I put the barrel attachment to turn my plasma rifle into a shotgun but it seems to be much weaker than before. Does each of the pellets do full damage or is it divided between each one and you have to hit all of them to get the full damage rating?

I'm sure it's divided....all the splitter mods would be horribly overpowered if they directly multiplied the original damage. From my experience with using the Beam splitter on a Laser Pistol, the splitter mods aren't very effective at ranges beyond 2x melee range (about 10 ft away). You can certainly kill something, but would do so with a focusing mod using less ammo.

If you want good DPS you can put a beam splitter on a 6 crank laser musket

Tempting but I'm using plasma to try and get more nuclear material. Guess its back to the automatic barrel.

Wait what... automatic?!?! You're the first person I've heard of who changes to an automatic mod...
I don't know if Beth meant to do this but with the damage reduction auto's take, they're just horrible compared to every other mod, even the beam splitter, for multiple reasons...
- Very easy to throw away ammo, especially valuable ammo such as gamma rounds with the gamma class weapons, with bad accuracy
- Successive bad accuracy forces you into a very VATS-dependent playstyle, which is no good
- Getting used to semi-auto makes you a more accurate shooter overall, and so better at conserving ammo and only using VATS when needed to preserve AP
- The near double damage all other mods give makes them much more useful when you can spam semi-auto just as fast as auto
- Inifinitely more useful when you can pop a jet or any other slow-time affect, and spam semi-auto at near 2000 relative rpm, whereas the auto is stuck at default relative speed

The damage reduction from switching to auto isn't that bad on it's own, but if you've taken more ranks in Rifleman than you have in Commando of course it's going to tank - I feel like that's what most people are noticing.