you should do just fine for 2 3/4 loads. attached is a picture of their original catalog, not sure if this is your gun, but all their 520 models were either "bored for nitro" or "proofed tested". nitro loads of the day produced similar performace in modern day loads. however, only some of their models indicate proof testing, check for proof marks. im not great on my history, but they may not have been required to proof back then. i wouldnt worry about it, especially since black powder loads werent the standard when this gun was produced. besides its based off of one of brownings durable design. i wouldnt hesitate to fire 2 3/4 loads. but if you still feel nervous, fire it with a string. i have about a spoonful of cold blue left to touch up that fresh cut barrel if you decide to cut it.
http://img.photobucket.com/albums/v3.../No5201912.jpg