Nice build, especially like the mag. Looks like your trigger fits in nice and smooth, probaly moves smoothly too. I'll say this, you do a better job with foam than I ever could. My medium has to be rigid. Iwanted to put a straight moving trigger on my pistol, but the magazine took up all that space.
As for your bullets, I think I prefer the necked case, that conveys to me power and accuracy, like the .357 SIG (I so want one of those) Stopping myself from going off on a gun tangent, the bullet should be like the hollow point, with a pointed steel core in the middle, giving it the advantage of a hollow point and a penetrator, thus semi-armor peircing. I might do that to my bullets after I get them casted, maybe, I dunno, we'll see. I like what you did with the ball berings.