Ah yes....a good topic.
The answer.... it depends. [:D]
My equipment:
ASA upper and block (early) on a CAV 15 lower w/ a DPMS 223 parts kit using the factory 223 buffer.
Issues I have encountered:
mags (or you can say, mag blocks). I have noticed that some mags have the mag catch cut too low, which raise the mag into mag well. This, I have seen, allows the extracting round to hit one of the lips and knocks it out BEFORE the ejector gets to it. Other issues with mags are the body size (under sized or over sized). Mags I have seen are "loose" in the mag well adapter. Tight can be delt with easy, but a loose mag requires a "funky" fix.
This is what I do to mags that are loose. Go to the hardware store (big one) and look for heating tape (aka furnace tape). NOT duct tape. This is a metal tape (aluminum or steel). What I did was make strips and put it on the mag to "shim" it to fit the block. I put it on the non mag catch side and front/back (depending which way it has to go). Does it look funny, yes. Does it make the mag work, some. And the tape is less then $5.00, so why not.
Mags-type:
I have-
Uzi (and clone mags) notched to work w/ the 9mm (hit and miss). As said, location of notch and body size are the main issues.
Also cheap USA Colt/Uzi mags (made one to work actually!!! And well). Amazing? Not really.
[b]Curved Madsen 9mm mags modified to work. The ones I have encountered work well, as their mag catch location is IDEAL.[/b] The only issue is, the bottom of the mag block has to be curved/recessed a little to allow the curve of the mag to fit. If I had to notch any uzi mag, I would have a smith use the location of the Madsen notched mag as a template (for my 9mm at least).
South African Double Stack, Single Feed (looks like Sten). The work great. BUT, I have encoutered some that were undersized and required the furnace tape shimming. But they work great now (and don't drop out).
Ammo:
I have shot FMJ out of it using most, if not all of my mags. They fed fine. Nose diving mag usually indicate a weak mag spring. Hollowpoints, some mags feed better then the rest. The South African double stack, single feed (DSSF) feed much better w/ hollow points then the traditional double stack, double feed (DSDF) mags. A Hollow point that looks like a FMJ nose will feed more then something w/ a huge hole.
Mods to gun:
Mine-none. Bolt is stock, feed ramp and throat are factory. I just dinked w/ the mags.
Conclusion:
It all comes down to the mag block and mags. I have heard that some mag blocks are tight while others are loose. Loose mags can be shimmed up w/ that heating tape. And those mags make great range guns. Heck, maybe I'll post a pic of my fix (as I do have a dig. camera now).