how are they going to know it was a 7.5 inch that killed the blast baffle, it could have been attached to a  shrike for 50,000rds

The .223 cartridge was designed specifically for a 20" barrel. Anything shorter is a rapidly worsening compromise in velocity, lethality, kinetic energy, and bullet stability. Our experience has been that with barrels shorter than around 11", bullet yaw increases dramatically.

You can frequently get away with shorter barrels, but the chances of excessive yaw are such that no manufacturer wants to warrant the suppressor. Damage to the blast baffle will primarily be a combination of excessive erosion and evidence of bullet strikes. Because of the far larger aperture, a .308 suppressor will tolerate significantly more yaw than a dedicated .223 suppressor.

There are generally only 3 causes for bullets striking the blast baffle:

1. Misalignment, often due to bad threading of the barrel.
2. Excessive yaw/tumbling due to too short a barrel, bad crown, etc.
3. Bullets disintegrating due to excessive velocity of highly frangible projectiles or disintegration doe to core melting from excessive full-auto firing and heat build-up in the barrel from friction.

A question always comes up in my mind as to the benefit of a .223 barrel shorter than 10.5 inches. At 8 inches with 55 gr ammo, the muzzle velocity is around 2,000 fps and the projectile drops below the speed of sound at 300 yards. Heavier bullets will have lower velocity.

Warranty or not, I would advise against using any can on a barrel less than 10".  In my personal experience it is a matter of "when" and not "if" you get baffle contact.  

Mark