Sorry, been traveling on Business...
Wow, alot of good questions. I don't think I can answer them all. For example---how did my gunsmith regulate this double.... hmmmm, I'd have to reply: like anyone regulates a double. Setting the barrels to where your years of experience tell you you should set them, soldereing them in, and going to the range and shooting it with various approx. powder charges of a desired load, trying to get them to hit poi at a certain distance. Correcting when it is not perfect and then resoldering and shooting again... In this case it was 75 yards. The bullet was a 535 Woodleigh. The charge hew as going for was somewhere between 70 and 82 grains of Reloader15. It took him about 9 months to get it where he thought it was right and the targests he sent me had a less than 2 inch spread for both barrels at 75 yards and with that accuracy---I am quite happy.
He has made several 50-90 double guns so he knew where to start, and he has built other large caliber double conversion guns as well.
Also, I strongly believe it is a myth to say a double is only capable of shooting ONE load accurately. As Graeme Wright related in his famous book "Shooting a Britsh Double" --- 'it is incorrect to think a double can only be regulated to one bullet and one load'... with experimentation you can find more. I think you are only limited by your patience and your fortitude... and support from the double gun gods.... As someome who used to formulate epoxies I can tell you the variables are endless and you can get to the same point many ways... The same holds true, I contend, with getting a double to shoot various bullet weights and bullet designs using various types of powders and various weights to hit at POI at your regulation distance. Heck, my first try and I had 3 different loads (2 different bullets/ two different powders) hitting within a couple of inches of the bullseye and I was shooting standing and shooting in very low light! 12 shots of all three loads would have taken down whatever I was aiming at. But the point is, and I am hardly the first to discover it---I double can shoot more than one bullet ie one load to POI. Is the factory (or gunsmith/builder) responsible for doing more than ONE? No, it is customary to do one.... but the owner of the gun can find the other happy loads.
Let's face it, even if the factory regulates a gun with a certain load but you take that gun/load to Africa or other locale with very different temp/humidty the gun/load combination can shoot off, ie no longer regulated ... and thus you need to find a NEW load that is regulated to THAT climate, and you can!...
The way I see it is I have at least 7 different candidate bullets and at least 5 different powders and yes that means quite a number of combinations and opportunities to find loads that hit to POI with both barrels at 50 yards. (which is where I regulate to).
Working pressures --- If memory serves me I think I am shooting in the 20,000-23,000 PSI range with the loads I have worked up. And this is well within the capability of this beefy Merkel with beefy Pacnor barrels within the 12 gauge tubes. I understand the Merkel 500 Nitro Express is built on their 16 gauge: this is what I was told. If thi IS true, this supports the notion Merkel shotguns are strong enough for conversions.
Yet bear in mind with the 50-90 loads I'm working with, these are more like a Light Nitro. I am not in the pressures ranges of the 500 Nitro Express.
And regarding the area of the base---The way I see it is the chamber has been reamed out of the 50 caliber barrels for the flanged 50-90 cartridge it is not the area of a 12 gauge. The 50 cal barrels are within the 12 gauge tubes.
