RE: Piston action v. Direct Impingement/Delayed Blowback.
I was always under the impression that the piston system would be a bit more accurate as there was less gas being vented into the gas operation, therefore upping the barrel pressures and giving the round a bit more speed to help with the trip out to the target. With delayed blowback, well, you know how accurate they are, first name to come to mind is well, you know...
I also recognize there are other factors involved, such as rifling, spec'd headspace, barrel length and specific round performance to name but a few. Sorry, I thought I gave the impression it would be a minor improvement, not anything noticable beyond the bench.
Regarding the OICW, when you are a soldier carrying a weapon as bulky as the OICW around, you will not be as effective. The US Military recognized there was a critical loss in capability
despite the increased available firepower. Also, I don't care how big a guy is, after he carries the thing at the ready for an hour accurate shooting will be a rare beast. Lugging the biggest & baddest doesn't mean a damn thing if you can't deliver the goods.
Trust me, I know. I used to lug an L7 with 200 rds ready on patrol at one time. It weighed about 12kg (26lb). There was a reason we would only use it when braced or on the bipod/tripod.