I've had one for about six years and haven't had any problems out of it. That said, it take a few additions to get it the way I think it should have come from the factory. They have the folding rear sight but now front sight, so I had to order one from the factory, also it came with a solid side folder while the 5.56 version has an adjustable folding version which I lucked out a a customer traded me straight across for.
75 round drums will not lock in, but I haven't had any problems with any twenty, thirty, or fourty round mags.