It's a 6-48 if its not been altered. Just running a tap through it may or may not fix it. If that doesn't fix it, see a competent gunsmith. Taps are available through Brownells, MidWay and machine shop supply companies.
A 2 flute bottoming tap as it can be used in more different situations than a taper tap can be. Buy high speed if you can. But for just 'chasing' threads carbon steel will probably be OK. Might be cheaper in the long run to call a local gunsmith and ask what he'd charge to 'chase' that hole with his tap. Any gunsmith worth his "salt" will have the commonly used gunsmithing taps.
Right. I'm guessing the obvious , I think, the bottom is good for a hole that has a bottom and the taper is easier to start on a hole but wouldn't work on a short hole with no outlet. In my case the whole goes straight through and it's a really short hole with enough room for only about 4 threads so either would work but like you said the bottom could be used on either. The taps are 15-17$ plus shipping so I might get it anyway as it must be a fairly common gun thread in case I need it later
I wouldn't touch a gun with a $17 tap. If you have the room use a quality taper 2 flute tap from MSC. Your in the ballpark at $35 and up. They cut so nice and last. Those sub $20 taps are for scratching threads into a mild steel home shop project. If it doesn't clean up and I doubt it will take it to someone with a mill and a DRO and have the holes milled out and corrected for straight and spacing then taped in the mill. If you try it with a drill bit, the bit will just follow the old holes and never end up where they are supposed to be.
Bottom taps are strictly for cleaning up the last threads in a blind hole that is already tapped. You cannot cut new threads with one. Not nice threads anyway and you stand a good chance of breaking it.