Here's some more info:
There were only TWO makers of the Auto-Ordnance semi-auto firearms....Numrich Arms/Gun Parts Corporation, and now Kahr Arms, who bought the design from Numrich.
All the other "Thompson" guns are "look-alikes" that only vaguely look like a Thompson.
These other guns ARE NOT Thompson guns, and are made of stamped sheet metal.
For the most part, these guns run more to the "junk" side of quality.
The Auto-Ordnance as made by Numrich and now Kahr is made of solid milled steel and walnut.
You will get a variety of opinion on the Quality of the Numrich versus the Kahr. Various people say one or the other is better, or worse quality.
The M1 version of the Thompson is a semi-auto version of the later WWII M1 Thompson submachine gun.
The 1927-A1 is a version of the original Models 1921 and 1928 Thompson gun as used by gangsters, cops, Feds, and early WWII British commandos and American forces.
The M1 will accept ONLY 20 or 30 round box mags, has a fixed peep rear sight, no cooling rings on the barrel, and no Cutts Compensator muzzle brake.
The 1927-A1 has an adjustable rear sight, which is NOT adjustable for windage like the original sights, will accept 20 or 30 round box mags, and the 50 and 100 round drums. (Note: The 100 round drum was designed for the early 1921 gun which had a MUCH higher rate of fire. Due to this, many 100 round drums WILL NOT function reliably in the 1928 full-auto OR the semi-auto guns).
The 1927-A1 also has the cooling ring barrel, the original Cutts Compensator, and the top-mounted cocking handle.
Both models use many original WWII era 1928 and M1 full-auto parts.
Original full-auto magazines can be used, BUT, you have to either modify the magazine slightly, OR replace the semi-auto magazine catch with a full-auto catch.
To modify the magazines involves filing the mag catch hole SLIGHTLY higher.
There are adapter plates that can be installed on the semi-auto guns that allow them to accept a full-auto 1928 detachable butt stock.
Many people complain about the semi-auto's weight, reliability, and long barrel, and trigger pull.
The gun is made like the original of massive, milled steel receiver and trigger group. The gun is HEAVY, just like the original. Those movies you see where people are swinging around Thomspon's are made with rubber guns. Milled steel and walnut are heavy.
The reliability problems are almost always traced to worn-out or defective military surplus magazines, or magazines that were improperly modified to fit the semi-auto gun.
The long barrel is a factor of Federal law.
The trigger pull weight is made necessary by the design which makes it difficult to alter it to full-auto. The trigger pull CAN be lightened and smoothed.
If you approach the Semi-auto Thompson as what it IS, a civilian-legal replica of a full auto collectors item, it makes sense.
What it ISN'T is a light-weight modern assault weapon, a target rifle, or a hunting rifle.
Look at it like one of the current replicas of a Winchester or Henry lever rifle: a legal, shooting version of a historic firearm.