Something like you show in your sketch would be fine. You could even cut further up but if you do I'd keep the cut line in the form of a flame look. As long as the upper tube is back to solid about a 1/4 to 1/2 inch behind the rear Oring when it's in the forward "fired" position all will be well. Mind you at that point the Orings will be traversing flame bits so radius and polish or ramp the points where the Orings pass by an edge of metal so that the metal smoothly wedges the Orings down into their grooves.
I had a "Paintball God" moment once.
Like Al Bundy's Polk High football moment I live on
for the slim chance of repeating that one time
miracle in some small way.........