Poison with Barrage Support?

The extra attack time cuts into barrage's effectiveness so LMP is usually a bust, also if you are doing poison there is no reason not to use gmp as projectile damage doesn't increase/reduce poison so its just a choice of 2 or 4 projectiles.

Barrage overrides your projectile behaviour is why you don't see the spread, normally arrows are added in a cone to widen the cone, if you use barrage they are changed to fire in sequence but are allowed to hit the same target with every projectile (hence the massive penalties on the gem).

Basically Barrage is intended for single target only and is heavily focused on arrow stacking. LMP isn't being disabled, its adding 2 projectiles the same as always its just they are added in sequence not to the sides like usual. Also this build sounds split focus which doesn't work in most cases, focus poison or ignore it it really doesn't work as a half stat.