more. All steel parts in the case of the TSMG would probably have resulted in an unacceptable rate of receiver wear, so it seems likely a bronze alloy Blish lock was chosen to limit the wear to this replaceable part. If this is the case, then it's clear any change in critical angle from the pressure/release curves disclosed in the patent was simply to compensate for the different frictional coefficient of bronze. It's easy to see how marketing might have woven a mystique around the "magical" adhesion properties of a "special" alloy, but the facts - and the patents - seem to demonstrate this had nothing to do with it. Your observation about the commercial availability of titanium caught my interest, so I went searching for an answer. I didn't find one exactly, at least as to date, but titanium IS sometimes specified as an alloying component in aluminum bronze. However, it does nothing to increase strength or wear resistance as most would assume. Small amounts of titanium (0.2-0.5%) serve to minimize porosity in Al/Bronze castings and alloy welds, and also to slightly improve corrosion resistance (for example in seawater heat exchanger tubes and the like). The addition doesn't make the alloy mechanically stronger or more durable. I can only imagine (if it was used at all) that the addition of titanium might have made for higher integrity of parts by minimizing porosity. Any metallurgists out there? Bob

While researching photos of finned barrels from which to model a dummy short barrel for my '27A1, I came across these pictures depicting a Colt 1921 (offered for $29,500, no less). The barrel on this piece intrigues me, since there are only 24 ribs (including the wider end-ring that abuts the receiver) instead of the 29 or so on other '21/'28 barrels. The serial number is in the 14000 range so this gun wasn't a prototype. Is this some aftermarket adaptation, an incorrect replacement, or a really rare barrel? What gives? Photos at: http://www.hallowellco.com/colt_1921_thomp...bmachinegun.htm Gun description at: http://www.hallowellco.com/class_iii.htm Also, the closeup photos seem to show the gun's markings as having rounded line-bottoms (this is particularly visible on the photo titled "Left rear markings", where even the line-ends appear rounded as from a rotating cutter). I would think this sort of engraving profile is more characteristic of machine engraving than the sharp line-bottoms and square corners typical of production roll or die stamping. I can easily see how very early models might have been engraved one-up on a machine, but were they still actually machine-engraving production guns of this serial range? If so, that's a helluva lot of one-up engraving. And here I thought MY carpal tunnel syndrome was bad! Bob

Although intended as a response, I'm posting this as a new topic because the images in the original post are so big they take forever to load and run the text way off the page. Despite having no practical experience in the phenomenon of Blish lock wear, the images do hint that the locking mechanism may not be operating ideally. I came across the following surprisingly detailed description of lock design and operation, excerpted from an evaluation by Philip B. Sharpe published in the journal American Institute of Criminal Law, 23 (1932-33): 1098. (Complete article available at http://www.saf.org/LawReviews/PSharpe1.html ) While most of this description won't be anything new to veteran Thompson folks, it may help shed some light on JimFromFL's question. QUOTE: A wedge-shaped piece of metal weighing one ounce locks the entire mechanism purely through friction on sliding surfaces created by the breech pressure while the bullet remains in the barrel. When this pressure is relieved, as happens when the projectile passes to the open air, this wedge is released and unlocks the mechanism. The idea is not new. Naval men discovered many years ago that their heavy fixed pieces played them pranks. They noticed that in firing the ten and twelve-inch rifles with service shot or target projectiles, the. guns behaved according to the handbook. But when blank salutes or light practice charges were fired, the heavy interrupted-screw type breach block cammed itself open automatically. Naval ordnance experts set forth to solve the puzzle mathematically¾and did. Commander John Blish is credited with arriving at the correct solution, and his principles were applied to several pieces of ordnance during the late war¾most common of which was the three-inch anti-aircraft rifle which unlocks its breech and ejects the empty cartridge case as the projectile starts skyward. (jump) The lock is perhaps the most interesting piece of mechanism, since it is around this item that the entire mechanism functions. First of all, its appearance is outstanding. Shining like a piece of gold, it is in direct contrast with the polished steel bolt, or the heavily blued actuator into which it fits. It is formed of titanium aluminum bronze, an alloy selected for the obvious reasons of coefficient of friction, strength, toughness, resistance to wear, and others. This lock is designed like an "H" with oddly shaped lugs on either side. The sides of the "H" engage with grooves in the bolt, the bar with the actuator, and the lugs with the receiver grooves. The surfaces between the bolt and lock are at an angle of 70 degrees to the axis of the gun. Those between the lock and receiver are at an angle of 45 degrees to the axis, so that there is formed on the lock an intercepted angle of 25 degrees between the bolt bearing and the receiver bearing surfaces of the lock. The chamber pressure is transmitted to the lock through the [Page 1105] bolt, and the angle of the receiver and bolt slots with the corresponding lugs of the "H" wedge have been figured so that the coefficient of friction of the alloy plus the breech pressure of the cartridge causes the adhesion of the lugs to their corresponding slot surfaces in the receiver walls. At the proper time this pressure falls to a point wherein it is just sufficient to recoil the bolt to the rear and operate the mechanism. This reduced pressure will not permit the locking surfaces to adhere, the wedge slides upward, releases its locking effect, and the action functions by reason of the energy of recoil still remaining. Should any police departments have a Thompson gun in which the lock has been "altered" or "smoothed up" by some over-energetic gunner or repairman, it would be wise to replace this lock with a new one. The various angles have been carefully determined, and must not be changed. Should they be filed down too much, the gun might open early, with considerable surprise to the gunner. However, Thompson engineers state that the action is built sufficiently strong, to resist the slam of the bolt even though the locking surfaces were removed entirely. Design of the weapon forestalls damage of the lock while within the action; should it become damaged while outside, replacement rather than repair would be the best policy. Sharpe states that adhesion due to the titanium aluminum bronze alloy's coefficient of friction exerts a positive locking action during the time before the bullet leaves the barrel, and allows movement only when pressure reduces to the point where it "is just sufficient to recoil the bolt to the rear and operate the mechanism." The type of wear and scoring evident in the photos suggests (if Sharpe's statement is to be taken literally) that positive locking isn't being sustained long enough (if at all) and instead is allowing operation before blowback pressures are appreciably reduced. Certainly this must be the case for lock #4 (furthest to the right), which shows grease deposits that must interfere with adhesion. If the (relatively) non-precision lock surfaces are forced to operate at high pressure - even in a layer of grease - they will score and release metallic particles that can only add to the abrasion. On the other hand, it's never made much sense to me that a system designed to exploit the frictional coefficient of a particular alloy should operate well in an oil-saturated environment such as the upper receiver of a Thompson. The other two used locks show a different sort of wear, including what appears to be some galling and maybe even a little looseness or misalignment evidenced by the asymmetrical upper wear surfaces. Almost looks like there may be some "cocking" of the lock as it disengages, but I'll let others with hands-on experience speak to that. Sorry I haven't answered the question as to whether the depicted wear is "normal", but maybe this will help get discussion going.

Bill in VA, as I said earlier you may be perfectly correct, but speaking only for myself I'd just as soon not become a martyr in order to eventually be found innocent of wrongdoing. Far easier (and cheaper) to just buy a $175-$200 L drum when it becomes legal to do so. If I know anything at all about the law (and admittedly that's not much) it's that anything "debateable" is grist for the legal mill. If it's not CLEARLY spelled out, I steer clear. I'd sooner not have to "argue" anything, and I'm too old and tired to put before a jury the question of who has the better lawyer! Virginia's machine gun presumption isn't quite as simple as "being in possession is proof of possession", it's more like "being in possession is not only proof of possession but also prima facie proof of intent to USE it to commit a violent felony." Might seem a small distinction, but with "Law and Order" judges and mandatory sentencing it could easily spell the difference between 3-5 years in ordinary confinement versus 25-to life in SuperMax! As for "nit-picky prosecutors" (and regulatory/enforcement agents in general), when did anyone hear of one who WASN'T nit-picky? They wouldn't be doing their job if they weren't. Remember that a prosecutor is an advocate - it's not a prosecutor's job to rationalize innocence, it's his job to rationalize guilt! Don't get me wrong, Bill, in a way I'm playing devil's advocate here. If I ever did find myself in this sort of legal bind, I'd certainly want you in my corner! http://www.machinegunbooks.com/forums/invboard1_1_2/upload/html/emoticons/wink.gif Bob

Bill in VA: Not to argue, because I'm certainly no authority, but this reasoning puzzles me because if true, even a temporary lapse of the ban would be tantamount to a general amnesty on any and all currently illegal converted X drums out there (and for that matter any other enumerated violations under the assault weapons ban). Let's assume that someone has an X drum that was illegally modified years ago. The law says that since that time he's been guilty of a serious crime. Okay, the ban expires, only to be reimplemented (let's say) following elections. Can this 2-month window possibly let him off the hook for years of breaking the law? Your point about requiring "post xyz" markings is well taken, but I can't believe the government would just give a free pass to people who have violated the existing ban for years. That would essentially encourage violations of any ban with a sunset clause - if the ban expired a violator could simply claim the illegal conversion wasn't made before then, and if it didn't he could just continue to keep the "evidence" well-hidden. Again, I'm no legal expert, but there ARE exceptions to the "innocent until proven guilty" thing - for example when certain presumptions are written into the law (I remember reading in the Virginia Code a few years back of the legal presumption that mere possession of an unregistered machine gun is prima facie evidence of the intent to use it in a crime), or when a government agency has absolute discretion to determine whether merely having certain parts on the premises constitutes "possession" of an illegal weapon. You may be perfectly right in your analysis and in fact I hope you are, but I'd hate to have to become a test case to find out! I've seen a few such pyrrhic victories in my lifetime- people who ultimately prevailed but lost everything they had getting to that point. Sometimes a huge disconnect between ideal and reality, theory and practice. http://www.machinegunbooks.com/forums/invboard1_1_2/upload/html/emoticons/sad.gif Edit: Not to put too fine a point on it, but isn't it also the case (not being a legal eagle I don't know this for sure) that an affirmative defense shifts the burden of proof to the one making it? For example, if a defendant doesn't dispute committing a proscribed act but contends it wasn't criminal because of special circumstances, isn't it his burden to prove (not simply to claim) that those circumstances in fact existed? Bob

My advice would be not to convert an "x" drum at all. Even if you were successful in replacing the spiral feed guides in the drum and cover (difficult but not impossible), the spring is probably also different, and the stampings and winding instructions would forever mark it as a conversion. This in itself could land you in a world of trouble - the burden would be on you to prove you converted it when expiration of the ban made it "legal" (or maybe not even then) to do so. Assuming the ban expires only to be re-implemented at a politically "less sensitive" time, you'd be in possession of an illegally modified magazine. Try explaining that to a judge: "But, but, your Honor, I did it in the six week period before the ban was reinstated ..." "Sure ya did, that's what they all say. I hereby sentence you ..." http://www.machinegunbooks.com/forums/invboard1_1_2/upload/html/emoticons/sad.gif

Thanks for the sound advice folks. I'll be calling the firearms tech branch next week. Guess I can understand how using a dewatted real barrel could be a problem, since the potential for "rewatting" could vary so widely depending on how and who. Good to know a dummy non-barrel is still an option though - and at least there are already legal precedents (Thanks Bill in VA and Tman for those examples). Goes to show, once again, that only the law-abiding worry about this kind of thing. I'll bet the BATF phones aren't ringing off the hook with inquiries from drug dealers concerned about staying strictly within the law when they want to do full auto conversions. Of course with a hobby like Thompson collecting, having a drug dealer's income wouldn't hurt! LOL - I can hear the defense now: "Members of the jury, my client was forced into dealing drugs to support his machine gun addiction ..." http://www.machinegunbooks.com/forums/invboard1_1_2/upload/html/emoticons/laugh.gif Thanks again guys Bob

Thanks guys for all the good advice. I think the rocker lever holdback wins hands down for reliability and just plain sexiness! PK, you have a real winner there. PK: You're of course right about the steel not being up to the task - the hardness of my receiver averages HS 24 on the Scleroscope (corresponding to about 2 on the Rockwell C scale) and the trigger frame averages only about HS 23 (below the Rc conversion table limit). No idea what steel Numrich used, but it's pretty mild stuff - definitely not the 4130 specified for the originals. Heck, I've seen readings higher than that on generic cold-rolled! Zamm: Thanks for posting those - a picture is worth a thousand words! The magazine that came with mine also was modified as shown with a ramp instead of a stop. I've long wondered about the consequences of the bolt slamming home on an "un-ramped" mag, but since I've never fired the piece this wasn't a pressing question. I wonder if Numrich still has the parts for the "magic pawl" retrofit. That'll definitely be needed if I ever take this shooting.

Work and planning on my Numrich '27A1 continues - one day it'll be a beauty! Since in the almost thirty years I've had the piece its only "use" has been as a display gun, I was thinking it would be neat to fit it with a dummy 10.5" barrel - or an actual barrel welded closed at the chamber. For safety and to prevent damage to the piece I also would fit a stationary half-sleeve in the magazine opening immediately below the bolt (this would be a curved piece of spring steel or some such that would simulate the closed-bolt condition and thereby obstruct the feed path). In the unlikely event someone (a child, space alien, mother-in-law) were to insert a loaded magazine, the uppermost cartridge would be inaccessible to the bolt and thus prevented from feeding (this obstruction would be removable and only used with the dummy short barrel). Assembled with the non-barrel and safety sleeve it would be impossible to feed, chamber or fire a round - in essence, a "non-gun". Meanwhile, no permanent modifications would have been made to prevent returning the piece to it's original long-barrel configuration. The mechanical end of it is fairly simple, but would there be legal difficulties to making this kind of modification? Logic would say not, but laws and how they're interpreted are sometimes far from logical. I guess the key question is would the BATF interpret mounting a short solid or blocked barrel on an otherwise-functioning firearm "manufacturing" a "Short Barrel Rifle"? If there are any red flags, I'll call the BATF (I probably will anyway), but thought I'd get some opinions first. Bob Assuming this would CLEARLY not be a problem, would anyone have a shot-out, bulged, or otherwise unserviceable finned barrel available? This could be any length, long or short, since work will have to be done on it anyway to "dewat" it and fit it with a compensator.

Norm, It did occur to me that reliability would be reduced as the squarely-cut surfaces become more rounded. Since my 27A1 is as-new I thought I'd forestall this possibility by undercutting the forward edges of the receiver hole to a slightly negative angle (say 5 degrees or so from the perpendicular) and turning a shallow matching groove at the engagement point of the actuator shaft. This would provide positive engagement and eliminate any rounding due to wear. This would probably work to restore the holdback function to badly worn surfaces as well, and could be done (carefully) with nothing more than modelmaker's files. Ralph, No way of making a comparison as to the spring strength, but must say it's no lightweight - takes a pretty good pull. Does your Kahr have the described actuator knob holdback feature?

I was examining my Numrich 1927A1 and taking measurements (thanks Craig!) with a view to moving forward on some of the modifications I've been considering. As I nodded, nearly napping, suddenly there came a tapping ... Ah, but I digress. In the course of my ponderings I discovered something I never noticed before - an apparently undocumented feature of the 1927A1. (On the other hand, maybe the only one who never noticed it was me - wouldn't be the first time!) Among the improvements I had planned for my baby was the neat rocker-lever-actuated bolt holdback feature pioneered by PK in Lionheart's "Project X", but I discovered the '27A1 (or at least my 1975 Numrich model) already HAS a bolt holdback feature that I've never seen discussed and isn't mentioned in the original documentation that came with the gun. It's not hidden or anything, just sort of "hiding in plain sight". Just forward of the large hole at the rear of the actuator slot (the hole through which the actuator knob is withdrawn for disassembly), is another smaller hole that matches the diameter of the actuator shaft just below the relieved area that rides in the actuator slot. If the bolt is drawn back so that the actuator knob aligns with this smaller hole, and pulled about 1/8" in an upward direction, the actuator shaft engages this hole perfectly and locks the bolt in the open position. To release the bolt, a slight rearward pressure is placed on the actuator knob while simultaneously pushing it back down to the point where it disengages the hole and is free to slide forward. It may take a little wiggling to get this to work easily if the sliding surfaces aren't smooth and lubricated, but it works! I don't know if later Numrich production had this feature (mine is in the serial number 800 range), or whether the current Kahr models have it, but it seems to do the trick just fine. Makes both the rocker-lever holdback modification and/or the "third hand" accessory unnecessary for the '27A1. Can't understand why Numrich never documented this, not even with their drum magazines, since obviously it was designed for this purpose as well as for easier cleaning. Damn! Now I've gone and ordered the parts, what the heck am I gonna use that fire selector lever for? Unless ...... !!! Oh well, I can dream, can't I? http://www.machinegunbooks.com/forums/invboard1_1_2/upload/html/emoticons/tongue.gif '27A1 owners?

If a spent shell casing were jammed in the compensator it could certainly result in the failure seen in the photos. However, I don't SEE what looks like a shell casing or even part of one, all I see is some metal other than lead that I took to be the bullet jacket. It DOES look "brassy" in the pictures though. Maybe Sgt. Fox could best answer the question of what metal it is and how thick. That ought to tell us something. Edit: I DO see in the rear-view photo what looks to be the rim of a shell casing. Pretty unmistakeable now that I'm looking at it. Guess the only mystery now is how it got into the compensator or chamber.

Those pictures ARE unbelievable! You must have damn near had heart failure! I've been trying to work through what might have happened based on the photos, but barring a physical obstruction by some foreign object I can't think of any reason for the failure unless the fit between barrel-end and compensator was sloppy to begin with. Thinking through that theory (and it's ONLY a theory), let's assume the original fit was loose enough so that the only thing providing rigidity was the cement used to fix the compensator in place. During assembly, the compensator is glued and pinned while the glue is still soft. Before the pin is inserted, the viscosity of the glue tends to maintain compensator/barrel concentricity. However, if the pin-groove on the barrel isn't machined to the PRECISE required depth or its position relative to the compensator pin-holes isn't JUST RIGHT, the driven pin will bear heavily against the barrel, acting as a wedge and biasing the entire fit error to one side. This would result in a "cocked" or off-axis condition that could be either eccentric or axial. This off-axis condition COULD become progressively worse with firing, since the high forward pressure created by the choked internal profile of the compensator exerts a one sided camming action between the pin and barrel with every round discharged (sort of like clamping the barrel in a vise and tapping the compensator wall opposite the sight with a hammer). Gradual breakdown or displacement of the cement/glue would make the situation worse, and the metal itself is stressed each time, gradually making the fit even looser. Once the off-axis condition reaches the point where the bullet jacket is grazing one side of the compensator exit-bore, a galling or even shearing action commences that dramatically exacerbates the effect and results in the sort of catastrophic failure shown in the photos. This would explain why the bullet jacket appears split in the "head on" photo - the jacket itself bound in the compensator exit bore like a hole-punch stuck in its die, causing a "backup" of lead that was displaced through the compensator ports. It might not be politically correct to say so in a gun forum, but clearly there appear to be some product liability issues here.

Just wanted to say thanks for all the thoughtful (and useful) advice. Was able to find from several suppliers a few of the things I'll need, but still having difficulty tracking down the paddle-type safety and rocker levers. Tried Sarco but they're out of stock (they have the pin type). I still have a bit of a puzzle regarding the correct drilling location for the rocker lever. The machinist's shop drawings I have indicate a through-hole diameter of 0.185" enlarged to 0.300" on the left wall of the frame, and 2.570" rearward from the magazine slide surface. Unfortunately, as detailed as these drawings appear to be, the vertical dimension is missing. Does anyone know the spec dimension from the bottom edge of the trigger frame housing to the selector lever centerline? Thanks again.

Brent and Jack, It's great to know the pivot plates and levers are interchangeable. Wonder why Numrich shows different part numbers for the different models - maybe the only distinction is that some are parkerized and others blued? Dan, I'd LOVE to go for a short barrel, but hate to go through the ATF rigamarole for just a repro carbine, and one of questionable quality at that. Heck, for the same trouble and fee I'd prefer to go the whole Class 3 route and really have something. Never been able to understand the reasoning behind the SBR restriction on Thompsons when, (a) they use pistol ammo and the barrels on garden variety 1911s are considerably shorter and in skilled hands just as deadly, (http://www.machinegunbooks.com/forums/invboard1_1_2/upload/html/emoticons/cool.gif there are so many modern guns out there that are shorter, lighter, and make better "gangster weapons" yet somehow are outside the NFA restrictions. Just seems rather arbitrary, but then logic seldom applies in matters legislative. Why is a Thompson any different than a "broom handle" Mauser pistol? Ah well ... Wonder if it's legal to fit a solid dummy short barrel for display and exchange the long one when and if I ever take it shooting? (Had it since new in 1975 and haven't fired it yet!) As for the slide mount adaptor, I removed the lug and milled grooves in the frame to fit the old 21/28 slides. I'll probably get one from tommygunner.com, but still need to find a lip-over buttplate to use one of the inexpensive boltless '28 stocks available from GPC.

Hope someone can help with this. I have an Auto Ordnance 1927-A1 Thompson carbine (Numrich, not Kahr). I've already made a few improvements, such as installing a new knurled actuator knob, polishing the bolt and feed ramp surfaces, relieving the magazine entry points and re-contouring the furniture in the style of the old '21s (looks terrific if I do say so myself). Can't wait for the ban to expire so I can afford a drum or two http://www.machinegunbooks.com/forums/invboard1_1_2/upload/html/emoticons/wink.gif . I'd like to further modify the piece to more closely resemble a Model '21/'28. The modifications I'm considering include changing the safety lever to the paddle type, installing a new pivot plate and adding a rocker pivot lever (configured to act as a bolt holdback), installing a Model '21/'28-type stock with slide, etc. I checked the Numrich/Gun Parts Corp. website, but discovered the various parts I need are numbered differently for different models. For example, the pivot plate is apparently different for the '28 as opposed to the M1/M1A1 - likewise the safety and fire control levers. A call to Numrich/GPC did no good - they told me they couldn't tell me anything about parts interchangeability (on their own products no less) and suggested I consult a gunsmith. The frame used on the 1927-A1 appears to be more similar to the M1/M1A1 frame than anything else. Am I safe in assuming an M1A1 pivot plate will have the same hole spacing/leg length? What about the knurled rocker pivot and safety levers - will the ones specified for the '28 model work? My shop includes a couple of South Bend Lathes and a Bridgeport mill, so I can handle pretty much anything in the way of machining. All I need is some good advice on what will fit, and a lead or two on where others who have done these modifications found their parts! Of course, if anyone has a few spares lying around ... Thanks in advance, and thanks for this great forum - I've really learned a lot just reading past posts!

Restoring old canvas? I have an old GI-issue canvas magazine case in overall good condition. We've all seen the type - dull mustard in color with blackened brass snaps and grommets that have begun to show some green corrosion around the edges. Problem is that over time the cotton fabric has lost its original moisture-repellent qualities and (like old books) has also become somewhat acidic. This threatens not only the canvas case itself, but the magazines it holds. In summer the material absorbs moisture from the air, and causes rust spots even though the magazines are well-oiled. I figured I'd neutralize the acidic condition by fuming with ammonia for a few days in a sealed cabinet - the same trick used by book conservators to de-acidify the paper in old books. However, I don't know how to treat the canvas to make it less moisture-absorbent. Does anyone know the formula of the original "waterproofing" used on webbing and canvas goods?