I bought New Vegas on Steam during the holiday sales, as well as the first 3 DLCs (the rest were not available in my region a the time).

However, I do not have the Gun Runners pack, yet I have the Battle Rifle available at places (sometimes Legion death squads carry some, as well as the GR vendotron has it for sale). I have the Weapon Mods Expanded activated, so that might be the reason. AFAIK that gun is sold in the GR DLC and should not be available otherwise.

Its added with Weapon mods expanded.

Antistar added this gun before Gun Runners Arsenal was released. If I remember correctly, this is non-unique version of This Machine or La Longue.

This Machine is the unique Battle Rifle. La Longue Carabine is the unique Cowboy Repeater.

Thanks guys for clearing this up.