The Brain Exploder starts his Simple Airsoft Upgrades with Part 1 being Inner Barrels... "Taking you through some simple upgrades that can greatly improve any gun and don't require a degree in mechanical engineering. The guide is overly detailed but covers the step by step process as demonstrated on this Krytac CRB, the same process and reasoning will work for most other airsoft guns but the hopup units vary as you would expect however for an M4 I would highly recommend the Krytac hopup design as it's great to work on and to use in the field."