Early rolling block rifle. Bug?

[Joe Pro]

So I just started a mission and when changing weapons I noticed I had a new one, the Rolling Block Rifle and I was confused as I never noticed picking it up. Upon Googling it, it looks like it's given to you in two different missions that I've yet to complete. Has anyone run into this before?

[Joe Pro]

So it's given to you in one mission, or it's possible to find it in the mission that I just started. I haven't even gotten to the area where I can obtain it but for some reason upon starting the mission it put the weapon into my inventory. I checked video of the last mission I did and it certainly was not in my inventory.