Hey, KevinB since we seem to be on this topic and you are the man on the ground anyway, do you see any reason for the 6.8 without a weapons system re-design?
I mean, is there any point to switching calibers if you are not going to make a better gun, so to speak?
To get a dead to nuts reliable 6.8 carbine in direct impingement, the following is ideal:
1. Mid length gas system
2. Fat tube (optional but important)
3. Barrel lengths 12.5 and longer
Can you build a reliable impingement 6.8? Yes.
Short stroke piston 6.8 eliminates the shortcomings of DI in 6.8. This relates primarily to 6.8 being a lower pressure cartridge with a shorter more agressive pressure curve. DI weapons become increasingly finicky when dealing with short gas pressure curves. Compensation is made by opening the gas port and allowing for volume of tapped gas to overcome the curve. This leads to increased bolt failures, wear on springs, increased recoil, increased fouling, increase heat transmission to the bolt and carrier which effects ejector and extractor spring temper and leads ultimately to failure. You have faster opening which relates to a higher cyclic rate and harder extraction. Bolt bounce is an increased issue which is why you need a H buffer or heavier. All of these same principles apply to 5.56 impingement carbines which is why the M4 has a significant decrease in service life comparitive to self regulating short stroke piston designs. It is why a 20inch C7 goes and goes while an M4 has issues with small part replacement requirements and requires a buffered extractor spring to prevent extractor skip or bounce.
Short stroke piston weapons have comparitive lower chamber temps because the upper receiver and rail essentially heat sink the chamber. Do a 100 round CMAG dump from a Short stroke piston gun and the bolt and carrier are just warm.
Impingement guns by design have an opposed unlocking. The gas pressure in the carrier delays or opposes opening to create a delay allowing the case to contract and make extraction easier. It also slows cyclic rate and makes operation safer. However, opposed opening puts stresses on the bolt that just are not present during the unopposed opening of a short stroke piston gun. The delay is built into the piston system self regulation. The piston does not move off until pressure and inertia are equal to the opposing mass and spring tensions.
Suppressor use is greatly enhanced with a self regulated short stroke piston. There is no effect from the increased back pressure caused by the suppressor. Essentially the suppressor makes all of the above mentioned impingement carbine system problems much worse. It increases cyclic rate, stress on the parts, heat injected into the carrier and receiver, fouling etc. The carrier velocity is increased which basically cancels the muzzle brake effect the suppressor has in reducing recoil. A short stroke self regulating piston basically faced with an increase in back pressure overcomes the mass and spring tension in the action then immediately vents all excess. No more or less stroke velocity is used than is required to cycle the action. Fouling is far less and is primarily associated with back pressure coming through the chamber. A puff of gas does foul the bolt and carrier, but it is limited to the 1st 3/4 inch of the carrier/bolt and is all exterior, none of it being pounded into the interface between the bolt and carrier. The other thing is you don't have a cooking hot carrier and bolt that cooks on the carbon and lubricant in sustained fire. So cleaning a Short stroke piston carrier is like wiping soot off of a teflon frying pan instead of the bake on carbon associated with a DI gun. Of note, almost none of this is present when firing sans suppressor.
There are far more advantages to short stroke piston designs than may be apparent to the user or infantry guy. Last longer, require less maintence and small parts replacement. Bolt fracture becomes a non existant issue. They can be operated almost completely dry with no oil other than 1 drop in the cam pin slot. Cleaning becomes a 30 second affair. As a matter of fact, all you really need to do is pull through the barrel and chamber with a bore snake.
Examples of self regulating short stroke pistons:
Leitner-Wise (total piston movement is 6/10ths of an inch)
HK 416, G36 series (Similar piston movement to the LW)
Non regulated short stroke tappet systems
Colt 1020
POF416
Of note the LW 308 shoots like a .223 in recoil and muzzle flip. An AR10/SR25 is more like a 12 gu shotgun. Members of this board have shot it and can attest.
