You can modify tapered pin reamers into leade reamers.... They are just about ideal for the job, the taper is very shallow, just 1/4" per foot (1.2 deg.)... You just grind away the big end of the reamer behind the chamber diameter you want.... Bob
With firearms you normally want at least a small jump distance, or else chamber pressures can get high. Not sure if the same is true for airguns, just without the pressure spike since the pressure is the pressure with air.
Pushing a few .300 slugs, NSA & others, there is no groove contact, only land.....As to the seal or a slight lacking of is doubtful to creating much loss or air being the total area on .30 is so great.