I’m guessing the oring design will probably work better, being rubber it will most likely seal better than the plastic (these chambers are good, but not perfect, they still have a weld at the bottom, it would seem like the elasticity of the rubber would create a better seal than the considerably harder plastic).
I don’t know if you’ve seen it or not, but Mark did some porting work on one of these, along with relocating the magnet, with good results…It’s in this thread somewhere, but may take some digging to find it.
I don’t know if increasing chamber length would have much effect. It looks like shot tube length has more to do with it. Standard chamber capacity is something like 1.2 cu. in., shot tube capacity is about .3 cu.in. Even though chamber volume is going to decrease some with compression, it’s evident that the bb is gone before the piston reaches the end of the stroke (best guess is within the first 1” to 1-1/4” of travel).
I ran an experiment a while back where I doubled the abutment to shorten the chamber to 1-3/4”. the chamber capacity was reduced to about the same as a model 25 (if I remember right, something like 1.056 cu. in.), but with the stroke shortened by 12.5%…the theory being that if a shorter stroke can compress the air more quickly, I might get a little more speed at the point where the bb leaves the tube.
Results were pretty inconclusive, it wasn’t any worse than a standard chamber, but gain (if any) was marginal. I can’t help but think that increasing chamber length would, if anything slow the rate of compression…but please note that I have been wrong before.
Best of luck regardless!
Rex