Have you ever heard of anyone cracking an M1 receiver?

[Doug Quaid]

If I remember correctly, USGI ball ammo is supposed to be 230gr FMJ at 830 fps.

But if you handload, it is possible to get much higher velocity than that without exceeding the SAMMI pressure limit.  For example, current Unique powder reloading data shows 966 fps at max charge weight.

Since a lot of guys like to load ammo on the high end of the spectrum, or even past the high end, I was wondering if anyone has ever managed to crack an M1 type Thompson.

I remember reading something about people cracking 1928 receivers by using a "speed bolt".

 

[BillyDixon]

what do you mean by cracking ? on the M1 no real locking , bolt just slams back, i suppose with a diet of super insane .45 reloads the bolt would bottom out at back of reciever possibly leading to strech cracking . ive heard of some aftre market m1 recievers that were made of something like 1018 free machining steel with no heat treat the bolt with gi ball ammo started to stretch reciever making hard to press in take down plunger to dissamble possibly leading to stretch cracking, just sayn

[Got Uzi]

The other part of this equation is the fact people don’t maintain their gun but changing out recoil springs. The recoils spring is the first line of keeping your gun running, while not letting it beat itself to death. Springs wear out and compress over repeated use. 

[Rabbit57]

Got Uzi, what is the rate you change your recoil springs out? I have to admit, I never thought of that.

Mark

[Got Uzi]

Typically 3,000-5,000 rds or when it starts to feel sluggish. 

[reconbob]

       Back in the 1980s there was a magazine called  The Machine Gun News. There

was a regular feature called "Tinkering With Thompsons" written by a guy who had a business

called "Gunmachines". Many of his articles were devoted to increasing the cyclic rate of Thompsons

by lightening the weight of the bolt by drilling holes and other similar modifications. Of course, no

good came from this and the combination of his Gunmachines "speed bolts" with West Hurley receivers could

result in cracking at the rear inside corner of the receiver. I believe the West Hurley receivers

were made from 12L14 free machining steel to cut costs. The 12L14 has 3% lead and can be machined

twice as fast with longer tool life than any non-leaded steel. BUT, because of the lead content you cannot

weld 12L14.  I believe PK has photos of cracked receivers.

     I have never seen or heard of an M1/M1A1 receiver that cracked. The receiver is much thicker and

the geometry is more resistant to cracking, but I am only one guy. Maybe others have different info.

     The spec for a recoil spring is 11 5/16" long, +/- 5/16. So one way to monitor is to measure. If the

spring becomes too short it is becoming fatigued and should be replaced.

Bob

[maxfaxdude]

Check out this past thread:

https://www.machinegunboards.com/forums/index.php?/topic/24634-cracked-m1a1-receiver/

Although not the same thing, I have seen MP5 receivers crack at the machinegun rental ranges we have here in Vegas.  They just weld the receivers back up and keep going.  Many of the guns have more than a million rounds through them. 

[Dan K]

I run my M1A1 with loads that just run the gun, i.e. book minimum. For making noise in the backyard, I see no need to run anything hotter, I have rifles for serious work.

[Blanksguy]

This might be a really good time for someone to post the "free-length" of both a new "US-Military" Thompson SMG recoil-spring..........and the "free-length" of a new "US-Military" Grease-Gun recoil-spring...............prior to them taking a "set" from use/over-use (??)...................maybe even the number of "coils" and the diameter of the spring-wire used (??).

[Doug Quaid]

On 6/1/2024 at 1:15 PM, Dan K said:

I run my M1A1 with loads that just run the gun, i.e. book minimum. For making noise in the backyard, I see no need to run anything hotter, I have rifles for serious work.

I plan on doing that too going forward.

I've been trying out various minimum (or "starting") charge weight hand loads and my M1A1 runs just fine on them.

I have found that quick burning powder is best for this application.   Some medium burn rate powders produce very high velocities out of a 10" barrel, when you use load recipes for 5" barrel guns.   


 

[Got Uzi]

5.7 gr of Unique with a 230 grain ball using a Lee factory crimp die….works in every Thompson and 1911 I’ve ever owned. 

[pstidan]

1 minute ago, Got Uzi said:

5.7 gr of Unique with a 230 grain ball using a Lee factory crimp die….works in every Thompson and 1911 I’ve ever owned. 

Yep.  That's exactly the perfect 45 ACP load for me too.

[68coupe]

Anyone use Alliant Power Pistol? If so, got a favorite recipe?

Karl, 68coupe

[StrangeRanger]

5.4 W231 230 Plated RN duplicates factory

5.6 W231 185 Berry's hollow-base RN, VERY mild, slows a Reising from 910 to 845 RPM

[Maine-iac]

7 hours ago, pstidan said:

Yep.  That's exactly the perfect 45 ACP load for me too.

Noted! I'm a big fan of using unique under my powdercoated cast bullets. 

 

On the subject of OPs question, I have seen one Thompson receiver "crack".  It was a mediocre reweld that spontaneously disassembled itself like a... Slinky, the sections held together by a spiraled thread of flux core weld lines. 

 

Not exactly a common failure mode, hah!