I was just wundering, people say 5.56 out of AR pistols is not effective for defense/hunting, how is 7.62? Any ballist gel test? Any cronograph tests? Any real world hunting?

762 has much better performance out of shorter bbls. than 556, which relies on velocity for acceptable performance. That's why the shortest I'll go with 556 is 14.5".

Standard 7.62x39 doesn't fragment, so you'll probably see less penetration, but not a huge reduction in effectiveness. There's a reason they invented the .300 Whisper/Black for short barreled AR's.

Out of a longer barrel, I'll take 5.56 all day, but out of a short barrel, it's 7.62x39 hands down.

5.56/.223 got its bad rep for depending solely on speed for performance from FMJ projectiles like m193. Modern ballistically enhanced ammo like Barnes stx, Hornady TAP, Amax, Vmax, Nosler, GDSP... etc has greatly increased the effectiveness of a short barreled AR. I still wont go below 11.5" barrels but i have no worries about taking down a whitetail at 100 yards with a good 64g to 77g round.

Most FMJ 7.62x39 ammo doesnt do all that great either ( i still certianly would not want to get shot with one), there are certain loads that yaw pretty quickly but you can do much better by going with a hollow point like the Wolf military classic version or a ballistic tip like the Hornady SST

10" is supposed to be pretty good for 7.62.

Another reason why 7.62 or 300 do well in SBR's is the bore is approx 0.3" compared to 0.22" of 556.  The little bit of extra diameter translates to something like 40% more volume per length.  This greater volume gives the powder a faster burn rate.