M1 vs. M1A1 bolt...apparently not the same...

[reconbob]

I just rebarreled a Savage WW2 gun with an original Savage mint condition barrel.

Of course, the next thing is to test fire to insure that the barrel is ok - does not have a jugged

chamber, wrong headspace, etc. It is very rare to have these problems, but as I have

posted previously occasionally defective parts are found.

 

When test firing - at least here - the drill is to first fire one shot. One shot just in case

the gun short recoils, and also to check the case for problems. After this two shots are

fired full auto, followed by two rounds semi-auto.

 

When the first shot was fired in this gun the bolt did not recoil far enough to activate

the trip to hold the bolt open, although the cartridge did eject. This short recoil is of

concern because it is possible for the bolt to recoil far enough to eject and feed

cartridges, yet not far enough to be held by the sear the result being a runaway gun.

This is rare, but it occasionally happens.

 

So why the short recoil? Underpowered ammo? Too-strong recoil spring? Some

friction in the cycling of the bolt? I changed recoil springs - same result.

 

I had tested this gun with an M1A1 bolt (I was sent the stripped barreled receiver

only). I have always considered that a gun would be more vigorous and reliable with

an M1 bolt so I tried an M1 bolt in the gun and the gun worked properly in every respect.

Full recoil to activate bolt hold open and semi auto function.

 

To double check I switched back to M1A1 and the bolt would not recoil far enough

for the bolt to stay open.

 

This all reminded me of the Jim C (of Medal of Honor pinned post fame) rate of fire

study where he compared M1928A1, M1, and M1A1 rates of fire. The M1 had the highest

rate of fire of the three being 19 rpm faster than the M1928A1 ( and almost 150 rpm faster

than M1) Now of course this is not an exhaustive study but if I had the choice between an

M1 or an M1A1 bolt I would choose the M1A1.

 

Thompsons are famous for being reliable guns and its interesting when an anomaly like

this is found. It would be interesting to see what effect WW2 military ball ammo would have

on this but none was available.

 

Bob

[jim c 351]

Bob,

Did you have a second M1A1 bolt to test???

Jim C

[reconbob]

I do have another M1A1 bolt but I am beyond more test firing...next time. Also

I made a mistake in my post (which I corrected). If I had my choice I would take

an M1 bolt over the M1A1.

 

Bob

[jim c 351]

Bob,

My guess is the M1A1 bolt is the problem.

The gun doesn't know whether a fixed firing pin or a movable firing pin is discharging the cartridge. Both bolts accomplished that. And both bolts should weigh the same.

So, if the M1A1 bolt isn't smoothly and freely recoiling , then the problem is the bolt.

As far as preferences go, I have an M1A1 bolt in my M1 gun.

Jim C

[Robert Henley]

I do have another M1A1 bolt but I am beyond more test firing...next time. Also

I made a mistake in my post (which I corrected). If I had my choice I would take

an M1 bolt over the M1A1.

 

Bob

 

And I recall from a thread some time ago the M1 bolt is preferred for left-handed shooters.

[richard w.]

So, what would you guess the problem is with the M1A1 bolt?

 

Aren't they getting the same blowback from the discharged round?

[jim c 351]

Rich,

Hard to tell at this point and we will never know for sure.

Something is causing excessive friction.

Perhaps the spring hole was drilled off center. Perhaps the bolt raceway in the receiver was undersize and the bolt was oversize.

Perhaps the gun will come back to Bobs shop someday and he can test with another M1A1 bolt. Or maybe he will try the bolt in question in another gun.

You'll just have to stay tuned.

Jim C

[reconbob]

There is something different between an M1 and an M1A1 bolt. The M1 bolt in JimC's test

had a rate of fire almost 150 rpm faster than the M1A1 bolt. If the bolts weigh the same, etc.

it has to be something...

 

Bob

[jim c 351]

Bob,

This phenomenon probably involves more than one factor.

One factor could be time of ignition.

The M1A1 firing pin is fixed in the foreword position.

The M1 firing protrudes more as the bolt closes.

A longer firing pin will cause detonation while the bolt is still moving forward.

A shorter pin might cause detonation as the bolt stopped moving forward.

I did a later test using an M1 Savage bolt and an M1A1 AOB bolt. The load used was my reload,--225 lead RN & 5.5 gr Unique.

M1 Savage bolt ---775 RPM

M1A1 AOB bolt----578 RPM

Same as you mentioned above.

Then I followed with a new experiment. Using the same M1 Savage bolt as above , I shimmed the firing pin in the forward position, creating an M1A1 bolt.

I expected the timer to read 578 RPM as with the M1A1 bolt . I was quite shocked when the timer read 990 RPM. That's as fast as a Colt 1921 TSMG.

Unfortunately I failed to record the amount of firing pin protrusion with the modified M1 bolt.

 

Of course other factors, mostly involving friction, come into play.

Bottom line,----If a person wants to know the ROF of his SMG, he must time it with his ammo.

Jim C

[MG08]

I have found my M1A1 guns to be much faster ( no timer so this is subjective) than the 28a1. I will have to see if I still have an M1 bolt in spares and see if there is a noticeable difference in my guns. Ammo also makes a huge difference. I pretty much use my reloads - 230 gr FMJ or RNL with 5.0 gr HP38. When I used another reload with 6.0 gr of Unique ( stout load by the way) it ran significantly faster in all 4 guns (2 1928A1s and 2 M1A1). Factory ammo like TZZ or USGI also ran faster than the reloads. What do most of the guys use to measure ROF (as in what brand/ model timer) ?

[Robert Henley]

I was wondering how you measure the ROF too. Time how long it takes to shoot a 30 round magazine, or some other more precise method?

[jim c 351]

MG,

These tests were done several years ago and the timers belonged to two other shooters.

I can no longer remember the names of the manufacturers. Several companies make such instruments and I'm sure they are all good,

 

Sorry.

Jim C

[gunhistorian]

Be interesting to use milsurp ammo. I wonder if the M1A-1 bolt is somehow -- instead of being pure blowback -- causing "advanced primer ignition" which would cause the short recoil in the rearward travel of the bolt. But WHY would this be happening?

[mnshooter]

Another factor in play could be the resistance of the extractor.

Any significant change in the extractor tension will change the total time into battery.

Granted, it shouldn't be much, but it could add up; maybe worth checking out.

When the weather finally allows, I'll do some bolt comparisons with different extractor tensions.

Never hurts to have one more reason to get to the range.

 

As far as timers go, many of the better shot timers will also give ROF.

But, I have seem them fooled by echos, so some care is required in the setup.

Edited March 6, 2015 by mnshooter

[Frank Iannamico]

See AT II pages 490 >

 

For info on M1 VS M1A1 cyclic rates.

[Kocapuff1]

See AT II pages 490 >

 

For info on M1 VS M1A1 cyclic rates.

Frank,

I'm sure there are many of us that do not have American Thunder II. Do you want to post the information please? I and many others are awaiting your American Thunder III for all that great information. Any update Frank on when AT III will be published and out for sale?

Edited March 10, 2015 by Kocapuff1

[Dolphinvet]

 

I do have another M1A1 bolt but I am beyond more test firing...next time. Also

I made a mistake in my post (which I corrected). If I had my choice I would take

an M1 bolt over the M1A1.

 

Bob

 

And I recall from a thread some time ago the M1 bolt is preferred for left-handed shooters.

I'm a left handed shooter so that may explain why my M1 fires so well with me.

[jim c 351]

Kocapuf.

If this subject interests you do a search under the following,-------------------

"Report on reconbobs heavy 28 actuator".

That will provide even more info than American Thunder.

Unfortunately I am no longer able to copy and paste.

It would probably be a really swell idea for dalbert to put the chart in the pinned section. Perhaps he will add it now. The subject does come up every so often.

Jim C

[TSMGguy]

It's a small test sample, but the M1 bolt yields a faster rate of fire in my WH M1 that a M1A1 bolt. With the M1 bolt, the ROF is identical to my M1928A1, a very even 750 RPM.

 

Reconbob, make certain that the M1A1 bolt that you used is not one of the WH bolts, which are longer and heavier than GI. WH bolts have no maker markings.

[darrylta]

PK stated that most all WH M1A1 bolts were machined out of spec and generally eats up the receiver.

Food for thought,

Darryl

[buzz]

it would be interesting if someone could figure out exactly how much "advanced primer ignition" takes place

 

if any

 

I have my doubts about it

 

it takes a decent amount of force to dent the primer enough to fire it

 

the advanced primer ignition cannot take place unless the firing pin is lined up with the primer

 

at that point all the bolt is doing is sliding the cartridge into the chamber

 

where is the resisting force supposed to come from?

 

the only available force is the friction force from the cartridge sliding along the chamber

 

is that little bit of force enough to fire the primer?

 

 

the way to test it would be to somehow keep the bolt from closing all the way, keep it open by say 2 mm

 

then put a dummy round in the mag with a live primer

 

if the primer doesn't pop when the bolt slaps it into the chamber, you'd know that you weren't getting advanced ignition

Edited March 12, 2015 by buzz

[buzz]

I thought of a better way to see if advanced ignition occurs

 

take a 45 case and shorten it by cutting off the case mouth

 

shave off the same amount of length as the depth that the firing pin usually dents the case

 

that way you would eliminate having the cartridge fire when the case mouth impinges on the end of the chamber

 

if you fire the gun and the shorty cartridge fails to fire, then you have no advanced primer ignition

 

if you fire the gun and the shorty cartridge fires, then you have advanced cartridge ignition

 

you'd have to remove the extractor hook to insure that the cartridge wasn't headspacing on the end of the hook

Edited March 12, 2015 by buzz

[gunhistorian]

Maybe that is a significant clue (the cartridge headspacing on the extractor hook). It is probably unlikely, but I wonder if the softness or hardness of the cartridge case makes any difference in the ROF? That is, would a steel cartridge case cause a higher ROF than one of brass? And would a cartridge case of softer brass give a lower ROF than one of harder brass? This, of course, goes beyond the strict issue of the types of bolts involved!

[buzz]

you'd have to test it

 

you can do very sophisticated calculations for motion, but you'd need to plug in a lot of data

 

all the data would have to be guessed at

 

the answer you get from mathematically churning 100 assumptions wouldn't be useful for anything

[Dolphinvet]

I rather doubt that the fixed firing pin vs. the floating firing pin and hammer are different in any way for ignition. Unless your case has enough friction to bind in the chamber somewhat, then I can't see where the ignition would be at a different spot in the chamber from fixed to floating. The case rim has to engage the extractor before it can engage the firing pin in either situation, and as it is being pushed along by the bolt lip on both types of bolts, the firing pin won't come into play until it is seated in the chamber. If this were not true, we would see OOB firing occurring on a regular basis. Additionally, by the time the bolt is nearly seated home on an M1 bolt, the hammer is contacting the receiver and pushing the firing pin forward. What I'm saying is that the firing pin on an M1 bolt should be protruded the same as it is on an M1A1 bolt by the time the bolt completes its forward momentum at the moment of firing. I would think the differences that occur would be due to changes in machining of the bolt itself, and of how early the hammer on an M1 bolt is contacting the receiver thus protruding the firing pin towards the primer on the case. Has anyone ever measured the protrusion of the firing pin on an M1 bolt vs. a M1A1 bolt? I would think machining tolerances and their differences plus perhaps earlier and deeper protrusion of a floating firing pin on the M1 bolt would affect cyclic rate.