Hi... Relatively new to the site, but wanted to ask opinions on a sales pitch thrown at me during a recent gun show. When I mentioned that I'd recently (the night before, oddly enough,) built an M4, a vendor asked if I'd "pigtailed" the gas tube (along with offering his gunsmithing services.) Apparently, the premise is that the platform and the round was specifically designed for a 20" barrel, and comparable gas tube length. His argument was that the shortened barrel of the M4 gave rise to compression issues, which would be rectified by extending, and looping, the gas return around the barrel, to simulate the original 20" length. He also credited misfires, jams, etc., with the shortened barrel & gas return system, but I'm not entirely convinced. Does anyone have any info or experience with either the rifle itself, or this particular modification to it? For reference: DPMS lower, 2 lb trigger, DPMS, chrome-moly M4 upper, 5.56, 1x9, and an itchy trigger finger. Only one round fired through it so far. I'm more familiar with my AK-47, and it's generous tolerances when it comes to various ammunition, etc. (not to imply that I don't maintain my guns.) - This is just the first AR-15 variant I've built, and I wonder if his claim has any merit, or if he was just trying to stir up business. Any info is greatly appreciated.