Model 1927A1 Semi-Auto Thompson
Barrel Removal
QUESTION : How does one remove a Thompson barrel, particularly from a parts kit receiver nose section?
ANSWER: Actually, removing a Thompson barrel is quite easy as long as the barrel is to be scrapped. If the barrel is to be saved, which is invariably the case today, there is just no easy way. The two worst aspects of the Thompson design are the 1 in 10 pitch square barrel threads and the grip mount not being removable without first removing the barrel. There is no excuse for the barrel thread design. It is difficult to manufacture and gage and requires tremendous force to tighten or, especially, loosen the barrel. Although Auto-Ordnance came up with several alternative grip mount designs that enabled the grip mount to be removed without removing the barrel, apparently, none proved to be as good as the one used. The result is that the grip mount is always in the way of trying to remove an already problematic barrel. The best advice I can give is to refer to an article I wrote entitled “Barrel Removal”.
Barrel Removal
According to Army Technical Manual TM 9-1215 on the Thompson Submachine Gun, “To remove barrel, disassemble the gun, wedge a block of hard wood in receiver to prevent springing of the side, clamp receiver in a vise with leather jaws and unscrew the barrel from receiver, using a strap wrench. If a barrel is to be scrapped, a pipe wrench may be used.”
Follow those instructions and you will very likely destroy your gun. The receiver is just not strong enough to withstand the twisting force of barrel removal when held in a vise as described. And no block of hardwood will prevent a vise from collapsing the sides of the receiver. The writer of those instructions was most likely relying more on imagination than experience.
Some barrels, particularly on World War II guns which have seen abuse, are very stubborn and refuse to yield to anything but great force. When that situation is encountered, there is little possibility of removing the barrel without proper equipment. Other times, the barrels may be little more than hand tight.
There are two approaches to removing the barrel: 1. Hold the receiver and turn the barrel or, 2. Hold the barrel and turn the receiver. The method chosen is primarily based on the tools and equipment available. In the home shop and without professional tools, it is best to hold the barrel and turn the receiver. This is because it is easier to hold the barrel in a vise than to grip it with a wrench. Holding the barrel is the main problem. Strap wrenches tend to slip or crush the fins on finned barrels. A pipe wrench does work well but goodbye barrel.
To begin, strip down the gun to the bare receiver/barrel/grip mount assembly. Using the biggest wrench vise available, grip the barrel in the vise jaws between blocks of soft wood like pine. The receiver assembly must be upside-down with the vise jaws close to the receiver. With the receiver upright in the vise. Note that the grip mount is trapped between the vise jaws/blocks with very little clearance on either side. Since the grip mount is attached to the receiver, it will turn with the receiver as the receiver is turned. It is obvious that before the receiver has turned very far, the grip mount will be stopped by the block and will be bent. Therefore position the receiver assembly so that the grip mount is above the blocks and next to the block away from which the grip mount will be turning as the receiver is turned. Tighten the vise. In the case of finned barrels, the wood blocks should go all the way to the base of the fins with the fins cutting into the blocks.
Grasp the receiver nose with a large adjustable open end wrench with a piece of leather between the receiver and wrench in order to protect the receiver. Tighten the wrench to avoid having the wrench engage the corners of the receiver nose. NEVER GRIP THE RECEIVER ANYWHERE OTHER THAN BY ITS NOSE END. With luck the receiver can be unscrewed approximately 1/8 of a turn before the grip mount contacts the other block. It may be necessary to reposition the receiver/barrel/grip mount assembly in order to gain another 1/8 turn which should be sufficient. If the barrel will not turn, it is sometimes helpful to soak the receiver/barrel assembly in gasoline over night. If a parts kit cut-off receiver nose is involved, the receiver nose can be heated red hot with an acetylene torch. Care must be taken not to apply the flame to the barrel or grip mount and not to allow those parts to be over heated. Let the assembly completely cool before again trying to unscrew the barrel. Some barrels will resist all efforts toward removal without special tools. Thompsons have a 1 in 10 square (not acme) thread that is a fairly loose fit, so a 1/4 turn of the barrel should loosen the barrel sufficiently for hand removal. A strap wrench can be used at this point because the force applied to the barrel will be minimal. Remove the assembly from the vise.
Screw a horizontal foregrip screw into the grip mount. Position the claw of a carpenter’s claw hammer around the grip screw as though the screw were a nail to be removed. Pull back on the hammer just enough to force the grip mount away from the barrel and unscrew the barrel by hand.
Barrel Torquing Specification
QUESTION: What is the proper torque requirement when installing a Thompson barrel?
ANSWER: There is no torquing specification that I am aware of and it would not be relevant if there was one. There must have been some figure used at Savage and Auto-Ordnance when the guns were made. It would only make a difference if very powerful and expensive machines were used, and they must have had them. From a practical stand point, it is virtually impossible for an individual to over tighten a Thompson barrel. This is because of two factors. The 1 in 10 pitch of the Thompson square barrel thread does not allow any “tweaking”. The barrel goes from loose to tight in just a few degrees of rotation. The barrel does not get tighter and tighter as is the case with V-threads where the external part (nut) actually expands as the tightening force is increased. If it were possible to measure the torque it would go from 0 to off the scale instantly, making a meaningful measurement unobtainable. The other factor is that it is impossible with reasonably available tools to hold the barrel tight enough to be able to over tighten it. Getting the barrel as tight as possible becomes the goal
Worn Barrels
QUESTION : How does one know if a Thompson barrel is worn out?
ANSWER: When the lands and grooves are no longer distinct, the barrel is worn out. Another indication is that shooting accuracy has fallen off. However, very few Thompson guns will reach the point where the barrel is worn out if military specification barrels are fitted. Unfortunately, most new-made barrels are not made to military specifications, so wear may be more of a problem in the future. A much more likely problem will be bore damage. WW2 guns will often show the affects of corrosive ammunition. This is indicated when the bore appears to be worn (actually eroded) at the breech end rather than over its full length. What is a more worrisome possibility is a ringed or bulged bore caused by defective ammunition. Being a machine gun, a bullet that does not make it out the end of the barrel is immediately follow by more bullets that jam in the barrel, destroying it. At the prices Thompsons are selling for, damaging a barrel becomes a catastrophic loss. Therefore, if a nice condition, all original gun is to be shot, I recommend replacing the barrel assembly so that there is no chance of damaging the original one.