Issue was previously reported last year for ticket T194277 but since the old feedback system is in read-only mode now I am reporting it again so it's known that this is still an issue.
Short and simple, you cannot utilize multiple barrels properly. Only one barrel will replicate across the network, more specifically when changing Max Salvo under MuzzleComponent
Description reads as: "Number of projectiles spawned in one simultaneously, should be <= to MaxBurst and <= number of muzzles (-1 to use for all muzzles). Problem is if you set -1 to utilize all muzzles or manually specify it only one muzzle will replicate the projectile. If you set the value to 1 saying there is only "one muzzle" it will replicate and rotate between all the muzzles on the weapon.
Steps to reproduce
Create a weapon and specify multiple muzzle end positions and projectile spawn positions