[quote=@Dead Cruiser]
My character is coming together. Might write up the sheet tonight or tomorrow.

He's modified a .50 cal sniper rifle into something the size of an assault rifle. His approach to "decapitating" vampires is more akin to "turning the entire top half of their body into salsa."
[/quote]

Might I suggest the GM6 Lynx or the 50 Beowulf. Both are bonafide "T-Rex Guns", but the Lynx sound like it's what you're looking for.

https://www.youtube.com/watch?v=oGmcf6qjI7g