"probably wont consider delayed blowback is what it takes to flute a chamber"
Why, you use a fluted reamer, of course
Here's a short recoil action I just found about today; the 14.5mm KPV heavy machine gun. Obviously yours would be smaller
D), but it's a rotating bolt short recoil op gun with gas assist (so some flexibility for loads/barrels/cartridge power levels)
"the 1941 johnsons were as accurate as garands firing the same ammo"
Yeah, but is that really saying a lot? Neither were 1MOA guns, which is what you'd want to at least aim (pun) for if you hope for an accurized design. Accurization wasn't really the chief consideration during the rifle trials (
a consideration, yes, but not a primary one for the job it was meant for)
"how would you tune individual barrel assemblies to function in the same rifle without serious reliability issues?"
You primarily operate the gun by muzzle-boost rather than straight recoil. That way the booster can be modulated to work with a wider array of rounds, but I still don't think you'd have luck getting a 9mm to operate a 308 capable gun (I'd just use a straight blowback bolt for the little pistol-cal rounds like they do AR uppers). The trouble here is that the ATF will at some point declare your booster a silencer (and only The Shadow knows where that distinction lies...)
"as for the side mag.. it has some pretty big benefits, longer barrel in a shorter package, longer recoil track over a bullpup, conventional trigger packs could still possibly be used without linkage, much easier to operate and reload when prone, and if the magwell and the plate on the opposite side with the ejection port are swappable then its ambidextrous with only needing to cut out two sides of the receiver, and not three (one for the magwell plus two for the ejection port).. ergonomics while carrying are the only issue"
Couple thoughts on the mag setup;
-the pros you list are true for top-mounted magazine setups, but none of the downsides. A Calico/Bizon helical mag up top would keep plenty of ammo on hand (even long rifle rounds, unlike the PS90 mag), and the mag bodies would be the same for all calibers (the internal fins would be the same, and only the helix would vary), or if you are feeling ballzey, a protruding BREN or Madsen like magazine has certain benefits (and costs) as well. Namely they make drum/pan/belt conversions easier (as do bottom magwells)
-Another option would be a grip-well; only instead of attempting to wrap you palm around a 308 box mag
D) you keep your thumb on the strong side and only wrap you palm around the front of the grip. Radical departure, but would get extremely-long mags in the same package as a bullpup while having slightly better balance and (maybe) ergonomics of mag changes. Find a way to load them from the top like stripper clips and you'd have all your benefits and none of the costs.
-If a side magazine is what you end up running with, consider making the gun
automatically ambidextrous. When the mag is latched on one side, the latch automatically shifts a linkage that causes one of two extractors to act as an ejector (see the ARX160 by BRNO), and if you're really clever, pops the charging handle out the other side of the gun (like the ARX160 does with manual manipulation). The gun could freely be fired either way with no other action by the user, which remedies all issues except for the awkward carry. On that note, I'd suggest a 'carry magazine' which only holds a few rounds and is flush with the (also flush) magwell opening, and doubles as a dust cover. Not an awesome solution, but a solution. Either that, or an L-shaped magazine that drops down alongside the weapon to at least minimize the snagging annoyance. If the receiver is wide enough, the bend of the L would be flush with the sides.
-Be sure to make the magwell flush with the gun; good for carry, and you'll have extra width to work with seeing as you have guide rods/pistons located to the sides of the barrel. Your receiver will be fat and short, as opposed to the AR's tall and skinny; take advantage of that where you can.
"if you used some kind of an op-rod on the bolt carrier to allow the bolt carrier to also be used as a striker in the FG-42 fashion, you could also eliminate all linkages in the trigger mechanism and your trigger would be no more stiff or long than firing something from an open bolt"
That's because you'd be firing from an open bolt.
The open bolt regs aren't there (just) because the ATF is a big meanie, it's because what you describe is laughably simple to convert to full auto; just remove the sear and it's "go" time, same as any other open bolt. And that's what the Bureau seeks to prevent by requiring a firing system independent of the firearm action, so that if the FCG is removed, you get hammer/striker follow and no ignition instead of fun times in the Federal P.
Besides, you can do better than that, anyway. Without the huge running start a full-length open bolt has, you'd need a very strong spring to kick the carrier the last little bit of its travel fast enough to ignite a primer. 90% of the charm of open bolts is they can be cocked easily, and you'd lose that. You'd still get the heavy/awful trigger and 'open bolt lurch' and long lock time, but at least an out of battery slamfire would be less likely. You get all the benefits and little cost besides a small bit of extra complexity if you go with a hammer or striker setup (which you wouldn't even be designing if you use an existing one)