I recently purchased both the 68gr and the 75gr Hornady OTM for reloading and was surprised at how close they were in length (.979" vs .986"). This may explain why some 1:9 barrels will still stabilize the 75gr projectile.
Anyway, I was wondering how the bullet were so close in shape and size and still had that much weight difference so I contacted Hornady. They said that the jacket on the 68gr bullet was thicker. I expect this explains why the 68gr bullet requires more velocity to fragment reliably.
Which leads me to my question, what is the low fragmentation velocity for the 68gr bullet? I’ve found lots on the 75gr but haven’t seen as much for the 68gr despite it being a recommended bullet for 1:9 barrels.
I’m still hoping someone can put a number to this. I’d love to see a velocity threshold number but I can work with a frag range if I know the muzzle velocity associated with it so I can work the numbers back.
In that he says that the 68gr Black Hills load (which uses the Hornady 68gr OTM) will fragment out to about 65 yards. With the muzzle velocity of this ammo being just above 2600fps that should mean that the 68gr bullet fragments down to about 2450fps. So definitely higher than the 75gr bullet.
ETA: That means my handloads should give me about 95 yards and a true 5.56 pressure load should work to about 120. Unfortunately Hornady doesn’t make a 68gr TAP round for shooters with 1:9 barrels, much less a 5.56 pressure load.