30+ years of Gunsmithing/machining, 20+ years of smokeless ML starting with a setup similar to the Remington UML that used shortened 45 cases.
I’ve converted quite a few break actions to smokeless. There’s a lot of ways you can do it, and I’ve tried most of them.
I converted a CVA Optima Elite with a 45-70 barrel that I used for 7-8 years. Did the same with a couple of H&R single shots.
The front of the plug used savage type liners, the modules I made from 416 stainless about an inch long, rimmed so a hand primer could be used to seat small pistol magnum primers. It worked with low pressure loads. 42-50 grains of 5744 under various 225-300 grain saboted bullets. Killed quite a few deer, hogs, and predators with it. The problem with it was gas cutting of the modules. You couldn’t contain the gas for long. The same happened with the original Savage ML that used cartridge shaped modules. I think McWhorter uses a similar system as well.
An O-ring cannot contain the pressure of smokeless, at least not the 42k psi levels of a 275 grain bullet at 2600+ fps. I would warn you not to try making an all in one cartridge shaped breach plug. The gas cutting will make it a ticking time bomb.
Whatever you do, you’ll have to have some type of bushing to keep pressure from blowing out whatever primer you choose.
I’ve switched all of mine to Hankins HIS or DIS setups. IMHO there’s no better mouse trap.
