After having worked with a major ammo manufacturer for several years and witnessed their proof loading and loads, I can assure you that NO individual has the time, experience or equipment to do this properly.

Industrias Technos S. A. de C. V. in Mexico made up special pressure test loads for each caliber they manufactured to a specific pressure as dictated by CIF and/or SAAMI, different loads for each caliber depending on which outfit was in charge in the country they expected to sell the ammo.

They have state-of-the-art pressure barrels and testing equipment that you and I could not possibly afford and if we could we wouldn't know how to use it.

After witnessing hundreds of proof loads fired in calibers from 22 short to 50 BMG, I can assure you it is not a project for the faint of heart. The occasional blowup of a firearm in their armored testing vault was simply par for the course... and they did not make guns, only ammo.

Call me chicken, but, I am not firing any rifle, pistol or shotgun that has not been proofed by a professional company. Way to scary for me.

I watched an idiot load a Ruger #1 in 7 m/m Rem Mag with a 175 grain bullet over a full case of Bullseye and pull the trigger with a long string. The rifle jumped about ten feet high (I'd give anything for a chrono, but, didn't have one), broke the stock at the wrist and had to be pounded open with a deadblow hammer. The action was seriously sprung and destroyed for further use as a rifle... amazing to me was that it did not explode... I guess they are pretty strong.

Making double rifles from shotguns has been done a lot, but, remember your average shotshell only developes about 10,000 psi or a lot less and your average rifle caliber such as the aforementioned 44 Marlin operates at up to 44,000 psi, and the 375 H&H can get to 62,000 psi... a lot of difference for that shotgun action to take.

My advice, save your money 'til you can buy a real double rifle (it doesn't have to be a Holland) and you'll be much happier and safer.