There are two ways of looking at your JB question. Basically if you use JB you are doing a process called "lapping" the barrel. Barrel material and quality of barrel machining is one of the differences between less expensive rifles and higher end rifles. And some mfrs. hand lap their barrels before they ship them, and some don't. Now in the MZ business most do not hand lap the barrel.

There are two schools of thought. The first is that all barrels should be hand lapped before being shot. And the second is that a barrel should not be lapped unless it needs to be(rough barrel, patch tearing, or bad leading problems with properly sized bullets).

Personally I lap most barrels unless it is a high quality barrel. Again this is not the type of barrel you are liable to run into in an MZ.

Lapping a barrel will smooth it out and will usually leave less leading in the barrel. Again as long as you have properly sized bullets. Bad sizing of your bullet will cause more leading than anything else. I even like to use tooth paste after I use JB for a real final polish. Since toth paste is also an abrasive, but not as abrasive as JB.

For lead removal that is simple. Just buy any good commercial lead remover. Tom.