Rustymaggot - it's a captive piston
I read an article about this back in the 70's, standard gun mag, either Shooting Times or Guns & Ammo.
The cartridge is a captive piston - I don't understand your reference to "upper and lower" unless you're talking about a cartridge sitting on its case head, like on a table.
The piston itself is captive inside the cartridge, so it's a "push rod" that stops before it goes past the cylinder, leaving the shot to be propelled and then it bleeds its gas off via leakage, so instead of the gas whooshing out the muzzle, it stays behind the piston until it bleeds off. I don't know about it being effective - it may have been - but I know guys that were tunnel rats and few of them had a "full time job" at it, so most went in armed with a .45. Given how easy it is to get special equipment in a line unit, (haha
) I can imagine even if anybody knew it was available, nobody'd wind up with one before their short 13-month tour was over. :banghead:
If I'd been a tunnel rat (would have, too, at 5'5"/125lbs in those days) I don't think I'd have burdened myself with one more piece of special equipment I'd have to carry through the steaming jungle every day. The tunnel rat I know best didn't even carry that - he always borrowed the .45 from his lieutenant.
Not everyday you can get one of your officers to carry your stuff.