The answer is yes and no. I have never tried a factory one, however, I use to slide a length of heater hose on my barrel cockers, and this did dampen the spring pistons vibration on the barrel. Long story short yes it did improve accuracy, but only on spring guns.
The hose was just slightly longer than the barrel, or at least it would stick out over the muzzle a bit. Makes a great cocking handle.
However, if you are talking about one of those new dampa chunks of rubber, don't bother. I found that if the length of then barrel was not covered in did little for accuracy. Not that it dose that much otherwise! It helps a bit, with some piston guns, just a bit.
Jim
