What is the true rate of fire of a M1921?

[reconbob]

Has anyone used a timer on a M1921? A while back when we did the heavy actuator

JimC did some rate of fire tests using an electronic timing device. However, he did not have

a M1921 to test. His results with WRA 55 military ball ammo:

 

M1928A1 Auto-Ord - 767 RPM

 

M1 Savage - 788 RPM

 

M1A1 Savage - 641

 

M3 Guide Lamp - 404

 

I had a conversation today with a guy who is setting up a rental range and it seems

that in the rental business you get a better rate of return with a fast shooting gun since

you are selling the ammo to the shooters. I told him that M1921s shot faster than the

other models of the Thompson but I realized I have no hard scientific data to back it up.

 

Anyone?

 

Bob

[dalbert]

About 900 rpm...no scientific data, though...

 

David

[Arthur Fliegenheimer]

According to Charles Bronson's character Josh Corey in "You Can't Win Them All" (1970) it's 400 rpm. A Grease Gun can do better than that.

[Sandman1957]

I have repro 1921 internals but no timing devices. Recon Bob or Jim C, I can loan you my 1921 internals if you want to add a column to your stats.

PM me for details if you are interested.

[mnshooter]

930 in my last test.

GI ball

Richardson 1921 buffer system.

Wolff spring (which in this example tests to have about the same weight compression as what I am reasonably sure is an original production spring).

 

Strange, but an M1 with the sand clearance bolt wasn't that far behind, at 880.

(The M1 does have a neoprene buffer, which may be adding some return bounce).

 

Will do some more tests, with more Thompsons, backed with video, when winter is over or eight months from now, whichever comes first.

Edited November 19, 2014 by mnshooter

[gunhistorian]

Whoops! I posted a related comment/question in the locking cut discussion because I am curious just how much the Blish principle affects the rate of fire vs. the weight of the bolt assembly (complete bolt, H-lock, actuator or bolt and bolt handle - on the M1 or M1A-1s). I suppose one should factor in the contribution of the recoil spring, but I suspect that weigh of recoiling parts plays a greater role than resistance of the spring. I'm surprised at the rate of fire of the M1, having believed that it was generally in the 550-600 range. Maybe the state of technology enables us to be much more accurate in ascertaining rate of fire than was available "in the day" -- even 40 or 50 years ago (when the interest in machine guns and submachine guns was just catching hold. I wonder if H.P. White labs did anything? (Now there's a name from the past!)

[buzz]

Ok, my last attempt to explain this was pretty lame so i'll try again with a picture:

 

 

The operation of a Thompson, especially an M1A1, is very similar to the picture

 

the bolt is the mass

 

the recoil spring is the spring

 

the internal friction in the receiver of the gun is the damper. like the damper on a screen door that slows it down and keeps it from slamming. or the shock absorber in your car

 

The force F is the recoil force from the cartridge

 

 

if you change anything about the mass, the spring, or the damping, then the natural cyclic rate of the spring mass system will change

 

the picture shows a classic damped spring mass oscillator,where the mass is free to travel as far as it likes

 

since the bolt on a thompson bangs into a buffer and its travel is limited, that means a change in the recoil force on the bolt would also change the rate of fire

Edited November 21, 2014 by buzz

[buzz]

As an anonymous internet poster, I feel obligated to tell you that I'm not making this up

 

they use the same exact model for everything that oscillates, it's called D'Alemberts Principle

 

I know a lot about this topic since I use it for my job.

Edited November 21, 2014 by buzz

[dalbert]

I wonder if H.P. White labs did anything? (Now there's a name from the past!)

Gunhistorian,

 

Yes, they did, for Kilgore Manufacturing Company, when they acquired the Thompson, and planned production. We don't have any test data, but Helmer found evidence of their involvement during his research in the 1960's, and footnoted it in TGTMTTR.

 

David Albert

Dalbert@sturmgewehr.com

[reconbob]

Thank you for the info gentlemen...

 

Bob

[Merry Ploughboy]

In the July, 2006 issue of Small Arms Review at pp. 50-56 there's a ROF test of a 21 TSMG with Blish device (BD) v. Disabled Blish device (DBD) and an M1A1 as well. Tester/author was CPT Monty Mendenhall.

Rem 230g ball - BD: 934; DBD: 1161: M1A1: not tested

Norinco 230g ball - BD: 883; DBD: 1145; M1A1: 765

 

HTH

[gunhistorian]

That is INTERESTING. A difference of a little over 200 rounds per minute between the 21 with functioning H-lock and with a disabled H-lock. Wonder what the difference would be in a '28. The M1A1 rate of fire is about what I expected. That 200 + rounds per minute difference suggests to me that the H-lock has some but not a great deal of "locking power" (for want of a better term). Of course, the Thompsons with the Blish-type locks are called "delayed blowback" operated systems.

[Rob1928]

Not a '21, but my 1928AC with PK's '21 repo internals clocked 990rpm with Win White Box. It was just under 750 with the same ammo and original parts the one time I clocked it.

[Merry Ploughboy]

The Blish device is of a class of mechanisms that are generally called "lost motion" devices.

[buzz]

interesting

 

well, that answers the riddle

 

the blish lock appears to be working fine in that particular gun

 

 

you could do another experiment -

 

disable the lock and shoot handloaded ammo

 

keep lowering the charge in the cartridge until you got the same ROF as you do when the lock is installed

 

the pressure of the downloaded cartridge will tell you what the pressure is when the blish lock unlocks

 

 

for example, suppose a full charge 20,000 psi cartridge gets an 800 RPM with the lock working normally.

 

suppose it jumps up to 1100 RPM with the lock disabled.

 

then you keep running the gun with the lock disabled, and by trial and error find that a 12,000 psi round makes it run 800 RPM

 

then you would know that during NORMAL operation, the blish lock is locking the bolt up tight until the pressure in the barrel drops from 20,000 to 12,000 psi, and then it unlocks

 

and that of course is the purpose of the blish lock.

 

 

 

 

 

 

In the July, 2006 issue of Small Arms Review at pp. 50-56 there's a ROF test of a 21 TSMG with Blish device (BD) v. Disabled Blish device (DBD) and an M1A1 as well. Tester/author was CPT Monty Mendenhall.

Rem 230g ball - BD: 934; DBD: 1161: M1A1: not tested

Norinco 230g ball - BD: 883; DBD: 1145; M1A1: 765

 

HTH

Edited November 21, 2014 by buzz

[gunhistorian]

Hmmmm. Lost motion device. Looked that up on the 'Net and was not very satisfied. Would seem that ANY automatic firearms mechanism (locking or semi-locking but not the Becker or "advanced fire" principle) would be a "lost motion device". . . but then, I'm not very mechanically inclined. The one thing that I DID find was a reasonable explanation of "striction" on Wikipedia, which seems to encompass the Blish principle. The analogy, I suspect (of the Blish principle to experiences we may have had) is what happens when you have two pieces of plate glass stacked together. Sometimes you can't simply lift the top one off, it seems "stuck" to the bottom one. You have to slide it off to "break" the friction. I suspect that this is "stiction". Now why did I write this? Doesn't seem important, but thought it was sort of interesting.

[buzz]

It doesn't appear to be a lost motion device

 

there's no motion at all while the blish lock is doing its job.

 

if you took a rod and stuck it down the barrel of your gun and gave the rod a mighty smack with a hammer, supposedly the blish lock would grab and keep the bolt from opening

 

the lock would have performed it's job as usual without any motion at all

 

 

when the cartridge is fired, for an instant there will be a thrust on the bolt face of the bore area x the chamber pressure, which in the case of the 45acp is

 

0.45^2*3.141/4 = 0.15 sq in x 20,000 psi = around 3,000 lbs

 

as the bullet zooms down the barrel then pressure will drop and at some particular lower pressure the blish lock will let go and the bolt will retract

 

 

the mechanism of a 1911 colt pistol is actually a kind of a lost motion device, the barrel is free to recoil with the slide while the link is standing at 12 o'clock

 

then as the arc of rotation of the link widens, the link starts slowing down the barrel and pulling it downward

 

it's actually a very clever and foolproof device

[buzz]

a lost motion device is just a mechanism where the initial motion of the mechanism is taken up by some sort of mechanical linkage and then it finally grabs and starts everything moving

 

like if you took all but one tooth off of a gear, the toothless gear would only move the adjacent gear once every 360 degrees of rotation

[buzz]

this is a pretty decent write-up

 

http://en.wikipedia.org/wiki/Blish_lock

[Merry Ploughboy]

The Blish device is most certainly a "lost motion" mechanism. The two-part (up/down) angular movement of the H-block required to permit the fore/aft movement of the bolt is "lost" vis a vis the fore/aft bolt translation length.

[emmagee1917]

Just a little " side note " . The 1911 barrel link is past TDC when the round is fired . The barrel recoils and the link will pivot to TDC just about when the bullet leaves the barrel . Before this point , the barrel is depressed at the rear and so points up a bit . At TDC , the barrel is in it's maximum lock up and most parallel to the sights . Afterwords , the continuing movement pulls the barrel back down and unlocking occurs.

Chris