I am considering buying a 12 bore rifle. I've experience with rifle cartridge double rifles, but this is a new world for me.

How does one go about doing this?

Thanks!

Well Chaz, there are 12-bore rifles and there are 12-bore rifles!

Single or double? ...and all-up weight will give a good indication of the intended charge.
Nitro or black powder? Rifling type and twist-rate will tell you a lot too, ie whether designed for round ball or conical.

Is there a specific rifle for sale, or are you going looking for something in particular?

Sorry to answer your question with more questions, but that's how it is.

Post photos of the gun, include detailed photos of the proof marks, slug the bores, cast the chambers, and get back to us with the results of your findings. Note the bore/groove measurements, the type of rifling, and the rate of twist.
Also include as detailed an account you can of the sights, including the distance from the bore axis.


--Tinker