Best weapons to make into shield/armor breakers

Wondering what people consider best weapon types (including variants within the class?) to make into dedicated shield/armor breakers (by adding electric or acid dmg). Generally it would seem to me to use a fire weapon on thing with just health or exposed health bars. Electric/Acid otherwise for defenses.

Scrubs it kinda doesn't matter since they have relatively low shield/armor related to their health, but the problems come in from those guys with ridiculous shields (bulwarks, volge, etc).

So what kind of weapons do people like to use? SMG is fast, but lower damage per shot. And ammo restrictions (and no more spikes) can mean running out of ammo at a crucial point when trying to shield drain. (The dual viscera tag team inside the ark say hello).

LMG? Typically bigger ammo box and dmg per shot, bad reload, generally meh accuracy at range. AR is inbetween the two, but requires more often reload.

Or should you not look at rapid fire at all? go for something else? A pistol as a breaker, or a sniper repeater? What do you guys use?

edit: or should you avoid shield/armor damage and go for penetration instead?