Don't do it. All the Ackley Improved calibers need to have the headspace set so the action won't close on the "go" guage, but will close with a little effort on a virgin brass case. Your gun already has the headspace set to close on the "go" guage, as it should - but being a break action you can't set the barrel back a thread to shorten the headspace. Reaming it to an improved will cause incipient case head separations after only a reload or two, rendering the gun virtually worthless.