Double stack Glock mags won't fit in the USC magwell. S&W 59 mags do, however. Don't know they can feed from that angle. Maybe STEN mags?
ETA: Think I'm barking up the wrong tree w/ the USC lower, but the USC lower, next to a $18 EP Armory 80% plastic lower, gave me an idea. I can chop off a magwell from an EP Armory lower easier than I can mill one out.
I can cast a magwell even more cheaply - and I can sell magwells. I think the long term route for the Feinstein Project is as follows:
1 - thumbhole stock of bricks for less than the Hera Arms CQB.
2 - a chassis for an existing trigger jig - not the current animal, as the California boys who designed it didn't leave room for a standard magwell. Preferably a cheap plastic jig that also ships with triggers, like the Ruger or LaRue. The Strike Industries, much as I despise their font, might work here.
3 - production of inexpensive plastic magwells in the Freedom-15 mold, for sale. Alternatively, chop up an EP Armory lower.
When I get the new FCS module in, I'll plunk that into the pistol Lyndon and test it. The USC lower is intriguing, b/c it's cheap, and it would be fun to marry one up to an AR-45 upper. But unless you really futz w/ the magwell, you've only got a 10 round mag, and it would probably best work w/ an Olympic .45" ACP upper, as the ejector would be in the upper.