I would say the stats shouldn’t differ a whole lot from the firearms given in the books, except probably a boost to armour penetration. A wild west bullet is probably going to cause more trauma than an arquebus projectile, but not much more. When you increase the speed of a projectile, you kind of get diminishing returns, except in terms of armour piercing, although size does make a considerable difference obviously. The main boon of wild west guns over renaissance guns isn’t damage, but speed, accuracy, and resilience.
Also think of this from a game design perspective. When DR’s get too high, you come to a point where every shot that hits is causing level 6 wounds, those being extremely debilitating. However, we know from real life that, although bullets are very deadly, they can be relatively easily temporarily shrugged off, and later patched up, so long as it doesn’t hit a vital organ. You want to keep open the high possibility that a shot is taken, and powered through until the fight’s end.
In short, I’d say that your high powered rifles should go no higher than say 8 DR, but with a deal more armour penetration, say a generous maximum of 4. The additional effect that fireharms have (additional shock equal to half wound level) should be kept as is. These are what I reckon to be your upper bounds, for high powered rifles of the time.
I have never played with firearms, nor did I put a huge amount of thought into these numbers. They are simply my reckonings based on my feel for how guns compare, and the mechanics of the game. So, do a bit of pondering yourself over that, but don’t take me at my absolute word.
My most sound piece of advise would be this: be conservative with your buffs. It’s better to underpower your guns than overpower them. Even if you miss the mark and they end up being unrealistically weak, that’s not a problem. Fun takes precedent over realism; shooting a bad guy in the head, only for them to get back up and make an escape, that’s fun! Having a player bleed out after a shot to the arm that didn’t even roll that well, not fun!