FYI - I sent an Email to Hornady with question (in black), answers in blue:
Questions about the “5.56 NATO 75 Grain Interlock HD SBR Soft Point Hornady Black Ammo – 81296”:
1) Do you have any ballistic data for when this round is used in a 16-inch barrel? No, this bullet/powder configuration was designed specifically for Short Barreled Rifles.
2) If used in an AR-15 with a 16-inch barrel, will the action cycle properly (assume a non-adjustable gas block with proper alignment between the gas block gas hole and the barrel gas hole) ? Not recommended, since this bullet/powder configuration was designed for 10.5 – 11.5” rifles.
3) Do you have any ballistic data for when this round is used in a 7.5-inch barrel ? This has been function tested in 7.5” barrels to a limited degree, however we do not have any hard data to publish. As with any rifle (SBR or otherwise), the penetration depths increase as the velocity decreases, since it takes longer for the bullet to initiate expansion.
4) If used in an AR-15 with a 7.5-inch barrel, will the action cycle properly (assume a non-adjustable gas block with proper alignment between the gas block gas hole and the barrel gas hole) ? This is an unknown – unlike an M-4, where there is an actual specification to build the rifle around, there aren’t any when it comes to SBR’s and AR-10’s. Each company adjust their gas system accordingly during their design process, so depending upon the size of their gas port, the location of the gas port, length, etc., it’s impossible to say, which is why we primarily focused on 10.5” – 11.5” rifles.