Metal Storm's prototypes would be OK as active defense for a tank. Active defense systems are expected to destroy incoming RPGs, missiles, and possibly bombs or artillery shells. An example of a currently deployed system is one that fires a grenade-like thing upward and then has it explode with shrapnel going mostly downward. Metal Storm's prototypes would fit right in to this role.

Another use is as a replacement for the Claymore mine. It's a minor upgrade. The advantage is increased safety for friendly personnel in the non-target directions. Likely disadvantages are cost, weight, and bulk.

For conventional small arms, differing muzzle velocity due to different effective barrel length killed Metal Storm's accuracy. Eg, the top round in the stack may travel down 6" more of barrel than the bottom round; in a 5.56 gun, this could be a 500fps+ difference (https://rifleshooter.com/2015/12/223-remington-5-56mm-nato-b...).

How in the world do you keep something supplied that shoots 250 rounds per second? Is this a Mass Effect 1 rifle where you just slam a block of tungsten in the back?

It feels like we've mostly converged on the ideal form of a general purpose battle rifle over the past fifty years.

Single-shot spread with sniper range.

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).