Excuse me, but I'm not saying it's a fake signature. It's just don't think it's the signature of the Joe Long that made many rifles of a very similar style and detail.
And if you look closely at the signature, at the way the letters are formed and cut, your gun would have to have been made when he was a very young man, or very old man. Just zoom in on the signature of your gun and look at the hammer/graver tool marks and the letter style and compare those to the ones I posted.
Also, as Shelby points out, your rifle has almost none of the styling, inlays, details that Long put on his guns over and over again.
I agree, a J Long made your rifle, I just don't think it likely is by the prolific Joe Long that we are most familiar with. In the end, this is just my opinion and there is no need for you to agree with it. I would only suggest you study some of the many J Long rifles and come to your own conclusion.
Like I said, it's a nice rifle, and thanks again for posting it.