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Old June 27, 2017, 12:53 PM   #1
Tommy Gun Sucker
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Fixes for Thompson (AO) 50 round drum mags

After having purchased a 1927 A1 semi-auto several years ago, I finally decided to by a NIB 50 round drum mag. What a disappointing purchase. Typically jammed 2 to 3 times per mag. Not something I want to take to the range or let people shoot. As a Navy vet from 69 to 75, I was issued a Tommy gun or BAR every time we practiced repel boarders. Each time we went to sea, I shot both off the fantail. My ship had 25 each of 50 round drum mags and hundreds of stick mags. In all the drum mag shooting I did or witnessed, I don't recall any type of jam. Admittedly, the guns were well maintained as were the drums. Thank you gunners mates

Having purchased an Auto Ordnance 50 round lemon, I made it a goal to fix this mag. It took me bending and filing pawls and bending one of the two feed ramps. I've documented my process in a .pdf available upon request via e-mail. I'm not a gunsmith as can be seen in the photos but the thing simply works. I've put 250 rounds through the gun with no jams or FTFs. Not a single jam and 100 of them were 200 grain lead semi-wadcutters. I'll need to put a lot more rounds downrange to measure the improvement but my fixes helped this drum. NO GUARANTEE this method will work for you but if you currently own an all-for-show, not-for go drum, it's worth a try. Good Luck.
Attached Files
File Type: pdf 50 Round Tommy Gun Jams.pdf (922.4 KB, 428 views)
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Old June 27, 2017, 09:14 PM   #2
James K
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Just FYI, when Numrich sought approval from BATFE to repro the TSMG in a semi-auto form, one requirement was that the new semi-auto only gun NOT accept and function with GI magazines. AFAIK, that is still in effect, which means the old magazines are all "off" when one tries to use them in the new guns. It sounds like you have an original 50 rd drum and it may not work right with the new gun.

Jim
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Old June 27, 2017, 10:06 PM   #3
44 AMP
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James, its kind of a yes and no thing. Yes, GI stick mags 20,& 30rnd won't work in the semi auto Tommygun. The Drum mags will work in the right model semi auto Tommygun, because they latch in differently than the stick mags.

What Numrich did, to get ATF approval was redesign the bolt & trigger group to make it a semi auto, striker fired gun. The addition of the firing pin spring is the main reason the semi autos take 3 men and a boy to pull the bolt back, while the full auto can be cocked with a couple fingers.

A friend of mine has the M1 semi auto variant, and it is about physically impossible to cock the gun with the factory handle. He has a 2 or 3 inch piece of steel bar stock made to fit snugly inside the factory charging knob, which gives enough additional gripping area charging the gun goes from impossible to merely difficult.

I have the 27A1 deluxe, with the larger top mount charging knob, while it is stiff, it is do able.

The other main thing Numrich changed on the semi auto models is the "famous 1/4 inch". Two places. The inside of the semi auto receiver is 1/4" off from the size of the full auto gun, so that you cannot just stick a full auto bolt into the semi receiver. You would have to mill out the entire inside of the upper receiver in order to fit the full auto bolt into it, (and then you would have to rebuild the lower as well, to get the FA bolt to work) this satisfied the ATF that the semi auto Tommygun was not "readily convertible" to full auto.

The other "1/4inch" is the stick magazine catch. Numrich moved the round button catch approx. 1/4" higher (closer to the top of the gun) so GI stick mags would not lock in place. it is fairly simple to adapt GI mags to the semi auto gun, you just make the round hole in the mag body oval, removing a bit of metal at the top of the hole to fit the semi auto catch button's raised location. Once done, they are no longer any good for the full auto gun, though.

The Drum mags lock in differently. RIbs on the drum slide into slots cut in the receiver, and the drum is locked into position by the forward edge of the large magazine release lever on the left side of the frame. These slots are NOT cut into the M1 version, so drum mags cannot be used at all.

The internal button used to latch in the stick mags does not contact the drum, at all.

Stick mags will lock the gun open on the last shot. Drum mags WILL NOT. Numrich (Auto Ordnance) now Kahr Arms, supplies a "3rd hand" with the drums for semi auto use. It is a flat piece of steel that you slide up the stick mag guide channel (which the drum does not use) to activate the bolt hold open. Once the bolt is locked back, you can remove the drum. There is not enough clearance to remove or install a drum with the bolt forward. Proper manual of arms for either drum or stick mag is to have the bolt back when inserting the mag. I am not sure if there is enough clearance to lock in a stick mag with the bolt forward. Maybe when empty. Loaded, you are going to have to force the mag against the bolt pretty hard IF it will even go far enough in to lock. And that top round in the mag is going to be pushing on the bolt with considerable pressure, making the semi even MORE of a bitch to cock. Lock the bolt back, then insert the mag. If you shoot a tommygun, its what you do.

I have two 50 rnd drums for mine. One, I bought brand new, right after the 10 rnd limit expired (there was no way in hell I was going to pay that kind of money for a 10 round drum!!). The other I bought used, and have not yet tested it.

The "new" drum loaded and wound according to the instructions has "realiably" fired each set of 5 rnds between the impellers, and then the bolt hits the impeller. Cocking the gun clears the jam and it fires the next five then jams again. Does this all the way through the mag. I have only tested that drum twice, and am always meaning to tinker with it to see if I can get it working, but something else always seem to come up..

Modified 30 rnd sticks run flawless in my gun, and it feed everything, 230FMJ, 230LRN, 200 LSWC, and 185JHP. My gun, despite a "spongey" trigger will put 5 rnds in one ragged hole at 25yds (from the bench) and using the ladder sight will reliably ring the 200yd gong at the rifle range. Not bad for .45ACP!
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Old June 27, 2017, 10:57 PM   #4
Tommy Gun Sucker
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Fixes for Thompson (AO) 50 round drum mags

Hi guys, thanks for your quick response. Jim, you had me worried a bit as I didn't know about specific laws around the 50 drum mags for semi-autos. I'm sure this is a new drum since it came with the 3rd hand.

44 Amp, thanks for the background info. It sounds like you know your Tommy drums but your symptom sounds suspiciously like the drum not being wound tightly enough. When I first read the winding instructions "9 or 11 clicks", I was way under-winding. As I wind, there are about 11 clicks in 360 degrees. It took me a while to pick up on the sound difference between the different clicks. Actual clicks take place as the anti-reverse tab (for lack of proper terminology) reaches 12 o'clock, 3, 6 and 9 o'clock. All but the 12 o'clock tab can be seen on the back of the drum.

Anyway, I've been using 11 clicks (2 and 3/4ths complete rotations) because I'm not storing the drum in a loaded condition. The only way I can see the bolt striking the impeller (probably better terminology than my "pawl) is if the impeller isn't pushing the next round into place quickly enough, say between the 5th and 6 round.

Possibly high friction or bullet drag.
Best regards.
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Old June 29, 2017, 12:29 AM   #5
James K
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The "9 or 11" clicks is simply because the M1921 fires faster than the M1928; 9 clicks is right for the latter, while 11 clicks will move the follower around faster to keep up with the faster M21 bolt.

"The addition of the firing pin spring is the main reason the semi autos take 3 men and a boy to pull the bolt back, while the full auto can be cocked with a couple fingers"

That is not totally correct. While the different firing pin spring does make some difference, the main culprit is the recoil spring, which has to be made heavier since the new semi-autos fire from a closed bolt and the bolt has to be kept closed until pressure drops. The old guns (whether a 1927 semi-auto or a 21/28 firing in that mode) fire as the bolt is closing, a feature called advanced primer ignition, or API. By working that way, a large part of the forward momentum of the bolt is absorbed by the blowback force of the cartridge pushing the bolt back. Or put another way, the blowback of the cartridge has to first stop the forward moving bolt, then begin to force it backward (in the 21/27/28 the Blish lock does impose some retardation to the bolt, but not to a major degree; it is not a true locked breech, but rather a retarded blowback). So the guns that fire from an open bolt use API and allow use of a fairly light recoil spring; guns that fire from closed bolt must have a strong spring to keep the bolt closed during firing. As a practical matter, open bolt firing is better in many ways, but BATFE bans open bolt firing in semi-auto weapons because it allows such easy conversion to full auto.

Jim
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Old June 29, 2017, 03:21 AM   #6
44 AMP
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Thanks James, I always thought it was the addition of the extra spring that made the semi so difficult to cock.

Firing from the open bolt is better in many ways (including simpler manufacture for blowbacks) but its main advantages are in automatic weapons. I've always found firing semi auto from an open bolt disturbs precise aim. Not important when your target is close and large like human COM, but the bolt group slamming forward can be enough to throw off one's aim, at longer ranges or on smaller targets.

With a little practice, you can learn to compensate fairly well, I think.
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Old June 29, 2017, 11:23 AM   #7
Tommy Gun Sucker
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Repairing jamming Tommy gun drums

Hi guys. Wish I would have checked in Army Tech Manual TM9-1215 before coming up with stupid names for the parts of a drum. The assembly with the pawls or impeller is called the "rotor assembly". Each pawl is called a "finger of the rotor". Not a big deal but I forgot I had a series of these technical manuals.

Jim, thanks so much for the meaning of "9 or 11 clicks". I've never known and thought is was odd the word "or" was used. Today I'm going to hit the range with another 100 rounds, 230 grain ball reloads using cheap ball (CU plated rather than FMJ). They'll get 9 clicks. If I get through them with 9, then I'll know I don't have another issue I need to address.

Update ... made it to the range. Had two friends burn through 50 rounds each wound to 9 clicks. One has a "competition worthy fast finger" and blew through the entire drum faster than I've ever been able to do it. No failures in this additional 2 mags so I'm declaring success on the drum mag fix, and thank you again, 9 click winding ... until proven wrong.

Last edited by Tommy Gun Sucker; June 29, 2017 at 08:45 PM. Reason: Range status
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