Actually Ed says he won't modify the gas blocks as he knows a guy who used to do it and some failed. There's more to the story, but Ed prefers that if you want a certain bbl length, your best bet is to just buy the original length you desire as the gas blocks differ from length as well.
A while back I purchased a G36 Clone with a shortened gas block. I was all excited to to go out and give her a good shake down. I brought the rifle to an LE range day with all of us firing our service rifles (Colt Mk 18) and the G36 clone. In all about 140/150 rounds went off without a hitch, my brethren were impressed. That evening took her apart for a good cleaning and to my horror I discovered a very badly bent piston rod. The rod was a custom made piece from an AR custom builder.
Doing a ton of measuring and re-measuring I ended up purchasing a few G36K barrel assemblies and using the rod from one of the barrels. Took it back to the range with no problems.
Just be sure whom ever is doing the job knows what they are doing, or else the results may be disastrous. Just as chopstix kid stated you are probably better off purchasing a G36K barrel.
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