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!