Maybe I should have said "lined" rather than sleeved.

I'm not really too concerned with the current chambering, I just was thinking that I could rechamber it to 400/350 if it would be safe.
IMO, the dimensional differences between what it is now and the 350 are not much so likely would be OK.
I was also wondering if there are any particular limits on a lined barrel as far as velocities and pressures go.