I just like to stir the pot a bit. Rather than further hijack the excellent XCR to SCAR-L comparative review thread, I decided to start fresh.that's not what long stroke actually means.
It sounds like you (Mikey) might ascribe to the definition "long stroke: piston moves the same distance as the bolt carrier (or just operating group), whether it's attached or not." I know this is a very common definition, and it makes good sense. I have yet to find the source for this definition, though.
For discussion's sake, the alternative definition is along the lines "long stroke: piston moves under pressure the same or nearly the same as the length of the operating group." ...where the key words are "under pressure". This definition also seems to make good sense.
And for comparison, the former (like Mikey) would say "short stroke: piston moves less than the distance of the operating group" (or perhaps some percentage less?), and the latter would say "short stroke: piston is under pressure less than the length of the operating group, and the rest of the group's movement is due to momentum."
The key difference is really the definition of stroke. Is the piston's movement itself the stroke, or is the stroke merely the distance that the piston is being accelerated? Both seem reasonable. Both have supporters, though I imagine that more people ascribe to the former.
Most rifles are generally agreed to be long stroke, like the Garand, or short stroke, like the M1 Carbine or AR-18. The discussion seems to center around the AK and the XCR, as both the former and latter sets of definitions would include these two.
So, is there a mechanical engineering dictionary we can go to to determine which is actually correct? Do other dictionaries agree? Chime in; lets see what comes to the top.