I know that for many years SR4759 has been a standard in duplex loading of black powder cartridges. I have an Ideal No. 6 Duplex measure. 

IIRC, the standard formula was 10% BULK measure of smokeless, filling the balance of the case with GOEX FF or F, depending on total volume. For example, in a .45-70, 7 gr. BULK in an Ideal No 6 measure, topped with 63 Gr FG.   

I recently started working with .38 Extra Long in a Whitney Phoenix. 158 gr heel bullet, (this design:  (You need to Login or Register to view media files and links) ) mouth crimp applied, external lube. I'd like to clean up the burn a bit as that outside lube bullet doesn't really carry well to the muzzle.  Are there some other smokeless powders that work well when duplex loading, especially in a smaller volume cartridge (the BP volume in the .38 EX Long is about 30 grains) I still have a decent quantity of 4759 to work with, but just wondered what current practice among those using duplex loads in various cartridges.

rodneys wrote on Mar 16^th, 2024 at 2:19pm:


Just to clarify, 5% by actual weight, or by BULK measure using the Ideal No. 6? I know that density varies greatly  between black and the wide variety of available smokeless powders.

I have only used two powders to duplex, 4759 and 296. The 296 was used in my 45/70, with 430 - 490 gr bullets.

I think you need to stay in the burn rate, between 4759 and 296. That would include 4227 & #9.

You can use bulk volume for 4759 but, I would weigh all other powders. I found that Rem 1 1/2 and 2 1/2 primers, gave the best accuracy using 4759, duplex or by itself.

Also, I use up to15% smokeless, as that was what the old Long Range, ASSRA rule stated as max.

Thanks, all!
Anyone duplexing smaller cases? .38 Long or .38 Extra Long?

I have tried duplex loads in a No. 3 Ballard chambered in .25-25 Stevens.  Since this is a cast action, I have limited my use to real black powder..Swiss 3fg.  The smokeless loads were 1.0 to 2.5 grs. IMR4227 & PB.  The PB loads left the bore looking like a very clean burning smokeless load had been fired.  However, for match accuracy, I still needed 1 barely damp patch thru bore leaving bore wet.  Since this was the same procedure I achieved match accuracy with straight black, why bother?  These were fixed loads with the Ideal 25730 seated with only base band in case mouth, 1st. band in firm contact with rifling lead.

In my 50-70 I am using 5gr by weight of D060 (Lovex) topped with 55gr (by weight) Swiss 1 1/2F under a 425gr Govt bullet. Absolutely spotless bore.