Joe H

Paul, The FA pivot plate will fit the semi. One common modification for the semi is to install the FA selector (which uses that extra finger ) and the FA pivot plate and use the selector as a bolt hold open. It requires a number of mods to the dis-connector, sear & pawl so they clear the selector in addition to bolt hold open work. Here is link to the modification: http://www.machinegunboards.com/forums/index.php?showtopic=5767&hl=bolt++hold++back Joe

Bob, I was at the range this weekend and a guy was sporting his brand new never been fired Kahr 1927A1. I looked at the barrel and it had that thick flange between the threads and the first narrow fin. It appears that is typical of new production Kahr barrels. Joe

When I load up less than a full drum I use the method described by Jim with no problems. I've cleaned from the muzzle with standard equip. I guess after the bolt is removed you could put the rod through the pilot hole then put the brush on and clean from the breech. I'm not sure if the pilot hole lines up exactly with the bore. I've never expeirenced any problems with the Cutts on my semi. I don't know if the same can be said for FA. Joe

Carl, I used to have a WH 1927A1 but it wasn't as old as yours. Mine had the trip & pawl. Jim is right. Try this link: http://www.machinegu...?showtopic=2355 Joe

T-Bone, Can't tell you why Kahr does or doesn't do anything but Green Mountain Barrels makes a 14+" barrel so that a cutts can be attached permanently and not be a SBR: http://www.gmrifleba...l.aspx?id=TMG-3 If you are concerned about the Compensator you can cross drill & pin like the originals. Joe

Carl, Sounds like you need a trip. When using the stick mags the bolt should stay open after the last shot if the trip & pawl are working properly. Joe

OMG Reduce the resale value of a Kahr Tommy??? Wear gloves to shoot it???. Why not just fix it or have it fixed? Kahr never did what any good machinist would do - Round the sharp edges after it came out of the CNC machine. I guess you could try to send it back or you could fix it youself. Round the sharp edges with a file or emery paper over a wood block and use Caswel's black oxide room temp gel kit to refinish the edges. Birchwood casey dark cold blue may work. The last Kahr (steel) I previously owned was black oxide. A call as recommended by mnshooter may yield a different approach but I doubt it. The work should have been done at the factory. Sorry for the rant, just my .02 cents, Joe

Bob, I've got a Green Mountain M1 barrel on one of your recievers. It's a semi SBR. The barrel is the correct length. I don't have the gages to check the details but it appears to be built to the original drawing. It looks and shoots great. I bead blasted it as I did your reciever. It is a good match in black oxide. I spoke with somebody from GMR a while back and they told me the intial barrels that they were selling were from stock they bought from a previous manufacturer. It is my understanding that barrels now available are from their current production. They are button rifled. If I was lucky enough to own a original FA M1 and needed to replace the barrel I'd spend the money for an original. Joe

Paul, Modifying the rails will in no way affect reliability. All it does is allow the frame to drop off before the frame latch hits the sear. Here are some links to modifying the 1927A1: http://www.machinegunboards.com/forums/index.php?showtopic=13560 http://www.machinegunboards.com/forums/index.php?showtopic=13713&st=20 Reconbob is right you can modify a 1927a1 frame for using the FA selector as a bolt hold open. It will require drilling the frame in the correct location, milling a pocket on the inside of the frame, a new FA selector, a new FA pivot plate, making the new hold open parts, and modifying the pawl, disconnector and sear to clear the new selector. Not a job for a novice. Joe

T-Hound, They the actual weight of the bolts in my builds. They are modified TSMG bolts. Joe

Jim, I used to have a Kahr semi so I don't have a bolt to weight. It is 0.1" less in height than a TSMG so I would estimate it at 1lb 8oz or less. It travels 5/8" less than the TSMG, at least mine do. I've built a couple of semi's. The 1928A1 maintained the blish lock. I also built an M1. On the 1928A1 the force of the return springs on the closed bolt is 7#. On the M1 the force is about 9#. Both of my semi's use the reduced bolt travel. On the 1928A1 the reduced travel is a geometric necessity due to the length of the return spring guide rods. My semi bolt weights are below. BOLT WEIGHTS (recoiling parts): TSMG 1928A1 1 lb 9 oz SEMI 1928 1 lb 8 oz TSMG M1 1 lb 13 oz SEMI M1 1 lb 10-1/2 oz I have noticed any unusal ejection of the spent cases but then I really have no basis of comparison. Joe

Bob, Anybody that doesn't believe in advanced primer ignigtion should read Engineering Design Handbook, Gun Series, Automatic Weapons. it's a phamphlet (348 pages) put out by the Federal government and its free. It's AMC Phamphlet 7C6-260. Just Google 7C6-260, it's about the third hit. The first example is a straight blowback open bolt .45 cal. Based on a very complicated design ( including calculus) the bolt needs to be 3 lbs and the firing rate would be 400 rpm. The force of the return spring is ignored but used only in the calculation of the cyclic rate. Using the advanced primer ignition concept the bolt becomes 1.3 lb and the rate inceases to 800 rpm, sound familiar? Assuming the calcs are reasonable approximations, a semi would require 3 lb bolt to keep the action closed, actually allow the bolt to move no more than the case head thickness, as the pressure drops to a safe level. If the bolt is slightly over 1 lb and the return springs are over 2 LB. the semi should work. I definitely wouldn't cut it that close but the 7 lb (my guess) from the ez pull springs plus the bolt weight have been proven to work. I just can't see any reason for the Kahr "gorilla" springs. As far as the Blish Lock if you don't believe in "metal adhesion" just the geometry of how it works provides a "delay". Joe

I built a 1928A1 semi using Bob's receiver & a parts kit. It's striker fired very similar to NK/Kahr. I also utilized the blish lock & oiler. The striker spring is the original TSMG spring cut to 6 - 1/4" . The return springs are IMO very similar to the EZpull springs and include the additional buffer. The return springs bear on the actuator, not the bolt. The spring load on the closed bolt is about 7# which is about the same as the TSMG. The pull is easy & smooth. Nothing like the Kahr. I also built an M1 but I used slightly heavier return springs 8+# on the closed bolt. IMO Lancer is right, the blowback is controlled by the weight of the bolt. Althought the advanced primer ignition may come into play in the FA Thompson, IMO the closed bolt semi relies pretty much on the bolt weight. IMO the Blish lock does delay the blowback but it is not totally necessary. The Orelikon 20 mm cannon is a classic example of the advanced primer ignition used in design. It does not have a locked bolt but does have massive springs. I don't think I'd want to pull the trigger on a closed bolt semi copy of that monster. Joe

Doug Richardson's book "New Production Semi-Autos" is a good source of info. http://www.1927a1.com/ Modification to use the FA selector as a bolt hold open: http://www.machinegunboards.com/forums/index.php?showtopic=5767&hl=bolt++hold++back Have fun, Joe

HP, I think I would investigate the disconnector operation. You have replaced just about everthing else. Take off the frame and with a piece of steel push the disconnector flush with the frame. With the disconnector flush with the frame the trigger should not move the sear. The little spring activated claw (lifter) on the trigger assembly should not engage the rear of the sear. Let the disconnector up totally and the trigger should engage and lift the sear. If this is what happens check the slot on the bolt to be sure that it is not clogged and allows the disconnector to be in the fully up position when the bolt is in battery. The slot in the bolt is located so that the disconnector can rise above the frame surface only if the bolt is in battery. Check to make sure the spring that operates the little claw on the trigger assembly is in place & working. If the firing pin was coming forward with the bolt and not catching on the sear the next cartridge would jam on the extended firing pin. This was an ATF requirement. Good Luck Joe

Yeah, but the trouble is you don't become a pro if you don't do it yourself. Gunsmithing is a hobby in which I want to keep increasing my skills. I am already way to far into it to go to "pros". I love knowing what is wrong and how to fix it and it eats at me when I don't. When I retire, which is not too many years away, I may become a SOT gunsmith. I agree with OCM go for it, you have to start somewhere. For what it's worth I used to have a Nk/Kahr 1927A1. The pull was awful. I bought a pak of 5 springs and a 6" x 6" sheet of polyeurethane from Mc Master Carr. I cut 2 springs to size & cut a square out of the poly for the buffer. I had about $30 invested in the supplies. It worked great. It gave me about the same force on the closed bolt as the TSMG. Obviously you will want to dry cycle the action with dummys a number of times to be sure it strips the cartridge and goes into battery. If the disconnector is working properly the trigger won't drop the sear unless the bolt is in battery. If this is a problem it's a problem with or without the lighter springs. If you are not comfortable doing it youself and being able to determine if there is a problem get a Pro. I have to agree with Reconbob, it's not "rocket science". And no I haven't worked on Thompsons for twenty years but I did stay at a Holiday Inn Express last night. Joe

I've researched this subject to build a semi Thompson and 2ndArmored couldn't have said it better and just as PK noted don't change it. When you are looking at the face of the bolt, when it is retracted and held by the pawl, after the last shot in the clip, the firing pin should extend past the feedlips. If the sear is disabled or the disconnector isn't functioning it will stay in that position as the bolt pushes the next cartridge forward and will jam on the extended firing pin. If caught by the sear the firing pin will retract into the bolt and feed the next round. When the bolt is in battery the firing pin will be below the bolt face. When fired, a lip on the firing pin assembly will hit the face of the receiver and limit the protrusion of the firing pin past the bolt face and into the primer to the required .040- .050". The ATF requires that if the sear is disabled or the dis-connector doesn't function that the rifle cannot fire more than one round. The Kahr fire control mechanism meets this requirement. Joe

Weapons Sgt, Doug Richardson details the use and modification of the TSMG lower for the Kahr in his book " Thompson - New production Semi- Autos" . One thing that is not mentioned is that you should weld a plug in the fire control recess that will prevent the insertion of an FA sear. If you do use the TSMG frame here is a good way to use the FA selector as a bolt hold open so the "the third hand" is unecessary. http://www.machinegu...767&hl=selector The SBR is a great modification. Green Mountain Barrels makes a GI spec. 10.5" M1 barrel for less than $100, the only difference between it and the original GI issue is that it is button rifled. IMO the M1 really doesn't look right without the 10.5" barrel. Have fun with your Kahr. Joe

TNT, Another mod to the Kahr 1927A1 is the use of the selector switch to provide a bolt hold open for inserting the drum or clearing jams (the third hand). The Kahr does not have the Full Auto / Single selector switch but if you use a 1928 lower from a parts kit it will. The switch can be added to the kahr and if you use a 1928 lower it must be plugged so that the original FA fire control cannot be inserted. Here is the link to the conversion: http://www.machinegu...767&hl=selector Joe

Scout, Don Bell at Omega has them listed. TM28-2 listed under Thompson 1928a1. Mount for the M1 & M1A1 are the same part. http://www.omega-weapons-systems.com/ Joe

I'll take them Joe

garandcollector, If its a military 1928A1 the compensator is screwed on and pinned with a 9/64" straight pin. I've never had much luck driving out the pin. It probably requires a good fixture to hold the compensator and barrel while driving out the pin. I've removed the pin by drilling out the pin with a .120" drill. I clamped the compensator in the mill vise to do the drilling. The pin will easily pop out after drilling. I replaced the pin with a standard tapered pin I made. Of course the hole will have to be reamed for the taper. The tapered pin can easily be removed if necessary. Joe

Floyd, The parts kit was black oxide if it had not been refinished. A black oxide finish will be the closest match to the parts kit. The metal surface finish is important if you want to match the parts kit. A bead blast gives good results. I'm certain that ReconBob uses a hot black oxide finish similar to the originals. Caswel's room temp Black Oxide kit does a good job and its easy to do at home. The hot black oxide is probably more durable. Attached is a pic of a Philly Ord. Colt 1921 receiver finished with Caswel's black oxide on a parts kit. Joe

Bob, I looked into doing that when I built my 1928A1 semi auto using your Receiver and a parts kit. Since I wanted to keep the blish lock, the distance from the rear of the bolt to the lock slot and the corresponding cuts in the bolt limits the length of the return spring guide rods and length of slot (for springs & rod) in bolt to a little over 3” (about the same as the NK/Kahr). My return springs bear on new tabs on the actuator and not the bolt. With the bolt closed the length of the guide rods determine the distance to the back of the guide rod assembly since the return springs will jam without the rod support. I built up the space in the rear of the receiver to the rear of the guide rod assembly with a spacer and additional buffer material since the receiver is machined to the original length. Using lighter return springs and the additional buffer material gives a cocking pull equivalent to the original FA TSMG. The system works fine with no issues. The M1 is a different story. The NK/Kahr is essentially a M1 action no matter with version you buy (M1 or 1927A1). The M1 type (NK/Kahr) bolt can be drilled deeper than 3” to allow for longer return rods and ability to use the full length of the Bridgeport bolt cavity. The springs also bear directly on the bolt. Certainly lighter longer springs could be used. This in addition to the longer travel length will probably give a much lighter cocking pull but so does the EZ-Spring modification. Everyone seems satisfied with the EZ Spring. One issue modifying the original GI M1 bolt will be drilling the 3” + holes in the hardened steel. A NK/Kahr bolt should be no problem. I read about this mod on DR's website but I’ve never tried it, as I didn’t feel any need for it for what I was doing. I've never heard of anyone who has. I hope someone else has and could give us a report. Joe

cplnorton2, What markings are you putting on? I think you started with a WH Semi-Auto reciever and added a TSMG frame. If that is the case you are not building from parts since you started with a legal semi-auto. You are modifying a currently legal rifle ( the firearm is the reciever). The rifle should be registered as a WH SBR with the serial number that is on the reciever, no new markings required. The serial no & maunufacturer as required in 4a on Form 1 are already on the rifle. If you started with a 80% reciever or a weld up then your name & location go on the rifle along with the Serial number. From Form 1 instructions: h. SERIAL NUMBERS AND OTHER MARKINGS. If an existing firearm is being modified into an NFA firearm, enter the existing serial number of the firearm into item 4g and the name and address of the original manufacturer into item 4a. DO NOT ALTER OR MODIFY THE EXISTING SERIAL NUMBER. If the NFA firearm is being made from parts, your name and address are to be entered into 4a and a serial number you create is to be entered into item 4g. Joe