[FrankenMauser]

With a 1:8" twist, you're going to have to back down on the powder charges. The bullets are "over-stabilized", and probably near the self-destruction threshold.

I have to make an assumption (as much as I hate to), that you're using a 40-52 gr bullet. With that assumption, I figure your starting load should be producing 3,050-3,300 fps. (Not compensating for slight velocity loss in the short barrel.)

At those velocities, your bullets should be turning at about 274,500 to 297,000 rpm. In my experience, that's too fast. The fact that they're making it to the target seemingly in one piece is surprising, in itself.

I've seen the best results with similar bullets from about 186,000 to 225,000 rpm. ...which would require 2,066 to 2,500 fps.

Some tougher bullets will do just fine at rotational velocities around 250,000 rpm, but most lightly-constructed projectiles (like DogTowns) start having issues. They might partially deform under the centrifugal force. They might come apart entirely. Or, they might just start showing imbalance issues.

For comparison purposes, as to why this bullet works in 1:10" and 1:12" barrels at those same velocities...
3,300 fps in a 1:10" barrel results in 237,000 rpm
3,300 fps in a 1:12" barrel results in 198,000 rpm
3,050 fps in a 1:10" barrel results in 219,600 rpm
3,050 fps in a 1:12" barrel results in 183,000 rpm

As you can see, they are much closer to (or well within) the 'safe zone', than your 1:8" barrel; even in the worst case (3,300 in 1:10").

Bullet rpm = muzzle velocity (fps) * 720 (**) / twist rate (inches)
Example: 225,000 rpm = 2,500 fps*720/8
To calculate, plug velocity and twist rate into: (fps)*720/(twist)
The result will be in rpm.
(** Conversion factor for feet to inches, and seconds to minutes -- it remains constant.)


My advice would be to work DOWN from 24.0 grains, until you reach your accuracy load. If it gets to the point that the AR no longer cycles reliably, before finding that load, you'll have to choose a different powder.