I think the rounds should automatically negate a certain amount of hardened armor. The rounds are high velocity, not the firing chamber.
I don't personally think hardened armor should be negated by volume of fire; this is what armor piercing is for, in my opinion.
A Vindicator firing APDS rounds is already at -8 AP, and using the Bulls-Eye action this could be increased to -12AP (4*3) or even -16AP ((4*3)+4), depending on reading. Even in full heavy mil-spec armor (20+3), -12 AP represents a reduction in armor rating by 52% while -16AP would represent a reduction of 70%. Expensive, yes, but taking out the metahuman equivalent of a walking tank should probably have a significant cost.
Miniguns spit out more rounds per second/ minute than any other weapon system due to the multiple barrells.
The velocity of those rounds is not much different from a normal weapon of the same caliber (i.e. light or heavy mg).
Traditional Gatling style weapons like the M134 Minigun and the GAU-8 Avenger feature high rates of fire in the thousands of rounds per minute. However, it should be noted that this is insignificant when compared to experimental technology like that developed by Metal Storm, which have a proven firing rate in the hundreds of thousands of rounds per minute, with the potential of exceeding one million rounds per minute (prototype 36-barrel weapons fired a 180 round burst in 0.01 seconds). Just saying
You are right on the mark about muzzle velocities, though. I agree completely on this point.
So I would not alter AP or damage codes but I would allow them to spit out DOUBLE or TRIPLE the number of rounds that a normal weapon could (although the recoil would be horrendous, they are meant to be vehicle mounted weapons).
And they should not be able to fire short bursts, full-auto or nothing for these things as the barrells do take a second or two to spin up to speed.
So yeah a short burst = 10 rounds, a long burst = 20 rounds and I like the suppressive fire change you note brakhaus but perhaps even increase the AREA the suppressive fire covers.
I agree on AP and damage codes. As for recoil, it might still be manageable, though it would take a Troll with Metagentic Attribute (Strength) to do so effectively.
Gas Vent 3 + Gyro mount 6 + STR 6 = 15 Recoil Compensation
Even with "just" the Gyro mount and gas vent 3 system you're looking at 10 recoil compensation from a bare minimum strength attribute. That would handle 10 round bursts in an of itself, though 20 round bursts would be almost unmanageable even for strong characters; a Strength 6 Human would suffer a -8 dice pool modifier. A 20 round burst would also impose a -19 dice pool modifier to defense, which quite frankly just seems excessive.
In short, I'd stick with the Arsenal rules, but modified for SR5 due to difference in firing modes between editions.
Edit:
Using the optional RG6: Damage for no defense rule might also be appropriate to represent how deadly MGs are. With a -9 dice pool modifier to defense many opponents are essentially defenseless, and with suggested increases to rounds fired this would be even more apparent with -11 to -14 or even -19 dice pool modifiers.
