The guts. Let's talk about them but first let us cover that's these are piston driven systems but differ in the sense that two are "long" stroke and the other two are "short" stroke systems. Long stroke systems in short have a piston that is connected to the op rod which is attached to the bolt carrier and follows the bolt carrier group through the full length of operation whereas in the short stroke system the piston strikes against an anvil (picture a pneumatic punch like in no country for old men) on the bolt carrier for usually no more than 1 inch of travel and then it is disengaged. Long strokes tend to shoot softer and have faster follow up shots and fixed ejectors.These are long stroke systems (AK actions)
This is the short strokes
At 12 o'clock you can see the ACR's op rod that is driven by the piston.
At 12 o'clock you can see the same setup with the LWRC
Gas adjustability.*The LWRC was said to be self regulating so it is not adjustable for suppress which I had to tweak it to get it to run good (tubbs flat wire spring and a spikes tactical tungsten powder buffer)The ACR has two adjustments suppressed and unsuppressed. This system also vents excess gas out of the port located in the front of the gas block. The top of the piston has a cork screw contour like a prop which causes it to rotate each time the gas hits it cleaning the system.*
The XCR vents under the hand guard and the system has 6 settings S,1,2,3,4 and off.
The bolts, we have two types of bolt design here. The three lug which is typical of AK actions and the seven lug (the ejector which has a lug doesn't truly act as such would make it eight but it could be machined off and not impede function) which is indigenous to the AR system.The ACR and LWRC are AR based*Seven lugs*
Three lug systems
The ACR and XCR are build to be piston systems. What this means is that the engineers or designers already compensated for the carrier tilt which is a side effect of the piston system by incorporating rails for the carrier to ride on. Most AR systems this was a moot after thought because the reliability was of far greater concern.ACR rail system
XCR rail system
LWRC (AR tubular system) you'll find no rails here
The YUGO M92
So in short the these two interchangeable caliber rifles have incorporated what the designers felt were the best operating systems. The ACR is obviously based on the AR-15 using it's bolt design with a plunger type ejector and a short stroke system and AR type barrels. The XCR is based on all the AK's strengths (less the magazine) the three lug bolt, fixed ejector and the long stroke system.*Caliber change.The ACR *is a tool-less quick change that only requires elbow grease.The XCR requires a tool (Allen wrench) I use an inch pound wrench, but is still a quick change system.I'm the beginning most thought that the ACR's system being tool less was the way to go but thoughts have shifted on this. I myself am really indifferent about it meaning I could give a crap either way because once the barrel is removed the zero will shift. Robarms maker of the XCR has found a solution to this with a non adjustable rear sight and a front sight which attached to the gas block and is adjustable for elevation as well as windage but they have yet to incorporate it into the design.