You might be surprised how much forward force there is on the barrel from the air pressure against the bolt.... At 3000 psi, with a 5/16" bolt, that works out to 230 lbs.... I had a .224 cal. barrel with a carbon sleeve that I fired right out of the breech the first shot because the set-screws ripped right through the carbon fibre, leaving three tracks, one from each screw, about 0.040" deep and 2" long while the barrel ended up flying through the Chrony (no velocity recorded) and hitting my target backstop 20' away.... I ended up using longer screws that tightened into shallow drill point depressions in the steel liner beneath the carbon shell....Bob
This is not a perfect simulation by any measure but it does shed some light on the forces involved. A 225lb. force is applied to the breech face, in reality, the force is acting on the bullet base in the bore. The barrel is free to slide in the bushing and the breech end is unrestrained. The ? mark indicates the maximum stress which is most likely as singularity (not real).Edit. I added one more M6 set screw for good measure. Factory uses two.