Seems to me that certain alterations in a barrel do not impact proof. Honing/lapping, for example, as long as the bore diameter remains within the parameters for that particular bore (not more than .010 larger than standard). Likewise, I'd never heard that opening chokes impacted proof, although both Dig in his book "Vintage Guns" and Salopian above refer to a gun requiring reproof if it's altered to accept choke tubes. In the latter case, significantly more metal has to be removed to accept screw-in chokes than is the case if a gunsmith simply removes a few thousandths to open a fixed choke. Thus, requiring reproof in that case would seem more logical.
