Safety, like any other regulatory program, can become a “growth industry” where more and more strictures are piled on until one can’t do anything, so therefore, one is absolutely Safe.
Ken Waters loaded hundreds of smokeless charges in dozens of antique black powder rifles and never blew any of them up. Dozens of other gun writers did so too. A careful handloader can do the same now. A careless one can blow up anything, antique or modern.
The Quigley has been going on for 27 years now with an average of 500 or so shooters every time, banging up over 100 rounds apiece in practice, sight-in and the Match itself. From my observation, at least half of them shoot smokeless loads, and probably some who shoot black powder shoot duplex loads. In all this time, exactly two rifles have blown up.
Making generalizations from those statistics as to the suitability of smokeless loads in general in antique rifles in general is like that story of the first two Model Ts registered in Kansas (or somewhere) that were involved in a head-on collision with each other. “You’ll nevvir git me into one of thim jalopies; nossir!”
I dunno; maybe I’ve just outlived my time. My load development and loading procedures follow Elmer Keith’s dictum that you approach the process like a three-legged coyote trying to get the bait out of a Number 3 Newhouse. But I do see a lot of “Anybody got any good loads?” questions posted on the Internet by people who apparently trust anonymous strangers more than powder and reloading equipment manufacturers with their guns, hands and faces. So maybe 21st Century handloading is now becoming a risky thrill ride rather than a careful, logical stepwise procedure.
Such people, to me, ought to shoot nothing but factory loads and hang their antiques on the wall to admire. Handloading and shooting are risky, but risks can still be managed. I don’t expect my guns to blow up, and I don’t expect to have a head on collision on the way to the range. It could still happen, of course, but I do everything I can to minimize the chances, and I don’t sit home and avoid the risks.
That said, I find duplexing only becomes a preferred option for me after the caliber in question gets smaller than .38-55 or so. This is why the established black powder guns in the old days were large bores; the .38 was the benchrest caliber, the .32-20 could fire enough shots for the rancher and woods loafer and the .32-40 size (until Pope and others started duplexing) was for the Shuetzen shooters who “liked their beer and liked their luck.” The small .25 caliber blackpowder cartridge was cutting edge technology equivalent to the wildcat smokeless developments in the 1930s.
At or above .38 caliber, with straight black, I can do more shooting than cleaning; smaller, the reverse happens, so I use the “nitro priming” (strictly according to established procedure) to blow the gunk out of the small-diameter barrel so I can continue shooting. However, this doubles the time I take in loading, because I look into the empty cases to check for stray ceramic cleaning chips, look again to see the level of 4759, and again for the height of the charge of black powder. I wouldn’t click-clack an Ideal #6 powder measure, stuff a boolit in the case and go shooting; I use Lee scoops for the priming charge and a #5 for the main one.
I realize in this litigatoratory age, where everything that happens is everyone else’s fault, that I can’t make specific recommendations, but if I wanted to try duplexing in a rifle in good condition, I would do so, myself. Must be the daredevil in me.