Serious question, what about that video makes you believe a weapon is being fired from that man? He isn't even looking at Charlie until the moment before you hear the shot.
Let me be clear: this entire thing stinks, and Robinson does feel like a patsy.
But there's videos from further away from the tent where you hear the shot, and it clearly sounds like a supersonic crack from a rifle.
I've done plenty of shooting, and immediately knew a rifle was used.
You would not get that sound from a guy on the ground near Charlie with some kind of concealed small gun.
Also how would he aim this concealed gun at Charlie? It just doesn't add up to this guy being the killer.

In the videos of Robinson climbing off the roof the rifle is visible. He even drops it(or just the scope) when he does the high jump to the ground.
The rifle does not disassemble, that is correct.
However they did find a screw-driver on the roof, and I THINK they're implying that he removed the scope before climbing off the roof. It doesn't make a lot of sense.

In the photos I've seen it could easily be a composite/polymer chassis, or wood painted black.
There are claims that he had it inside his pants/shirt while walking funny in some of the videos. However, that could be he is injured after that at least 14 ft drop.
I don't know if we have footage of him actually entering the school. It's possible he placed the rifle on the roof somewhere ahead of the event too.

Another issue with sneaking it in concealed in his clothes: a large scope like that could lose it's zero if it's being pulled on by clothing. At least enough to make a first shot a slight miss. Also when you zero a sight/scope you're typically doing it for a certain distance, and if you're closer or further than that distance you learn the appropriate 'hold-off': a spot in the reticle outside of the zero/center that you line up on the target to compensate for distance/height/wind.
This is why sniper teams usually have a spotter next to the shooter with his own optic. He sees where the round impacts, so that the shooter can adjust his hold-off based on the spotters info vs the sniper's stand-off.

The shot itself isn't difficult or impossible for a decent shooter, but that's not including the stress of firing on a human above a crowd of that many people. A hunter would be stressed about it, and nerves affect accuracy.
It would be a lot more believable to me if he had fired more than one shot.
But it just so happens that the first shot would have immediately sent everyone moving, and it wouldn't have been successful.
It does stink, there's things that don't make sense. But I don't think the guy in that video is shooting a gun.