Sigh...Speak...please pass the butter and salt when you have a second.
BARFCOM is your real friend...plenty of folks over there who'll gladly take up this argument and run with it for you ad nauseum. Just be sure you ask about the "Chart" and be sure to explain that you think it is shite and anyone who buys into that crap should be run through and their entrails used as grease for tank treads!
I have to agree with you 100%...it is, in fact, A STUPID ARGUMENT!!
Look...you are attempting an apples to hand grenades comparison. You are BOTH attempting to interject multiple variables into a situation and attempting to reach some pretty wild conclusions therefrom. Just because you are saying that x-rifle similarly equipped to y-rifle is automatically the same or comparable in terms of its performance (accuracy or otherwise) just on those "features" alone is poor logic. The "except the barrel" bit in your post in the most laughable. Barrels, along with quality FF tubes and triggers, are the very foundation for any ARs accuracy potential. Leaving that one variable open for "any old barrel from various mfgs will do" is just plain silly. Now, if you are just talking about any old AR-15 with any old ammo and any random 12" plate at 100, 200, 300yds...then you are at least somewhat correct in your premise that both rifles, if fired by competent semi-auto shooters, will be able to repeatably keep roughly the same percentage of rounds on said target (i.e. - they are "shooting the same" in that narrow sense).
For example, and not to "side" with anyone in your argument, but I have a rock-stock factory Colt 6920MP and a likewise equipped RRA CAR A4 (different twist barrel, but otherwise, at least from your viewpoint, nearly identical in configuration with the exception of the small parts like different mfg mil-spec triggers, pins, springs, buffers, etc.). The Colt will easily and repeatably hold ~1.5MOA with your basic ball ammo and not a whole lot better than that with most Match fodder. The RRA on the other hand doesn't shoot ball worth beans (>2MOA) but with match fodder up to 69gr, will get down into the sub-MOA range (doesn't like the 75gr and 77gr stuff one bit). Now...does that make the RRA a "better" rifle? Well...depends on your definition of "better" and what all you are asking the rifle to do and how long you expect it to do it before something goes tits up.
As for piston vs. DI and whether you can build a "precision piston AR-15"...yes, it can be done. That said, I wouldn't expect you to be able to "build" one in your basement from a piston upper/parts kit, but there are factory production piston operated ARs in existence today capable of predictable, repeatable levels of sub-MOA accuracy, including large-frame ARs as well. Your basic bolt-on kits from any number of different mfgs, well, not so much in my experience with various options over the last 6-7yrs.
