Which of the 3 is most reliable? Thanks.

No idea but I have the 7 and it has yet to give me any issues as far as reliability from the first round fired.

Quoted: No idea but I have the 7 and it has yet to give me any issues as far as reliability from the first round fired.

Which make do you own?

I have a 10.25" RRA - 800rds - 0 problems!

All things being equal, the 11.5 will win that contest. Individual weapons may vary.

For reliability 11.5" then 10.5" my brother has a Rocky Mountain 7.5" gave him nothing but problems!

I have both a 7.5" and 11.5" upper and both have been 100% reliable, and I do mean 100% with thousands of rounds through each.

YMMV,
MadDog

Quoted: I have both a 7.5" and 11.5" upper and both have been 100% reliable, and I do mean 100% with thousands of rounds through each. YMMV, MadDog

What brand 7.5 inch do you have?

The LMT 10.5 inch uppers run very well.

my 7.5 runs flawless, Its a Model 1 barrel, gas tube, and model 1 pistol buffer setup with a CMT upper, and CM AR15 BCG on a LAR pistol lower with an RRA LPK.

if pistol function is what you seek i think the gas port could be altered to make any barrel length work..however i believe function to be a sum of weapon reliability and bullet dynamics combined.  I've read on the forums and gotten notice from professionals that a minimum of ~10" should be maintained if you ever intend to use the weapon for "hunting"  There is apparently a rather abrupt drop in velocity at a certain range when the bullet is fired out of too short of a barrel.  It is my understanding that velocity is the .223 bullet's main damaging attribute.   I've also heard the accuracy does not diminish much so i suppose paper is paper and as long as you're not depending on the gun for defense go short! it looks neater.