We tested failure in barrels. Specifically PROOF. The liner is so thin that trying to cut the muzzle was detrimental to the integrity of the muzzle, the carbon, and in some cases caused issues with the carbon due to heat, it also changed harmonics.
The pressure testing at the breech and in the shank was the area we were most concerned with based on the thickness of the liner in the first 6 inches. If someone was going to shorten it, then the liner would also begin to get thinner quickly and with many magnum calibers, failure in the chamber area caused to the steel liner due to pressure was apparent. We even tested lodged bullets in the barrel and shot another bullet down after it. Failure and the carbon shot out everywhere like missiles, sticking in the wall and if it weren't for remote control shooting, probably sticking in the shooters face. Huge safety issues messing with the integrity of the barrel. If you are a gunsmith doing this, I am not sure what the liability risk would be but there has to be some. Might want to get a waiver signed if you do it for a customer so at least you are protected.
I own many CF barrels and would never shorten one - my life is worth more than 600 bucks to replace it with the length I want