A 24" barrel is a long tube to shoot subsonic loads. I shoot a 16" 300 Whisper and it takes either pistol powder, or fast rifle powder like AA1680 to obtain subsonic velocities while still cycling the action in my AR15.
With your long barrel you will have to also be careful about getting a slow moving projectile stuck in the bore. There will probably be a narrow window between just enough powder to push the bullet out of the barrel at subsonic velocities and not enough powder, leaving the bullet in the bore.
With your long barrel you will have to also be careful about getting a slow moving projectile stuck in the bore. There will probably be a narrow window between just enough powder to push the bullet out of the barrel at subsonic velocities and not enough powder, leaving the bullet in the bore.
