Unfortunately, aligning different bores is very difficult. Putting them in the same chunk of steel prevents them from diverging over time, at least, but doesn't solve the original issue.

They'll diverge as the gun heats up and the inner bores stay hotter than the outer ones. Both short term as you use the gun and the barrel warms up, and long term, when the bores wear differently for the same reason.

Good point! Hadn't thought about the bore wear - if they don't force successive shots to cycle through the barrels, that would be a huge issue.

Maybe. I'm skeptical that multiple simultaneous discharges wouldn't adversely affect the accuracy.

This is way too detailed, but describes how barrels behave while a shot is being fired: http://www.the-long-family.com/shock%20wave%20theory%20summa...

Based on this, it seems like multiple barrels probably wouldn't affect each other in different ways, leading to shot consistency as long as there aren't significant differences in ignition time or velocity.

Almost forgot: while not simultaneous, the AN-94 did not have issues with ultra-fast RPM in one barrel (https://en.wikipedia.org/wiki/AN-94).