safe duplex loads

How do you determine how much smokeless to use in a duplex load?Just starting to play with black in a 1895 marlin .45-70.Wondering if there is a formula or something to go by. thanks mark

I usually run about 10% plus or minus of the charge weight of BP with smokeless for duplex loads. In fact, I just came in from loading up a few 45-70s with 5.0gr. of 5744 and 65.0gr. Goex CTG. and the Lee 405 HB bullet. Why 5 gr? 'Cause that's what a Lee .5cc dipper gives you. It seems to work well, they're very accurate and can shoot all day without fouling buildup.

There are quite a few references to smokeless/black duplexing in older Lyman manuals, Sharpe's Guide, Pet loads, this forum, others, etc. Even some to black/black duplexing. BvT

Mark, I shoot a lot of duplex loads in my Sharps. Here's the ticket. First, you need a powder like RL7 or in a pinch, 2400. Second, use up to 10% of the BP charge in smokeless. So, if you were putting 60 grains of 2F in that 45-70 case, use 6 grains of RL 7 and then 54 grains of BP. Notice I ruduced the BP charge by the amount of smokeless I am using so that the total charge weight remains the same.

Where it get dicy is in the measuring. BP is thrown by volume not by weight. Smokeless is almost always measured in weight. So, using a good BP measure, find the charge in BP by volume. Subtract 10% and that becomes you max smokeless charge BY WEIGHT. Confusing? Well, it was to me many years ago when i strted doing it but after a couple of thousand rounds I have the process down pretty good.

In my 45-90 for example, I drop a charge of 2f that is right on 79 grains by volume. I have to compress this .125 to properly seat my bullet and to get a good clean burn. I back this down to 70 grains and then using a scale, measure out 6.5 grains of RL 7. Note that I did not choose 7.9 grains even though that would be 10% of the total charge. It just so happens in my gun that this is the most accurate load and if gives velocity tested by chronograph that is equal to a straight BP charge.

I drop the RL 7 first using a regular powder measure. Mine happens to be an RCBS. Then using a 24" drop tube, I add the BP. I have an old Ideal #5 for this but you could use any BP rated measure that you can set by volume. I used a Beling and Mull prior to this and set the meter with a BP measure I bought from CVA for muzzle loading. On top of this I seat a .030 fiber card wad. I compress the charge .125 and then seat my bullet. I still use BP lube. In my case it's either White Lightning by Dan Theodore or SPG by Steve Garb.

Ok, I believe I understand now. 10 percent seems to be the common denominator. I would use a weighed smokeless load and a volume bp load.70 grains of straight black would work out to 63 black by volume 7 grains of smokeless by weight, correct? mark

You got it Mark. To be on the safe side, I'd drop down to maybe 5 or six grains of RL 7 to start and work up from there.