Traitor Jacknife
Sorry stupid person here Whats a HW marine.
I assume they're talking about Hollywood Marines. They were in White Dwarf 300 and are more true to the fluff of Space Marines. They have something like strength and toughness 6 with 3 wounds and such. Thankfully they're not totally legal, you need your opponents permission to play with them.
As for Psycannons, they are awesome, being a heavy 3 weapon if you stand still for 36' range or an assault 3 18' weapon if you move. They rip a lot of things to shreads. I enjoy having a purgation squad with 4 of them in it. If my GK terminators aren't a threat at the moment (i.e. they aren't charging fairly soon) they instantly start worrying about the psycannons ^_^
Hilarious thing, last game I played I only had 2 psycannons in a 1500 point game against my friends Tau. One psycannon was on a terminator, the other was in my troops. The one in my troops immobilized a devilfish, and destroyed two other ones. It was awesome. I felt kinda bad though since my friend was rolling terrible that game and I was rolling so good.