Think like a crit that is only able to go off every 5th hit (or something), but instead of a crit, it's this splash. Considering you'll probably have have like 10 attacks per second eventually, 25% every 5th hit to deal splash in a 250 radius sounds really good.

Oooo, I have an idea for description! P
Explosive Rounds - The Marine knows he had some explosive bullets somewhere in the magazine, but he's not exactly sure where in the mags he put them. (% chance every Xth attack to deal splash damage with Y radius) (no?)

WHATS THE SOUND OF A DEVIL DOG!?

OHHHRAHH!

marines lead the way fk the rangers lol

So I must be misunderstanding what you mean. Because a 1.5x radius when we get like 1m+ agi late game would be beyond rigged. You could kill Azazoth (and the whole rest of the map too) easily.
*Attacks Horror Beast* *Explosive Round goes off* *All of the Voids die, Hell comes after you, Azazoth is mad, all of the Astrals are after you, all of the bosses are mad at you, etc*

Also: 200 radius is a 400 area diameter, so that'd be a fairly large AoE (for a chance to happen on attack), perhaps more like 100-150 radius.
Plus: I think we should just take out the every X hits, could be tricky to do and basically a "splash crit" would probably be so much easier to trigger.

Thought: To make it splash it could be he normally has splash but has an orb effect, any DC likes. Just something so that it overrides splash. When the splash hit comes around, it just removes the orb effect. It probably wouldn't work properly with any item orb effects though...so I doubt it'd be like that in the end.

example would be

every 250 hits marine fires off an explosive round that does splash right?

why not make it like the pr's ability then? explosive arrow lol

but add incinerate or something too it but make the incinerate explode instantly?

Couple of holes. First, you'd have to trigger the game to count his attacks. That'd get annoying for DC I bet. Which is why I said just make it a "splash crit"
Explosive Arrow is just single-target, and is basically a triggered crit that deals spell damage (I think) with an animation
Several problems with making it an orb effect. You couldn't have other orb effects. You'd never be able to have lifesteal on him (which'd be nice with the splash), and some orb effects might override the incinerate effect. Plus, Incinerate has stuff blow up when they die, which wouldn't help for killing that guy. Plus if you add other Incinerate effects. like the bonus damage each hit, that could be rigged depending on the damage it adds.

PS: I don't know exactly how skill/item orb effects stack, because I've seen vamp item orb effect with slowpoison skill orb effect work at the same time. (I saw the hero getting healed by vamp [the healing animation went off] and the poison still hit)