MOST TSMG JAMS ARE CAUSED BY FAULTY MAGAZINES
In the last couple of years, I have run about 10,000 rounds through 3 different TSMGs; and, as near as I can tell, I have had no failures to feed and fire that were caused by the gun itself (when using stock parts). Even so, I've had many, many dozens of jams, including some embarrassing ones at Tracies Shoot. All these jams have been traced to bad magazines; the following checklist should help if you are having jamming problems:
The curve of the magazine lips must be cylindrical, in the same shape as a .45 cartridge; there should be neither flattening or vertical flaring .of the lips;
Magazine lips must be smooth and have no dents, bends, or rough spots.. .not even very small ones; check by sight and by feel; rebending and/or a small file will correct;
The two magazine lips in the middle one half inch or so section must be absolutely parallel with gentle curves at both ends;
The large flat "U" formed at the front of the magazine, the "cut away" portion where the cartridge is pushed forward into the chamber, must be cut deep enough for the cartridge to feed out without the rim hanging up of the bottom of the cutout edge. Load in a half dozen cartridges and then "feed" them straight out with your finger; this hang up problem is usually encountered only with 30 round AO magazines;
The magazine body must be "square" and dent free; remove the bottom plate and ensure that the follower runs up and down smoothly without dragging at any one spot or in general. If follower swapping cant solve the problem, discard the magazine.
Most 20 round magazine spring tensions are OK~ but some brands of 30 rounders, primarily Seymours, have too many spring coils; make sure that a 20 round magazine spring is 12 by 13 turns, in a "W" shape, or a 13 by 14 "W" shape; a 30 rounder spring should be a 19 by 20 "W" shape. A decent wire cutter will trim a too long spring. A 1928 model will be most sensitive to magazine spring pressure.
Make sure that the "round" mounting/latching hole in the top rib of the magazine is only slightly vertically elongated, perhaps out of round by a few hundredths only, and that the magazine catch holds the magazine firmly in place with only a little slop. The stories of someone having to lengthen (vertically elongate) the holes (except for a semi WH) is really the story of a bad magazine catch and not a bad magazine.
Make sure that your mags are clean of crud from old storage grease (and perhaps storage paper debris); some "clean" mags are really nasty and dirty inside.
The best tool that I've found to adjust magazine lips is a small, needle nose style pair of pliers, but get one with wide lips (the tip will be about one quarter of an inch wide) and not the actual "needle" nose tip. ("Small" means the overall length is perhaps 4 inches.) The inside of the jaws should be smooth and should not be crosshatched for grip. Sears and most decent hardware stores will have such a tool for about $4 or $5.
If you know of some other "tips" about magazines, please let me know so that I can update this article. Good Shooting!
Contributed by Chris Martin