Of course, I have the luxury of having the original image and a loupe in hand, so I can clear up a few things - the muzzle is slightly out of focus but the barrel is not cut for a false muzzle - that is the nose cap that we are seeing.
Regarding there being a knife or gun below the subject's right arm, that area is the space between his upper arm and torso with a part of the chair back behind showing through. There is nothing else there.
The rifle looks real and is a right-handed rifle because the tintype image is reversed. I have seen rifles that were photographers' props but those are obvious copies in my 50+ years of collecting armed images.
No one here has claimed to have seen a rifle with this large diamond shaped side plate, so it appears that this is a reinforcement for a break in this area that I have always mostly thought. If this rifle does show up someday, then it will certainly be the same gun (rarely seen) and I would be the logical prospect...