Can the barrel of a Rem 11-87 SP be shortened from 21 to 18in without adversley affecting reliability? Will the gas system need to be modified? I know that it should be possible because I've seen Police model 11-87's with 18in barrels. I'm just wondering if it would be practical. The gun would only have to function with slugs and buck-shot. BTW, I've looked on line for an 18in barrel, but have never been able to locate one.