RedJag here,
My own outdoor plan is just what has been mentioned previously: Hiding an extention barrel under the supressor and upgrading the spring. Most people go Haha! when they see somone trying to use what most consider a CQB specialty item in the field, then zap 'em with what they didn't expect. Actually I'm planning a half extention so I can still get some effect from the supressor, quiet and powerful, :twisted:
If you've got issues of what your suppliers can get south of the 49th, contact one of the the larger Canadian suppiers (007, Rangers, or Double Edge) and have the part (spring) sent courier in a brown wrapper.
The G-36 most of the circle I hang with use is the Classic Army model and it readilly upgrades to the velocities your looking at. A fellow tested one at the chrono a little while back doing c.425fps but I don't recall the spring type (it was either a 110 or 120% but "isn't behaving like it"). 8)
Any way Lots 'o' luck
:zzz: