Hmm..Kynoch and Birmingham Proof house disagree that duplex loads increase pressures. In one example, straight from their records of pressure tested loads, in the 500 BPE the original load of 136 grs. of straight black under a 340 gr. bullet the pressure was 10 tpsi. A duplex load of 15 grs. of SR 4759 under 110 grs. of GOEX Ffg gave 8 tpsi under a 440 gr. bullet. The same was seen in the 450 X 3 1/4 BPE with several loads, less pressure with duplex loads than with straight black loads. Oddly, the 500 BPE duplex load was less than 10% and the 450 X 3 1/4 load was closer to 15%.
I can see no really good reason for duplexing in a single barreled rifle, unless as Ranch said one has less than quality powder, a big case and a need to control fouling other than wiping. In a double rifle such is often not the case. Rarely, very rarely, is enough velocity achieved to bring the barrles into regulation with a straight black powder load. After working with over a half dozen doubles chambered in BPE cartridges I have never achieved it. As with all things reloading one must pay attention to what they are about when utilizing duplex loads. I'm convinced they're entirely safe if loaded correctly. Kynoch and Birmingham Proof House were definitive for me.
