Gentlemen,

I want to report that yesterday I finally had the opportunity to test my project gun. For those not familiar, I am building a DR chambered for .444 Marlin using a former Beretta 411 pidgeon gun frame.
Given the previous discussions on proof loads and how to do it, I made my set up using a pallet and firing from a distance. During the test, the gun showed no looseness or any other issue. Later, at home, I measured everything again (having measured it before), and confirmed that it had no dimensional changes (gaps, chambers, etc).
The cartridges showed classic signs of overpressure, but even so I have no problems to extract the shells after firing.
Now my next step is to load a few more rounds (regular load development this time), and then regulate the gun.
I would like to thank everyone for the inputs on this procedure. I was much more confident and better informed after that discussion. I hope this also helps other gun builders in the future.

Fred