putting a flange on the piston to stop the spring like on the ar180 would make the piston assembly too tall to use an ar-style handguard.
and imagine the amount of whine from all the internet operators if the rail on their handguard didn't line up with the rail on their receiver.
so, on the wk and mcr they got rid of the flange and put a step in the piston to stop the spring. this is where the piston breaks as it creates a weakness in the piston right at the area of maximum deflection.
solutions so far are:
- a larger, monolithic receiver that allows room for the flange like on the crusader (and acr, and sl8/g36 ...).
- no spring like on the siberian; the problem being that without the spring there is nothing to hold a two-piece piston together and as a result they went with a one-piece piston which cannot be removed without removing the gas block.
- mount the piston closer to the barrel to allow room for the flange, which then requires a special barrel nut or other barrel mounting system (rwa and sterling use a mounting plate instead of a nut which requires a larger receiver to accept the fasteners).
so, the take-away?
the wk and mcr are the weakest.
the crusader, rwa, sterling are robust but heavy due to all the extra material.
the piston on the siberian appears to work fine (even without the spring) but it has issues with the gas block coming loose (can't pin it as it is required to be removed for service). but it's light.
