Report On Reconbobs Heavy 28 Actuator

[jim c 351]

Back in Nov.-08 I asked the question-- why were the relief cuts machined on each side of the M1928 actuator? I will try to find these dicsussions and bring them forward.

 

I ask this question after clocking my Bridgeport 28A1 #AO 134,xxx and learning that the rate of fire was 842 RPM.

 

As we all know, the US Navy was unhappy with the Colt 21 cyclic rof of supposedly 800 rpm. The Navy wanted a 600 rpm rof. Obviously my 28 was cycling , not only faster than what the Navy wanted, but also faster than the 21 rof.

 

No one knew the answer to my question. One person speculated that the cuts were dirt grooves. However, this would not have merit for the following reasons.

1. The depth of .125 inch is way more than would be needed.

2. The actuator/bolt fit is very sloppy.

3. When cleaning my 28 I've always found the actuator area to be the cleanest area on the gun.

 

At this point reconbob volunteered to make weights for each side of my Savage actuator to bring its weight back to what it would have been before machining the grooves.

 

Last Thursday, with the help of M60Joe, I ran some tests.

 

Guns used---M1 Savage & 28 Bridgeport.

Bolts used---28 Savage bolt

28 Remington actuator

28 Savage actuator --Reconbob heavy actuator--

28 WW2 buffer

28 Machinegun Mikes buffer

28 used bronze

28 new bronze

M1 Savage bolt

M1A1 AOC bolt

Ammo used-- WRA 55 military ball 230 gr ball

Federal American Eagle factory ball 230 gr--866 fps

Reload--230 gr Rainier--5.5 gr unique--856 fps

Reload--225 gr Lyman cast--5.5 gr Unique--954 fps

Timer--Pact

 

Ten round bursts were fired in test

 

At a later date I hope to publish all the times. For now I will list the total averages.

 

28 with standard Remington actuator and various combinations of buffers and bronze locks--821 RPM

 

28 with heavy Savage- Reconbob actuator- and various combinations of buffers and bronze locks---737 RPM

 

821 rpm -737 rpm =84 rpm difference

 

The heavy Reconbob actuator averaged 84 rpm slower than the standard Remington actuator. But even the heavy actuator does not meet the Navy requirements of 600 rpm. This makes one wonder how they clocked guns in 1928.

 

Conclusion-- At this point in time , without further testing or information, I believe that cutting the grooves in the 28 actuator was a mistake. It was a waste of time, money, and was counterproductive in achieving the Navy requirement of 600 rpm. I guess we will never know what Oscar Payne was thinking. I will also say I consider a 84 rpm reduction worthwhile enough that I will leave Reconbobs actuator in my gun. If any of you feel the same, contact Bob, and see if he wants to get into an additional line of service.

 

I also ran some tests with the M1 Savage.

 

M1 Savage with M1 Savage bolt----total avg--811 rpm

 

M1 Savage with M1A1 AOC bolt---total avg---692 rpm

 

811 rpm-692 rpm =119 rpm difference

 

The M1A1 bolt averaged 119 rounds per minute slower that the M1 bolt. This made a bigger difference than the 28 heavy actuator. I have a theory about this, which occurred to me after reading about a 20 m/m cannon detonating the primer as the bolt moved forward.

The M1 firing pin emerges gradually and therefore detonates the cartridge later in the closing cycle.

The M1A1 firing pin is always all the way forward and detonates the primer sooner and while the bolt is still moving forward. This creates the slower rof. If my theory is correct, then 2 M1A1 bolts of equal weight, both within specs, one with a firing pin at the long end of tolerance and the other at the short end of tolerance,-- the bolt with the longer pin will cycle slower.

 

Something to think about.

 

1928 w/std Remington actuator --821 rpm

M1 w/M1 bolt---- 811 rpm

 

1928 w/ heavy Reconbob actuator-737 rpm

M1 w/M1A1 bolt 692 rpm

 

Being a person who likes the slower rof, the heavy Reconbob actuator will stay in my 28 and the M1A1 bolt will stay in my Savage M1.

The buffers and bronze locks didn't seem to make much difference.

Respectfully submitted,

Jim C

 

[Merry Ploughboy]

Thanks for the report.

 

Was the same recoil spring used for all of the tests? I've seen a big difference in ROF on the same gun and same ammo just by changing the recoil spring.

[jim c 351]

Thanks for the report.

 

Was the same recoil spring used for all of the tests? I've seen a big difference in ROF on the same gun and same ammo just by changing the recoil spring.

 

Yes, same spring. When I got both guns I sorted thru my spare parts and installed the longest springs in both guns.

I did this not to obtain a rof, but I figured it would put less stress on rear of receiver.

Jim C

[reconbob]

Jim - This is most interesting. You make a very credible case for

the 600 RPM rate of fire listed in manuals being fiction and not fact.

Clearly back in the day no serious thought was ever given to this issue.

Maybe lead actuator weights would be a little better than steel to

get you down to closer to 600 RPM...

 

Bob

[jim c 351]

Jim - This is most interesting. You make a very credible case for

the 600 RPM rate of fire listed in manuals being fiction and not fact.

Clearly back in the day no serious thought was ever given to this issue.

Maybe lead actuator weights would be a little better than steel to

get you down to closer to 600 RPM...

 

Bob

 

Bob ,

Its even worse for the H&R Reising. Practically any book or manual you read, with the exception of Frank I's book, List the Reising ROF as 450. While testing the TSMG I also clocked the Reising at 900 RPM, Which is in agreement with Frank I.

I hope, if there is someone out there that has a 28 that has clocked in at 600 RPM with 230 gr Mil spec. ball , He will enter into the discussion.

Thanks again for your help.

JIm C

 

PS By the way Bob, I shoot my 28 at Tusco Rifle Club, which is near New Philidelphia, Ohio. If you ever come to this area for any reason, let me know and I'll give you a list of steel plate shoot dates. You can stop in and shoot your genuine Reconbob actuator.

Edited April 6, 2009 by jim c 351

[Merry Ploughboy]

Jim,

 

Thanks for the confirmation on the use of one spring. Back when I had a 28, there was one recoil spring I had that was so marginal that the brass only was thrown about a foot. I don't have means to check ROF but with that spring the ROF was noticeably lower than a full size Uzi with Winchester White Box 115 gr ammo which is slow running in an Uzi.

[dalbert]

All,

 

Jim asked me to post this spreadsheet with the results of his rate of fire testing. I found that the rate of fire experienced with the Reising was a confirmation for me, as I believe mine runs at about 900rpm. Many thanks to Jim for the detailed comparison with many different ammunition types.

 

You will probably have to click on the image to see it at the correct resolution.

 

http://www.sturmgewehr.com/dalbert/MGBoards/Actuator_ROF_Comparison.JPG

 

David Albert

dalbert@sturmgewehr.com

[jim c 351]

In addition to the above firearms I also clocked my Group Uzi with and without a Vector silencer.

 

9m/m Uzi--151 gr Rainier--4 gr Unique--1025 fps-- 569 RPM.

 

9m/m Uzi--151 gr Rainier--4 gr Unique--1025 fps--w/Vector silencer--609 RPM

 

9m/m Uzi--123 gr Federal factory---535 RPM

 

9m/m Uzi--123 gr Federal factory---w/Vector silencer--607 RPM

 

Jim C

[PhilOhio]

In addition to the above firearms I also clocked my Group Uzi with and without a Vector silencer.

 

9m/m Uzi--151 gr Rainier--4 gr Unique--1025 fps-- 569 RPM.

 

9m/m Uzi--151 gr Rainier--4 gr Unique--1025 fps--w/Vector silencer--609 RPM

 

9m/m Uzi--123 gr Federal factory---535 RPM

 

9m/m Uzi--123 gr Federal factory---w/Vector silencer--607 RPM

 

Jim C

 

Jim,

 

Your increased ROF is about what one can expect. The suppressor holds a little bit more of the pressure over a longer period of time and releases it (relatively) slowly, and in both directions. So besides the recoil from bullet acceleration, you are also getting the empty case and bolt moved rearward more sharply because some of that residual pressure is still being released from the suppressor, through the bore. You might also notice that, for the same reason, your whole bolt head and the breech area will be dirtier with the suppressor than without. Some people also find that they chronograph slightly higher velocities with the suppressor than without. (The physics to explain that is a little bit beyond me.) With your gun, I only see this with the Federal ammo, unless you may have miscopied the second velocity with the handload.

 

[TAS1921AC]

Jim, Have you ever played with these? I don't know that I'd recommend them for long term constant use but a friend tried them and you can vary the rate of fire to suit your taste. The second link is a test results page done by the dealer.

 

http://www.blackjackbuffers.com/index.php?...products_id=190

 

http://www.blackjackbuffers.com/index.php?...1&chapter=0

 

[jim c 351]

Jim, Have you ever played with these? I don't know that I'd recommend them for long term constant use but a friend tried them and you can vary the rate of fire to suit your taste. The second link is a test results page done by the dealer.

 

http://www.blackjackbuffers.com/index.php?...products_id=190

 

http://www.blackjackbuffers.com/index.php?...1&chapter=0

 

TAS1921AC,

I've read about them on uzitalk.com, but never tried them. To be honest, I'd be afraid to use them. I've read stories and seen pictures of Uzi s cracking out the rear receiver plate and I can't help but suspect that they were either using something like this or shooting +p+ ammo.

I think I read somewhere that when shooting a properly balanced load in the Uzi, the bolt stops just short of the rear end of receiver. Haven't run any tests to confirm this, but if true, then a Uzi receiver should never crack.

Thanks for the info.

Jim C

[PhilOhio]

Those aftermarket buffers are also spacers. Decreased bolt travel = increased ROF. As long as you use a reasonably pliable buffer, to moderate bolt impact shock, and as long as you don't use loads that are really dangerously hot, you are not likely to crack the receiver of any well designed, mass produced firearm.

 

Take a look at the extremely long rubber buffer/spacer which IMI used in the original Uzi Model "A", like I have; several inches, as I recall. It allows much shorter bolt travel than the buffers in the above url. It would be interesting to measure the very high ROF this would probably produce if installed in a full auto Uzi.

[Shooter]

Jim,

 

I see that there is a big difference between the rate of fire for a fixed firing pin M1A1 bolt and an M1 bolt that has a hammer, spring and firing pin as in the 28. I am wondering if the rate difference is due to the firing pins or weight differences.

 

What are the weights of the bolt assemblies?

 

Could you make a 28 bolt with a fixed firing pin to test?

 

I suppose this could be done by adding weld material or a small set screw to the back of the hammer where it rests on the bolt. This extra material could then be ground down to shorten the firing pin protrusion in order to test your theory about firing pin protrusion versus rate of fire.

 

TJ

[just4grins]

Those aftermarket buffers are also spacers. Decreased bolt travel = increased ROF. As long as you use a reasonably pliable buffer, to moderate bolt impact shock, and as long as you don't use loads that are really dangerously hot, you are not likely to crack the receiver of any well designed, mass produced firearm.

 

Take a look at the extremely long rubber buffer/spacer which IMI used in the original Uzi Model "A", like I have; several inches, as I recall. It allows much shorter bolt travel than the buffers in the above url. It would be interesting to measure the very high ROF this would probably produce if installed in a full auto Uzi.

My son has a full auto UZI, and a rate buffer made by Blackjack. His goes from about 600 rds/min without the buffer, to 1000 rds/minute with. You are correct - the shorter travel ups the ROF. He also told me that people have made tungsten inserts for their UZI bolts to slow them down, probably for the same reasons you are looking at on the Thompson. He also said that on full size UZI's there is no problem with cracking receivers because the bolt does not actually go back far enough to hit the receiver. On the smaller UZI's, that is not always the case and you might eventually get cracks.

[reconbob]

The difference in the rate of fire between the fixed firing pin M1A1 and

the floating firing pin M1 is that the bolt has greater velocity - i.e. its moving

faster on the M1. But its moving A LOT faster - 23% faster.

I believe the difference here is the difference between the cartridge

being fired milliseconds before the bolt slams shut (advanced primer

ignition) and the bolt being fired when it is fully shut (blowback).

Of course it is common for people - even engineers - to refer to all

of these non-locking gun mechanisms as "blowback". But some guns are

designed so that the cartidge is fired as the bolt is continuing its forward

movement and the forward momentum of the bolt works against the recoil

impulse which starts to push it back and has the effect of lowering the

rearward velocity of the bolt. (M1A1)

The extra milliseconds required for the hammer to pivot aganist the

firing pin when the bolt shuts on the M1 means that much less (or maybe

none) forward momentum is present to work against the recoil impulse so

the bolt recoils faster.

In general the forward movement (counter recoil) of the bolt would be

the same since its being pushed by the recoil spring. But, if the hard buffer

present in both guns causes the bolt to rebound then a faster moving bolt

will rebound more than a slower bolt, which would also keep the bolt velocity

higher and would contribute to the 23% faster cyclic rate.

I think this is one reason the semi auto Thompsons (West Hurley and Kahr)

have so many problems and why they have such heavy recoil springs. The

gun was not designed to fire from a closed bolt. With zero counter-recoil

movement the bolt is going to recoil real fast...

 

My $0.02

 

Bob

Edited April 12, 2009 by reconbob

[jim c 351]

Shooter,

I have no way to accurately weigh the M1 & M1A1 bolts, but Frank I weighed similar bolts and mentioned it in his book on WW2 TSMG. He found no significant difference.

I don't think a fixed pin would work in a 28. It may cause the gun to fire before the Blish is fully locked. I admit I haven't really checked into this, perhaps reconbob could field this one.

I don't have 2 M1A1 bolts to run a firing pin length test. I do however have 2 M3 bolts. One has a .003-.004 inch longer pin than the other. I guess if I was prodded enough I could run the test on the M3. Buy the way, I keep the one with the longer pin in the gun, and that is what was used in above tests.

 

bob,

I might be wrong , but I think you just agreed with me. Good show.

 

All,

I forgot to mention , but I really like the 600 rpm rof of the Uzi. Another reason for not using a long buffer. Another thing, -- I really like using the silencer. A really unique experience.

Last Oct at Knob Creek subgun match I asked Andy Blasek (sic), a shooter 1000 times better that I, if he always used a silencer. His answer---Always.

 

just 4 grins,

I recently hefted a tungsten weighted bolt for a 9m/m M16 bolt. WOW . What a difference.

Jim C

[PhilOhio]

...I believe the difference here is the difference between the cartridge

being fired milliseconds before the bolt slams shut (advanced primer

ignition) and the bolt being fired when it is fully shut (blowback)...

Bob

 

Bob,

 

Very good and interesting post, but whenever the subject of "advanced primer ignition" is raised, so are my eyebrows. For the most part, I don't believe it exists in many of the firearms where it is claimed to exist, primarily slam loaders. There can be such a thing, but only where it is carefully and intentionally designed into the mechanism, and in a gun of a different type. The Thompson is not such a gun. I've heard it used to explain high ROF in Suomis of the WW-II era, for example. Nope. Also wrong type of gun. But even the Finns believed they were doing it.

 

In WW-II, the Germans made the MG-42 an advanced primer ignition gun; turned out to be a very bad and dangerous idea. They back tracked, slowed things down, and turned it into a safe and reliable gun again, albeit still very fast. You simply have to allow for the possibility of unburned powder grains and dirt, partially obstructing the chamber walls, turning your cartridge into a mini grenade. When firing hundreds or thousands of rounds in a very short time, the odds grow against you very fast.

 

Everything depends upon whether the bolt (and only a bolt with a stud firing pin and recessed bolt face) has an extractor with reasonably strong and adequate spring loading. If it does, there is no way in the world that the firing pin can dent the primer until the live round is fully seated in the chamber, with its case mouth solidly stopped by the forward end of the chamber. Only then can the extractor snap over the case rim, and only after that can the pin dent the primer. And that is not advanced primer ignition.

 

Now, if you have a Thompson M1A1 with a very weak extractor (snaps over the rim way early), or if you have dirt and debris creating a tight chamber situation, you sure can get advanced primer ignition...REALLY advanced...out-of-battery-firing advanced.

 

So with an M1A1, the way I understand the mechanism to work, the primer begins to be dented only when the case is fully and firmly seated, with extractor snapping over the rim, but when the bolt still has, let's say, 1/64th" to 1/32" more to go. This distance represents the depth to which the primer will be deeply dented. And that means the bolt still has most of its inertia stored, probably helping to delay ejection slightly. In any event, pretty interesting stuff. To you and me, at least.

 

So my understanding is that, unless you have an extractor designed to not quite work (right), you can have only slam loaders that fire in-battery. Fully so. But you can design a moveable firing pin gun (not a slam loader) to trip the sear just before the action is closed and locked...if you like to live dangerously...or let somebody else do so. The Germans had a lot of soldiers lose their eyesight that way. First, they gave them goggles to wear. I imagine you could spot the MG-42 shooters by their brass fragment pocked faces. Then they started taking smart pills, and scrapped the advanced primer ignition concept. Maybe somebody is doing it again these days. I don't know. But I don't want to shoot one of those.

[just4grins]

I also forgot to mention that my son's Uzi has a suppressor on it. It stabilizes the gun and really makes it easier to shoot without muzzle rise. (He holds onto the suppessor and uses it as a foregrip) Firing it with subsonics is awesome. All you can hear, other than the bolt clanking back and forth, is the bullets hitting the metal silhouette targets. Edited April 12, 2009 by just4grins

[reconbob]

Hey Phil - I know we have respectfully tangled on this before, and I figured

you'd weigh in (as I did) and once again you make good points. Meanwhile

we have two virtually identical guns, but one fires 23% faster than the other

so something is going on. Also, I did not mean in my post that the Thompson

was designed to incorporate advanced primer ignition - I think it just happened

when they redesigned the bolt.

The cartridge is fired during the forward movement of the bolt. The case

seats in the chamber, and the bolt continues its forward movement as the

fixed firing pin indents the primer. The firing pin protrudes approx 0.05" from

the face of the bolt, but the cartridge fires somewhere before the bolt seats

fully. If it took a 0.05" indentation to ignite the primer and your firing pin is

only 0.05" long there would be problems with misfires.

So it may be only 0.02" before the bolt shuts - i.e. the front face of the

bolt contacts the front face of the bolt pocket - but that still means that the

forward momentum of the bolt is available to dull the recoil impulse, at

least thats the way I see it.

You've really intrigued me with your comments on the MG-42. I will

have to dig out the books and start reading up....

 

Bob

 

 

[jim c 351]

reconbob,

Just curious.--Has anyone contacted you for the purpose of making weights for their 28 actuator?

Jim C

[PhilOhio]

Bob,

 

As for advanced primer ignition and whether it relates to these slam loaders with stud firing pins, all of them must have a protruding lower lip on the (thus recessed) boltface. And all of them have a fairly stiff extractor spring. The extractors all have angled faces, which must cam completely up and over the cartridge rim and snap back down BEFORE the stud firing pin can possibly touch the primer. This is how the M1A1 Thompson works.

 

Because of the strength of the extractor spring, it cannot snap over the case rim until the round is fully seated...completely stopped against the front shoulder of the chamber. Then bolt inertia is dissipated by, in order, (1) the extractor snapping over the rim, (2) the pin denting the primer, and (3) the cartridge base seating against the recess in the bolt face.

 

This sequence is why I can't see that advanced primer ignition could take place with this type of firearm. It seems to me that it is a physical impossibility, except...

 

...if dirt, unburned powder grains, or brass shavings create an interference fit, so the round is aligned with the bore but cannot go all the way in. Then there is enough resistance so that the extractor can snap over the rim and the pin can fire the primer prematurely. We have out-of-battery firing, with the case either being severely bulged or exploding...and I've experienced it with a stuck semi firing pin. If by chance the case is seated far enough, but not all the way, the strong part of the case, the web, should still hold the pressure. I've recovered incredibly bulged 7.62 x 39 cases from a range where the web held full pressure, when some idiots were apprently trying to make some Sov Bloc rifle go full auto with some jerry rigged setup. My buddies and I keep a couple of these as souvenir reminders to urge young guys on what not to try.

 

Am I overlooking some technical factor here?

 

Now, I know that many things can be done to increase rate of fire, and I'm wondering if some designers have, in the past, incorporated some of them to boost ROF and have, mistakenly, attributed part of their success to advanced primer ignition? I have also seen the term used in a context which suggests that the author mistakenly thought all slam loaders work on the principle of advanced primer ignition, because he doesn't understand how it really works on something like an early MG-42. In that case, just as the roller locks are about to go into their recesses the striker is allowed to contact and fire the primer and powder burn begins. So, hopefully, by the time pressure builds up, the bolt will be locked. If not, you have big fireworks and brass fragments coming out of the ejection port. Often it did. So the Germans admitted to a rare "oopsy" and gave up on advanced primer ignition MG-42s. So far.

 

As for info on this phenomenon vis a vis MG-42s, there's a wealth of technical and historical information and documentation widely available, and it's really fascinating. Fortunately, the Germans documented everything to death, so all the knowledge they gained was not lost. You can still buy both kinds of MG-42 bolts, the old and scary kind, and the later safer and slower one, as used on current versions. I think some members here have 42s and know a ton more about this than I do. Wish I had one of those babies; got an H&K Lafette mount and optics just waiting for it...perhaps waiting forever.

[kruuth]

This all has got me thinking about that magic 600 round number...do we actually know how they got to the 600 round count? Conventionally we would think that they would drop a 20 round magazine and just time it. I'll use round numbers for this example:

 

20 round mag in 3 seconds = 400 RPM

 

Now, what happens if someone decides that they are going to fire two 20 rounds mags? Now you have to take into account the time that it takes to remove the original magazine, and ready the second one. Suddenly you have:

 

2 20 round mags in 10 seconds = 240 RPM

 

By factoring in the time it takes to reload, we've now lowered the ROF by 40 percent. According Merry Ploughboy the original Reising used 12 round magazines. Using this method of adding reload times you can easily hit 600 RPM. Especially if you set up a table with a bunch of stick magazines or drums and you have someone fire until they get to 600 as opposed to just multiplying things out. Since nobody was at the tests this might be how that number was achieved.

 

Just my 2 cents.

[jim c 351]

This all has got me thinking about that magic 600 round number...do we actually know how they got to the 600 round count? Conventionally we would think that they would drop a 20 round magazine and just time it. I'll use round numbers for this example:

 

20 round mag in 3 seconds = 400 RPM

 

Now, what happens if someone decides that they are going to fire two 20 rounds mags? Now you have to take into account the time that it takes to remove the original magazine, and ready the second one. Suddenly you have:

 

2 20 round mags in 10 seconds = 240 RPM

 

By factoring in the time it takes to reload, we've now lowered the ROF by 40 percent. According Merry Ploughboy the original Reising used 12 round magazines. Using this method of adding reload times you can easily hit 600 RPM. Especially if you set up a table with a bunch of stick magazines or drums and you have someone fire until they get to 600 as opposed to just multiplying things out. Since nobody was at the tests this might be how that number was achieved.

 

Just my 2 cents.

 

kruuth,

If I understand you correctly, You think that a subgun is actually going to fire 600 rds. No way. The 600 RPM refers to the cyclic rate, that is the rate the gun would discharge 600 rds if it had a 600 rd mag. Only a belt fed watercooled would actually shoot this many cartridges in one minute.

I hope I explained it well.

Jim C

[PhilOhio]

Put another way, what Jim is trying to say is that the cyclic rate refers to how many rounds per second the gun fires while you are pulling the trigger and it still has ammo.

[emmagee1917]

And just for the record----Reisings had 20 rd mags first , then 12rds ( a 20 with ribed sides to hold the rounds in a single , rather than staggered , column) to try to improve reliability. Chris