1) Is there a scope mod for the Brush Gun?

2) Is there a way to get La Longue Carabine without killing Sterling?

1) No. Brush gun has just 1 mod and it is the receiver or forged receiver. I think it just adds durability but I'm not positive.

2) You can steal it. Grab the ammo in Sterling's inventory, then leave the scene and come back. When you come back he shouldn't have the gun equipped anymore so it is available to steal now. I found it much easier to just kill him though, as I tried and tried and tried but always get caught. I've never tried to grab it at higher levels though since there's not much point once you have a Trail Carbine other than the look.

I stole it from him, but it took a while to get a successful attempt.

yeah happens alotwith named NPCs, it took me ages to get bennys suit and maria.

There's only the forged receiver mod for the Brush Gun. I begged Josh til I was blue in the face for him to put in a scoped buffalo rifle like a Remington Rolling-Block or Winchester 1885 for the .45-70 round, but alas, it was not to be.

I recently found Sterling quite pleased to trade (read: re-equip so I could steal) me his repeater for This Machine. I personally feel he'll be more effective with that anyways. Gotta help out your brothers in arms, ya know?

-Gunny out.