inner barrel size, barrel extenders

How big is too big for an innerbarrel? I finally got my g3 inner barrel for my g36c and I think its too short, 469mm or 18" is too small, 12" after the 6" is subtracted because of it being inside the gun. The gun will be used only in single fire mode so there is no worry of a vacuum inside the barrel and the spring that is going in will help create a balance from the inner barrels reduction in velocity from its length.

I was thinking something around a m16 length then trimed down to appropriate size its a 550mm barrel so something like 21.5" minus the 6" from where it starts to where it exits the gun. so 15.5" would be a decent inner barrel, as for the outer barrel I know of a silencer and extender that will cover a g3 size inner barrel so around 12". how retarded would the gun look if I put two similar outer barrels together?
http://brotherprice.com/ebayphoto/KAC%20Aluminium%201%20piece%20barrel%20extension%20.jpg

And I have access to a lathe so these can be machined down to just a smooth uniform piece. I just need the length of one of these pices so I can do the math but cant find any info on them. I someone has one or one similar then please do me a huge favour and get just the length of the extender and not the threads. Thank you and if anyone has any info please help out.

Thanks,
Jensen