How to Make M4 CQBR out of M4 (Airsoft)

One day I went airsofting with my AK-74U(Because my MK.18 Mod 1was too big). And on my way home I thought, since there is no M4 CQBR, why don't I make one by myself. So I went to market, bought myself a Colt M4 and start making it. 
Anyway here is what u need for making M4 CQBR
1-Airsoft Colt M4
1-Duct Tape
1-Office Knife
1-Lighter
1-Super Glue
1-Saw
1or 2-Rail
1-Sand Paper

Step 1: Measure the length of barrel.

Before u get into sawing and gluing measure the length of the barrel. Since there is inner barrel I measured it with pencil.