If the bore is to supposed specs, it should be free and clear of rifling at .223 inches. A number 2 size chucking reamer, will get you to .221, and the next larger size, a number 1, will do you to .228 inch.
A 5.7 mm reamer is at .2244 inches. Maybe even available from a decent tool supplier (not BusyBee or that ilk, but more along the lines of KBC Tools.
If you were to use a Number 2 reamer to clear most of the material, you could also set up and slowly (!) remove the last of the rifling, if there was any, with an adjustable hand reamer, though the problem of the long skinny reamer extension is an issue.
Rough and ready. Silver solder a #2 drill bit to an extension, stone the corners of the drills lips to a round shape with a hand stone, and run it through the barrel, then make up a lead lap, and go to town on it to lap out the barrel to smooth.
Were it mine, I'd look to building up a pull through style reamer to clean it up.
Dunno if the experiment interests you, but I've heard of guys putting a choke on a rimfire barrel, using a pipe cutter type arrangement to squeeze it down a bit at the muzzle. If it fails to improve things, you can always ream it back out!
Some ideas to ponder....
Cheers
Trev