We have a Character that wants a P-51 Mustang [Real or Reproduction is not important as how he gets it]
The P-51 has 8 .50 Caliber Machine Guns [We were just going to use Ultimax HMG-2]
I am not concerned about fitting it all in, Our problem is this
In the Real one the pilot pulls the trigger and 8 HMGs go off.
How is this done in SR4?

I don't know if there are inherently rules for it, but I'd lean towards combining Wide Burst and Covering Fire rules.

Easiest way to do it is call it a wide burst. the reason those old planes had multiple guns was to make hitting enemy planes easier in dogfighting situations, and cause enough damage to larger bomber type planes to actually bring them down. Wide and narrow bursts abstracts this appropriately.

dont forget the extra bullets that go flying. and if a target is large enough it should be more like fireing extra bullets insead of covering a larger area (not sure how wide burst works).

Another idea, you could simply say that each additional weapon linked adds 1 to the damage value or to the burst or even both.

If he's not paying for 8 weapon mounts, but instead paying for two mounts with 4 guns each, just treat them like two high velocity weapons I guess. If he's paying for 8 mounts, then it's 8 separate attacks.

Short answer: There's just no good way to do this in the SR rules. The system really wasn't designed to handle this sort of thing.

I would use a GE Vigilant Autocannon... it is treated as a Minigun (15 Rounds downrange (30 for Suppressive Fire, which face it, really is what the P51 is actually doing... SUppressing an area while a target moves through it)) and has an impressive damage capability...

Describe it as having 8 Ultimax in the Wings, But use the Vigilant Autocannon Rules (hell, put one in each wing)... the only real way to fly... (not really, but it sounded good anyways)

It will keep the options of having both Narrow Burst and Wide Burst Capabilities, and provides an impressive Suppressive Fire capability.

Keep the Faith