I'm new to Thompsons and just picked up a new AO 1927A1. I noticed the fit between the frame and the upper receiver is pretty loose and wanted to find out if this is normal. When holding it, I can feel the frame and the receiver clunking into each other if I just move it slightly. If I twist or move it from left to right you can see the gap between the frame and receiver get larger and smaller.

Below are pics of how much the gap changes between the frame & receiver when slightly twisting the rifle. Is this normal?

I used to have an M1A1 that did that. Drive me whacky. One of the dealers fixed it did me. Then I bought some parts kits. And noticed no 2 were dimensionally the same. Lots better QC  nowadays than in WW2. But they are cool.

I have a 27 sbr I bought back in '08, pretty sure it's normal, I'll dig it out of the safe and check.

Ya that’s normal. Look please do not.  Do not  alter magazine ok alter the magazine catch.  Your Thompson will run much better than screwing up magazines.  Go to machinegunboard. They will have all the info you need to do this mod.
http://www.machinegunboards.com/forums/ Attached File

Get a couple of Taiwan drums from Numrich, they work great, don't have to be altered either.

I own a legit Colt mfg.1921AC machinegun and it is just as loosy-goosy as you describe OP. It used to bug me until a true TSMG expert explained that the internal parts have to be allowed to work together without the upper and lower receivers adding a binding force to the geometry.

Mine still hums along after 100 years.

That is beautiful! I’d love to build a M1928A1 SBR like that. The full auto flavor is a tad out of my price range.

I sold all my surplus TSMG mags back during the Obama years and don’t want to try accumulating them again.

I purchase a 1928 lower and did the mod myself.. Ya the full auto stiff is out my price range also.