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Doublegunfan,
According to some of these guys, self proofing your newly converted gun is impossible for an individual. I say that they don't have any experience with doing a proof test to begin with, in other words, they are prejudiced against any converted gun. You should carefully measure all the various parts and places they talk about in here, but in the end, it is the proof loads and the firing of them that will tell you if you have done a good job or not.
I have proofed many converted doubles, several of them on Beretta frames by the way, and here is how I do it. You need to figure out what is the heaviest bullet that is likely ever to be fired in your new gun. In other words the heaviest standard bullet for the caliber. Then you need to know what the load will be that you want to regulate it at, including the powder type and amount.
Taking the regulation load, you must make up a bullet, or buy one, that is 20 % heavier than the load you intend to use. It may be hard in some cases to come up with a bullet that is 20% heavier.
You don't need to worry about what the bullet is going to look like, it just has to fit the bore properly and be 20 over the regulation bullet weight. If necessary you can purchase a cheap aluminum bullet mold and re-cut it to make the bullet 20% overweight.
Next you need a proof testing fixture to hold the gun, I will try and enclose a picture of the one I use. Mine holds the gun upside down but that works out very well. I load both chambers and fire the first one with a long cord, then I move the cord over to the other trigger and fire it off.
I do two proof loads through each barrel, then two regulation loads, then I check out the gun. I have never had one fail yet, and some of them have been proofed for pretty heavy loads. Calibers proofed so far include a .600 special, 2 -.577's, 2 -.405 Winchesters, a .45-120-500, several 45-70's, several 20 gauge rifled slug guns, and 2 -12 gauge rifled slug guns.
None failed the proofing tests and all are still working and shooting at this time. I realize that 20% overload is more than many people use, but I feel that it is necessary to be sure the gun is sound. Good luck with your Beretta conversion. What do you think is the average working pressure of your load? You should be right about up towards the top end of what should be used in a conversion gun. I take it that your Beretta has no crossbolt in it, is that correct? The gun shown in the picture is actually clamped in sideways, it is a Ruger 20 gauge Red Label converted to a 45-70. Normal doubles are put into it upside down to test. I also mark the regulation load on the barrel flats so that everyone knows what it is proofed for. Bob

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