Barrel threading, how to do it?

[Solvo]

For a rough job yes you can just use a 3 jaw, but most 3 jaw chucks are the self centring type and would make it damn near impossible to machine true to bore as you cant clock in. Not too bad for screw cutting but bad if you want to re-crown.

Weight of the breech block would depend on how you have the work set up, if you have the barrel clamped in chuck near the muzzle the offset would be a problem at any real speed.

Edit - for the balance i wouldn't use any fancy calculation. Last time i did something similar i used a grinding wheel balance to roll the round shaft on, then tied metal to the opposite side of the offset block block to balance it, when happy secured with some stout zip ties. Not perfect and i'm sure there are better ways but it removed the vibration.

[Rich]

A lump of steel the same size as the breech block, and lash it on.

Whatever machining you want to do at a fast speed on a lathe, you can normally do just as well at a lower speed, it just takes longer. My dad always taught me, and this was sixty years ago, when in doubt, slow down.