I'm only looking at the MA page here and I haven't seen the show in a while but it seems like the exact type of projectile was never explicitely mentioned on screen, but the script for the pilot claimed they were plasma weapons while Berman and Braga called them pulse guns in an audio commentary. I haven't seen that audio commentary but to me that sounds like they're just using a kinda generic term for the weapon, but then again I'm definitely not an expert on Trek weaponry, so I may be absolutely wrong.Technically the early ship-mounted cannons and EM-33 pistols seen in Season 1 of ENT were electromagnetic pulse weapons, not plasma weapons per se.
Looking at the MA page for plasma rifles it seems like pulse and plasma rifles are pretty much synonymous and that they were used by both starfleet and the ECS.