Barrel Extention

I'm pondering obtaining a silencer/barrel extention for my JG G36C, but I"m not sure which barrel to get that the right lengh (~420mm) heres what I've found so far:

Silencer/barrel extention:http://www.airsoftgi.com/product_info.php?cPath=31_55&products_id=3693

Inner barrel:http://www.airsoftgi.com/product_info.php?cPath=30_39&products_id=2357

My other concern is this, will the cut in the barrel be the right size for the G36C hop-up unit?

I dont know much about swapping barrels/hop-up myself, but I do want to caution you againt the Blackside mock suppressor. I got one a while back for my mp5 and its heavy as a rock being solid metal save for the slim hole through the center. It makes the gun, even my other rifles I fixed it to, very unbalanced as it obviously makes the gun end-heavy. I would suggest spending 10 or so more dollars and looking into the King Arms line. They come with screw out bases to swap out the direction of the thread so you never have to worry about which one to get to match your gun. Also, they are very high quality, very light (yet durable) and they even come filled with foam so at least the sound exiting the barrel will be dampened a little. They also come in different lengths to fit your style. Just do a google or check out www.precisionairsoft.net or www.airsoftsmith.com

As far as the barrel fitting in the hop up unit, it should work. I'm not sure what length you should get though.

Weight is not at all an issue for me i think the my gun could use some more heft but thats my opinion.