Thompson SMG Barrel Bulging

I have just heard a story regarding the earlier Thompsom SMGs, those with high cyclic rates, sometimes experiencing barrel bulging due to defective .45 ammunition. This is the first time I knew about this happening. Does anyone know any more about this story?


There are many documented examples of bulged barrels, the proverbial snake swallowing a rat. They occur in all types of firearms - rifles, pistols, revolvers, MGs. HATCHER’S NOTEBOOK has an entire chapter on barrel obstructions.

I’m not sure that the cyclic rate of the different Thompsons would make any difference.


Searching through some Thompson sites, it does seem that barrel bulging incidents were known to have occured with the Model 1921, although the cause is unexplained. The description provided is that a bullet from a defective round does not exit the barrel, and is immediately followed by several more rounds being fired before things come to a halt. As I said, the root cause was not elaborated upon, and I must wonder how a squib load would cause the action to function to enable firing three or four additional rounds. As the M1921 has a high cyclic rate, maybe the bolt mechanism functions more readily with light loads, i.e., a squib load will do it. And later, and lower firing rate, models will not function with lighter loads, so that problem may not have existed with them. In any event, I wouldn’t want to be holding a Thompson when this happened. I just thought someone might have been familiar with this eccentricity of the Model 1921.


It would be interesting if someone knows how many fps it takes to push a bullet completely out of a 10.5" barrel. I would think it would take a very low velocity for a bullet not to exit and, as you said, how would the mechanism function with such a sqib load. I used to own a Thompson and had a time or two when the bullet stuck in the barrel. But, the action did not function. When I extracted the empty case I was smart enough to know to check the barrel and not to fire another round behind it.

Most barrel obstruction incidents are the result of a squib load, not checked, and then one or more rounds fired after it. There are known examples of revolver barrels with all six bullets lodged in barrel. I suppose the gas escaped through the cylinder gap but it makes you wonder what the shooter was thinking when he didn’t see a bullet hitting anything. ??


To add one more comment. The M1921 and M1928 were known for producing squibs, not because of the ammunition but because of the design. They had a firing pin but it was struck by a “hammer” which was only a triangular piece of steel that struck the inside of the receiver and transferred the energy to the firing pin. Not exactly the ideal set-up, especially if the mechanism became dirty.

Later models eliminated both the firing pin and hammer (and the Blish sliding lock), none of which were actually needed for the gun to function.


Frost’s book “Ammunition Making” describes a situation where a cop’s revolver (I think it was a Colt New Service) in .45 ended up with all six bullets getting stuck in the barrel due to the use of ammunition which had remained loaded in the revolver for many years without being fired.

I don’t believe that bulging of a TSMG barrel due to an obstruction would be lmited to any one model. I have a picture of myself firing a tommy gun with a very clear bulge in the barrel between the cooling fins and the back of the Cutts Comp/Front Sight band. It belonged to the FBI and they used it for demonstations, etc. and just left the barrel bulged. It did not hinder the performance of the gun, which as I recall was an M1928, but even if a “21” definitely had the rate of fire reducing kit in it.

One of my customers stuck 18 or 19 bullets in the barrel of a Numrich Thomspon 28 Carbine (16" barrel semi auto. It finally split the barrel and we had a picture at the store of it with one bullet nose poling out of the top of the barrel.

One of our cowboy shooters uses a slightly bulged-barrel .44-40 92 Winchester, and it performs ust fine. Of course, CAS is not a precision shooting sport, but rather a speed sport with even rifles shot at pretty close ranges on very big targets. The gilt-edge accuracy of the gun, if any lever action rifle can be so discrbibed, was probably gone.

If I had not just busted my scanner some how, I would copy the picture of me shooting the gun - it is amazing how clearly the bulge can be seen even in an upper torso and gun photo.

We shot a lot of rounds through that gun with no obvious further damage to it, and I was told by one of the Agents that the gun had been with the bulged barrels, and used that way, for years.

I really enjoy these weapons but over time, many have been abused in more ways than one. Squibb’s, with out checking the bore, can do crazy things. As John says, multiple bullets in the bore are actually quite common. The weapon in the picture is my Colt 21/28 Navy. To give you an idea how long ago I bought it, the price was $900 and included 1000 rounds of USGI ball, two drums, and more magazines than I could ever use. The good old days!. JH

The M1921 had a light bolt which gave it the high cyclic rate. Perhaps a squib would still have enough power to cycle this model. The bolt on the M1928 was made heavier to lower the cyclic rate.

While the Colt 1921 does have a lighter actuator, the bolts are identical to the 1928. In addition, the 1921 has a more powerful recoil spring. Together with the lighter mass & a more powerful recoil spring, the rate is much higher on the 21. The “at rest” spring tension on the 21 assembly is actually a bit higher than the 28. It takes a standard load to reliably run either mechanism. I reload for and shoot both. Most bulged barrels IMO, are operator error. JH

Orange’s answer jibes with my line of thought thought that there may have been something about the M1921 which allowed full bolt movement (or at least enough bolt movement) with a greatly undercharged round, therefore following rounds could be fired, bulging the barrel. I suppose operator error would enter into it if there were a squib load, and then the operator performed immediate action to continue firing. I very, very nearly did that once with a .30 Carbine, but something told me to not pull the trigger, and I’m glad I didn’t.

At one time, I performed a series of tests using some condemned M9 pistols and M16A2 rifles to determine the effects of in-bore collisions with a bullet driven into the bore different distances ahead of the chamber. It’s been a long time, but in some stuck bullet locations the barrel bulged and in others it did not - I’d have to pull out my old test report for details. Anyway, I did not experience any barrel ruptures with any weapon. I do remember some case ruptures during the M16A2 tests, but no damage beyond blowing out the magazine. Of course, all firing was done remotely.

Personally, I have experienced only one accidental barrel bulge incident. Back when I was a kid, I had a Remington Model 550 .22 (a fine rifle, that I wish I still had). I managed to get some mud in the muzzle and inadvertently fired it. It created a slight bulge about 2" behind the muzzle. It was not visible on the barrel’s exterior, but you could see a dark ring by looking down the bore. It didn’t seem to have a noticeable effect on accuracy. I never did anything about it as it seemed to shoot just fine.

Orange - while there may be some, I cannot think of many instances when a bulged barrel is not the operator’s fault. The barrel should be checked before firing. Of course, I realize that under combat situations, this detail could be forgotten and mud or something could get in the bore. Still an operator error but 100% understandable.
Of course, and sqib load that might stick a bullet in the bore can be heard, and firing should cease. That is always operator error. The one extenuating circumstance there is that with full auto weapons, one may hear it, know he should stop firing, but simply can’t respons quick enough to stop that next round(s) from being fired. I’d hate to have to respond to the sound of a squib in a German MG 42! So, in theory, agree with Orange.

I recall that one pistol’s specs (HK if memory serves, either the USP or SOCOM Mk23) required the pistol to be fired in said condition (bullet lodged in barrel) with no rupture of the barrel or decrease in immediate reliability. One of the tech sheets showed a very slightly bulged barrel on a test pistol.

This of course is much less of a concern with pistol ammunition in a handgun vs. an SMG or a centerfire rifle like Dennis’ M1.

Our agency’s firearms have had no such problems, thank goodness, but I’ve seen a number of confiscated/evidence guns with bulged or split barrels.

I haven’t seen an HK 4 in so long that I forget what it looks like inside the slide, where the barrel goes. A bulged barrel from shooting out a single obstruction usually does not end the usefulness of a revolver but it usually does stop an auto pistol, as the barrel bushing, be it a separate piece like a Colt 1911 or simply the diameter of the hole in the front of the slide through whcih the barrel protrudes, will usually not pass over the bulge, but instead hangs up on it and the slide often has to be forced forward off the bulge before anything can be done to put the pistol back in service. I am not sure I would consider any self-loading pistol that is not of the general Luger profile - that is, the barrel is attached in front of the entire firing mechanism (Luger, Lahti, Glisenti, etc.) to be usable with a bulged-barrel. I have seen the result on dozens of pistols fired with obstructions - pistols with enclosed barrels - over the years, and in fact have witnessed three of them happen, two of which I tried to stop the shooter before he fired again after a load that from the sound obviously had problems. In all three cases (match pistols built on the Government Model shooting platform, one a .38 Special and the other two .45s), the slide jammed beyond our ability to free it on the spot without risking damage to the back of the sdlide (one guy wanted to pound the slide forward off the bulge by pounding on the back of the slide with a claw hammer he found in the range shack!). Squib loads and resulting bulged barrels CAN matter in some gun designs, even when dealing with low pressure pistol cartridges.

Mods, if this is too far off the original post, please delete or move.

John, I found the text from the tests of the H&K (USP). The USP series has a lot of ‘slop’ between slide and barrel compared to the 1911, as do most Glocks and several other ‘tupperware’ pistols. The H&K USP and Mk23 address this via a replaceable O-ring that fits onto the barrel to tighten things up a bit when the pistol is in battery. The text below:

"Barrel obstruction test Live round fired with projectile
positioned at forcing cone and
30mm into muzzle.

                                        No damage to pistol. 
                                        Accuracy unaffected."

Given the overall toughness of the design/specs on these particular pistols, I’m betting the bulge was slight enough not to interfere with the operation, at least in the short-term. My Google-fu is weak and I did not locate a copy of the bulged-barrel photo H&K used to have in their USP literature. I did however find plenty of catastrophic failures (barrel and slide) from the USP and several other handguns.

First agency I worked at had a Beretta .40cal that had been inadvertently fired with a round lodged in the barrel; locking block and barrel were damaged and the weapon had to be ‘unfrozen’ at the range shop.

I can see how any ‘tightly’ spec’d pistol or long gun, esp. a match gun w/barrel bushing, would suffer from even a very slight barrel deformity.

Getting into very specific details, a fired ‘stuck’ bullet would most likely have a tighter ‘seal’ against the inside of the barrel than a bullet driven into the ‘stuck’ position by hand. Either way, a bad day.

Of course, and I am sure most on this Forum know this, a stuck bullet of itself seldom damages a pistol or revolver. It is when the next round is fired that the damage occurs, and it occurs behind the base of the second
bullet, not where the nose of the second bullet strikes the base of the first bullet. It is the retardation and accompanied expansion of the gas behind the second bullet that creates the bulge.

Getting into very specific details, a fired ‘stuck’ bullet would most likely have a tighter ‘seal’ against the inside of the barrel than a bullet driven into the ‘stuck’ position by hand. Either way, a bad day.

I assume you mean that pressure behind the bullet would upset it a bit, making the bore squeeze a little tighter. I would not think that bullet upset would occur, especially with a jacketed bullet, as the pressure in a squib load would be minimal if it was not even enough to allow the bullet to exit the muzzle.

I remember reading something about one of the perils of African big game hunting being the presence of some species of wasp that liked to build mud nests in the bores of the large-caliber rifles, un-noticed by the hunter, with predictable results when fired.

The pressure of the squib load is minimal, and generally, or at least in my experience of people bringing guns to
us with projectiles stuck in them or with damaged bores from firing another shot, over my 36 years in the business. I am not talking at all about “bullet upset.” I am talking about the pressure spike behind the next shot fired after a bullet gets caught in the bore, for whatever reason (squib load, no powder, etc), which is what causes the circular bulge in the barrel (gas expansion) behind the second bullet. Many of our customers not really familiar with more than simply firing firearms assumed that the bulge in the barrel came behind the first bullet stuck, as the second bullet hit its base (usually driving both bullets out of the bore), and that therefore, the bulge was at the point behind the first bullet. It is not - it is at the base of the second bullet, the one behind the bullet that initially stuck in the barrel. Therefore, when a single bullet gets stuck, if another shot is NOT fired, then that bullet can usually be removed from the barrel with the pistol or revolver suffering little or no damage.

I’m sorry that I did not seem clear in what I said. It seemed clear to me, but then I am the one who wrote it. I hope the above clarifies what I was talking about. I only posted it for those with not much experience in this matter and that often think the damage occurs behind (at the base of) the first bullet to stick in the barrel.

Seems I remember from Hatcher’s Notebook, his prescription for removing a stuck bullet was to pull the bullet from another live round, dump out half the powder, insert the half-charged case into the chamber and fire it vertically. I’ve never tried that trick, but I ran across an old Stevens tip-up .22 rifle once that had a bullet (or maybe several bullets) stuck about midway, and no mater what I tried, it could not be driven out with a rod and hammer. I eventually had to use my propane torch and heat the barrel at the point of obstruction to melt out the stuck bullet.

I have removed dozens of lead and jacket bullets from barrels at the store, the results of people buying crappy commercial reloads (I am not saying that all commercial reloads are "crappy), or their own poor efforts at handloading. Never had a problem with most, but they should always be driven back down the bore from where they came, even if close to the muzzle, and not tap forward out of the muzzle of the bore. Any I could not take out, usually because I didn’t want to push my own limits - I am NOT(!) a gunsmith - our gunsmith got out easily. The only gun, ironically, that we couldn’t get an obstruction out of (a cleaning rod with too tight a patch) was a .25-20 Winchester 92 SRC barrel. The gunsmith sent it back with the barrel unscrewed and the rod still in it. I say ironically, because it belong to one of my dearest friends. I guess this case is getting outside the realm of this Forum, although it supplements the discussion of ammunition components stuck in barrels. Sorry about that.