Weapons this powerful, really don’t do extra damage against normal creatures. I believe the technical term is blow through. Bigger targets are another story. These weapons are capped at 13 damage, like other rifles in Feng Shui 2. While some of those rifles have a bonus to hit mooks, these rifles do extra damage against targets with toughness schticks: like the Big Bruiser’s Very Big schtick, or extra wound points. As always the GM is the final arbiter of what applies.
Double-barrel Big Game Hunting Rifle (13/5/6)
The weapon of choice for Victorian aristocrats on safari, and Michael Gross in Tremors. It looks like a double-barrel shotgun, but the barrel is rifled and it fires a variety of very large rounds — .600 and .700 Nitro Express being the two largest. These weapons were the expensive toys of the elites, and they remain costly to this day. A rifle like this does +2 damage to tough opponents.
.700 WTF (13/4/6)
For all of you who thought the Full Metal Nutball’s homemade weapons were a crazy idea, I present the .700 WTF. A home made gun, firing homemade bullets our of home modified brass. Actually, it’s still a crazy idea — just in a different way. The .700 WTF is a single shot weapon. Its bullpup design has two advantages. First, the weapon short so you can pretend it is concealable. Second, the bolt is right next to your head so if anything goes wrong with this homemade monster you are unlikely to suffer for long. It also does +2 damage against tough opponents.
Marlin Model 1895SBL (13/5/4)
This is the gun carried by Chris Pratt in Jurassic World, and it is pretty much ideal for demon hunting. It compensates for its small round (compared to the other two) by having a magazine capacity of 6. Its stainless steel barrel is weather resistant, and it has a military spec picitinny rail on top for mounting a wide variety of optics and other accessories. The 1895SBL will do an extra point of damage against tough opponents.
I would also allow the Maverick Cop’s Winchester Model 70 to get a similar +1 damage against tough creatures. .458 magnum is plenty big enough.