M1 Vs M1927A1/1928
Assuming that we are talking about semi auto rifles with the longer barrels here (short barrels = SBR permit from ATF), I know nothing of the full auto variants, aside from exterior looks.
The M1 Semi Auto will not accept drum mags. Rear sights are fixed (two types, Lyman "L" and the pressed sheet steel type with the dog ears. Barrels are smooth. Stick mags need to have their magazine catch hole enlarged, but you are better off modifying the magazine catch itself. Don't ask me how, I don't know. Butt Stock is fixed with two screws, not quick release, and forend is horizontal type only.
M1927A1, M1928 can be finned barrel or smooth barrel, with cutts compensator or simple band with sight like M1 style. Rear sight can be adjustable Lyman sight or fixed Lyman "L" sight. Both will accept drum magazines. Stick mags need to have their magazine catch hole enlarged, but you are better off modifying the magazine catch itself. Don't ask me how, I don't know. 1927 Buttstocks are similar to M1 buttstocks. Forends are horizontal or vertical grip.
Lately, Khar arms improved their m1927's by re-recreating the removable buttstock, hence the 1928 variant. This buttstock is simply removed by pressing a latch.
I have handled both the Khar arms M1927 and M1, and prefer their M1927. I have also removed the cheap adjustable sight that was made to simulate the lyman sight and replaced it with a fixed lyman sight. I fitted the drum magazine and I am very pleased with the performance of the gun as an average plinker.
I also have an original lyman sight and used it for a recreation of a M1928 dummy gun built on a solid steel receiver made by Philadelphia ordnance. It will not shoot of cource, but it is detailed in every exterior aspect to appear like a full auto original. The Original Lyman sight was the icing on the cake.