I have a 1892 win. in 44-40,would it be possible to get it bored out then get a sleeve to convert it to .357

It would be more cost effective to rebarrel rather than sleeved. You might check Gun Parts Corp. (Numrich) as I believe I remember them having new manufacture barrels for Win. '92. You might get a better response by posting your question on one of the cowboy action shooting websites as they're always getting their old Winchesters worked on.

That 44-40 barrel adds considerable value to the '92. Unless it's a POS.

Depends on the '92, but if you just want to change it because .44 WCF is hard to find, you'd probably be money ahead getting new '92 repro in .357.

This.

Also the M92 conversions to .357 usually start with a M92 in .23-30 or 32.20.
A .44-40 is not suitable for conversion to the smaller cartridge.

These conversions were popular years ago but now are not practical for a number of reasons. Trust me on this, I spent a few hours researching this after buying a converted Winchester 1892 in .357. Mine works well with medium level cast bullet loads but I would have been better off with a modern lever .357 ....