I'm getting ready for my first sbr and want a 7.5" upper for it. I haven't seen many topics on them, mostly 10" and 11". Are they unreliable? Any brands to avoid?
Mine is as reliable as any other AR, but its awful loud (even suppressed) and since .223 relies so much on velocity, it won't be as lethal. I think 10.5 inches is a good compromise for this caliber. You can go short in other calibers which don't rely as much on velocity to be lethal.
Tim La France perfected a gas tube set up for short barrel reliability. The shorter barrel seems to create short strokes/fte/ftf and can be picky. The 10.5set up I have seen set up on a postie m16 from Model 1 Sales ran better then the DPMS 7.5 with the same ammo and the problems were short strokes and magazine issues. The DPMS had a pig tail gas tube and the 10.5 had a custom 3bend gas tube. The 7.5 got very hot very quick and 30 round mag dumps left it smoking. The 10.5 was just as awesome but more reliable.
I just finished a 7.5" pistol build which will be SBR'd down the road and so far it runs like a top! I'm running a full auto carrier and a H2 buffer with NO cycling issues.
Had good luck with two them, always ran a #60 o-ring under the extractor and made sure to use a buffer with anti bounce (i.e. some of the shortest pistol buffers are solid).
Worked fine on handgun and the Colt M16A1 on full or semi.