You just convinced me to never allow that multiattack houserule, ever.
You're welcome.
Honestly I think it's unnecessary, Autofire is always going to be a better option. I think the true benefits of being 4 armed is being able to hold 2 things and still have hands free for other stuff, climbing for example. Or being able to hold a rifle and still have a Vibro sword to deal with anyone dumb enough to get close.
That's why you mount your rifle on your armor and carry the sword.
I now have a Besalisk in a game where I am aiming to go down this path. The GM and I have discussed this subject, and we have at least something of an idea of how we think it’s going to proceed.
TBH, jury-rigged plus auto-fire is always going to be way worse than anything you could do with multi-limbed characters using lots of weapons. That’s part of why I think it’s okay to do, so long as both the player and the GM understand and agree on the difficulties and consequences.
For me, I think it’s much cooler to have a Besalisk waving four OK-98 blaster carbines or four HH-50 Blaster Pistols, with four or eight barrels of mini-death pointing down range at the target(s).
Star Wars version of Deadshot is rifle for long range, wrist mounted blasters on each hand, monogoggle attached to see in worse conditions.
War Machine from Marvel comics is wrist mounted blasters, shoulder mounted blaster rifle, goggles for seeing in worse conditions, jetpack
Going ham with a 4 armed guy: near impossible but if you pull it off, priceless.