For the Buffalo Gun, I used the La Longue Carabine model and animations, but the sounds and settings from the Brush Gun and Medicine Stick to make a scoped levergun with decent power. I liked the versatility of the Medicine Stick but prefer scopes for long range. Since it's a long-barreled, scoped .45-70, I figured it would do a little extra damage and have more critical chance and damage than a regular Brush Gun, but made it heavier and with a regular 6-shot magazine to keep it reasonably balanced. It's sold by Vendortron but it's not cheap because it's a unique, end-game weapon like the Medicine Stick.

The shotgun is called the Terminator (little joke) and is basically just a 12-gauge lever-action shotgun. I just mashed up the stats of the 20-gauge lever-action shotgun and the hunting shotgun. More importantly, it works with the Cowboy perk, so it's actually the most powerful shotgun in the game if you have that perk. DPS-wise, with the perk and magnum ammo, it's comparable to the riot shotgun, but uses less ammo and with lower requirements. I figured this was a good way to keep shotguns viable at higher difficulties and levels without making anything unrealistic.

Requirements: FNV (of course) and Gun Runners' Arsenal add-on.

Installation: just drop the .esp file into your data folder and enable it in your mod manager. Uses native data, so no data files required.

Buffalo Gun
Req: ST 6, Guns 100
Dam: 78 (same as Medicine Stick)
Attacks/sec: 1.4 (same as Medicine Stick)
Crit. Dam: 100 (approx. 30% bonus)
Crit Multiplier: 1.5 (same as La Longue Carabine)
Ammo: 6 (same as Brush Gun)
HP: 200 (same as Medicine Stick)
Spread: 0.05 (same as La Longue Carabine)
Reload Time: 3.3 (same as La Longue Carabine)
Weight: 6.0 (heavier than Brush Gun or Medicine Stick because it has a scope)
Value: 15000
Works with the Cowboy perk.
Repairs with Brush Guns (or other lever/pump guns if you have Jury Rigging).

Terminator
Req: ST 4, Guns 50
Dam: 70 (same as Hunting Shotgun)
Attacks/sec: 1.8 (same as LA Shotgun)
Crit. Dam: 10 (same as Hunting Shotgun)
Crit Multiplier: 1.0 (same as both)
Ammo: 5 (same as LA Shotgun)
HP: 150 (halfway between LA and Hunting shotguns)
Spread: 1.3 (lower than unmodded shotguns, slightly higher than Dinner Bell, much higher than choked Hunting Shotgun)
Reload Time: 0.5 (same as LA Shotgun)
Weight: 4.0 (heavier than LA shotgun, much lighter than Hunting Shotgun)
Value: 4000
Works with the Cowboy perk.
Repairs with Lever-Action Shotguns (or other lever/pump guns if you have Jury Rigging).

Both guns just use models from the base game, but I put them in Vendortron so you need GRA and have to buy them. They both work fine with my testing, as they should since they're just vanilla guns with new names and stats. Enjoy!