BSA R10 mk2 barrel cut

If the barrel is choked, you'd have to take off length from the breech end unless you don't mind losing the choke. Depending on the way the barrel fits the breech, that may necessitate re-machining the transfer port and inlead. If it were me considering all that effort I might buy a replacement barrel and do the modifications on it, that way you still have a good barrel if things go badly.