A short barrel will not spin up a bullet enough for stability. Some guns have a gain twist like the .460 S&W so to cut it down approaches a smooth bore. I will never go too short. But I don't carry and stick a gun into a BG. I hunt deer. My best is a 10-1/2' Ruger SBH. My BFR's in .475 and JRH are 7-1/2" and the 45-70 is 10".
Twist rate is not affected by barrel length. All revolvers have enough twist for normal blunt bullets that are commonly used.
I had the same discussion with a guy on another forum regarding short barrels and velocity, and his ability to grasp the concept was as elusive as bfr's. He couldn't grasp that bullet spin is a simple formula based on twist rate and velocity.
Yes, a shorter barrel will give up some velocity, typically 25-30 fps/inch. And consequently the bullet will be spinning slightly slower. But if a given bullet length and barrel twist requires say 1000 fps to stabilize, whether you have a 4" barrel and 1200 fps or a 6" barrel and 1250 fps, the bullet will stabilize.
