Thread: Why does duplexing work?

Retrieved: 12/23/2014
Last Post: 04/01/2008

Why does duplexing work?

When you shoot BP you get fouling... fact

To maintain best accuracy you must keep the fouling soft or remove it... fact

Modern BP burns drier than ''old time'' BP and therefore any fouling nowadays is harder... this what all the 'experts' say in their books

Adding a small percentage of smokeless powder as a ignition booster reduces fouling... again what all the 'experts' say in their books

OK, if modern BP burns drier than old time BP and leaves a harder fouling how does adding some even hotter and even drier burning smokeless powder make the hard fouling less or even non existant?

...for example,does the increase in pressure created by the booster charge of smokeless simply blow the fouling out of the barrel?

...but on the other hand, if the smokeless burns even hotter won't it just make the dry BP fouling even drier and therefore harder and even harder to remove?

...what's going on here? ...I hope you boy's can explain the process to me.

...but accepting that duplexing does work, can you use the teensyist pinch of smokeless pistol powder... say 3 or 4 grains?

...or do you use just a tad more of smokeless rifle powder? ...say 5 to 8 grains?

e15cap
03-28-2008

Don't know why it works, but, Harry Pope, thought so much of duplexing that he built a powder measure that concealed a smaller chamber inside. Moving the lever one direction dropped a measured charge of smokeless and the return stroke dropped the black. Crafty old divil, and one hell of a shot in his day.

jonk
03-28-2008

I suspect part of it too is quicker, cleaner, and hotter ignition.

hyoder
03-28-2008

I would have to agree with jonk.

I have used 4759 in duplex loads for hunting loads in a 45-70. You can use up to about 10% 4759, i.e., if you use a 65 gr. charge of black you duplex would be 6.5 grs. 4759 and 58.5 grs. black. You should start with less that the 10% and see how much it takes to get a good clean burn - usually less that 10%.

Boz330
03-28-2008

What I heard was that the smokeless adds more oxygen to the burn which helps comsume the fowling in the ignition process. It does work though.

Swiss powder seems to be more like the wetter powders of old. Of course without those powders to compare to, it is conjecture. It is most definately easier to clean up though.

Don McDowell
03-28-2008

I've shot a couple of examples of the older black powder, lets just say that the glowing reports of how good it is, welll might be about as much wishful thinking as actual fact.

Duplexing was infact a good way to be able to better work with the black powder of old.

With today's black from all the manufacturer's duplexing just flat isn't required.

Powders nowdays are as clean or cleaner, and just as accurate if not more so than anything that's been around in the past.

Primers are better quality, more uniform in burn, and non corrosive. That also leads to a difference between now and even back to the 1940's.

Duplex all you want, but as near as I can tell its a waste of perfectly good smokeless powder.

DonH
03-28-2008

Quote Originally Posted by e15cap
Harry Pope, thought so much of duplexing that he built a powder measure that concealed a smaller chamber inside. Moving the lever one direction dropped a measured charge of smokeless and the return stroke dropped the black.

My understanding of Pope's duplexing is that he was using bulk smokeless for the main charge over a small "igniter" charge of black powder. The account I read of this (by which olrtimer I don't recall at the moment) said the bulk smokeless or semi-smokeless was harder to ignite. Better ignition = better accuracy, hence the BP igniter charge. Pope didn't want to "advertise" his secret so made up the tricky powder measure. The club where I shoot BPCR allows duplexing and a number of shooters do so. I don't know that their loads are any more accurate than my straight BP loads but I can tell you they don't use blow tubes nor do they wipe barrels between shots or relays. I think it is a little chicken but to each his own, I guess. For what it is worth, a friend duplexes with BP; 4F kicker and Ctg main charge. His load shhots nearly as clean as the smokeless/BP loads and also w/o blowtubing or wiping the barrel. If I were going to shoot long range competition where duplexing is allowed, I might choose to use a smokeless kicker under the main BP charge in my .45-70 just to stretch it's legs a little. One gent who won LR at Raton a few years back said he used 5 grains of 4759 as a kicker to boost his .45-70 a little ways toward .45-90 perfoemance.

Larry Gibson
03-28-2008

I mostly shoot duplex 45-70 cartridges in my TDs. Duplexing works very well, even with todays powders. I do not have to use a blow tube and I can shoot 100 round and have as little fouling as I have after 3 shots. Cant do that with straight black of any manufacture. Lots of theories as to why. Understanding that BP is primarily charcoal and that no matter how "hot" it burns the residue will be pretty much the same. Best explanation I've heard is that the column of pwder gasses of the smokeless powder is behind the column of BP gas and expells most of the bp gas in front of it out the muzzle. This makes the most sense to me. Especially since when working up the amount of smokeless to use you can actully see (with a bore mirror looking from the chamber end) the BP fouling progessively being blown out the muzzle end as one increases the smokeless charge. When all the fouling is blown out then there is really no need to further increase the amount of smokeless powder. The BP "fouling" is the long heavy dark black streaks of stuff you see, there will still be "normal" fouling from the lube etc.

Limey
03-28-2008

Yes but...

...guy's this is all very interesting stuff...

...even if nobody can say for definate how it works... but persaonaly I do go with the reasoning that the hot-poop booster smokeless powder literally flushes out the still burning BP so there is no chance for the fouling to deposit onto the bore...

...but the smokeless powder... is it rifle or pistol grade?... is it fast medium or slow burning?

I am pretty limited on powder availability over here in France... the local 'brew's' are made by Nobel.

I use Nobel Tubal 2000 in my smokeless 45-70 loads for hunting in my Marlin Guide Gun... for a 405 grain bullet you are looking at 45 grains as max loading... our hunting is close range, between30 to 100 yards max so I download to 26 grains and pack out the space with firmly packed cotton wool... works really well, bagged a 110kgs boar this January out around 100 yards, clean pass through, it was a running shot rear left hand hip in and front right hand shoulder exit... no lack of power even with just 26 grains combined with home cast bullets made from wheel weights.

And I use Nobel Ba 9 in my 38 and 357 loads in my Python... with a 158 grain hard cast bullet 5 grains is considered a top end 38 load while 7.5 grains behind the same bullet is a top 357 load.

So anybody like to make some suggestions of which of these two to experiment duplexing with?... I am not looking to gain any velocity... just make shooting and cleaning when using BP easier.

xtimberman
03-28-2008

Be careful. If you toss out that word, duplex, around some BPCR shooters, you'll be scolded and accused of heresy. This has always puzzled me, because late 19th century gun cranks discovered this trick as soon as bulk smokeless powders became available. That's over a hundred years ago, so cries of modern advantage ring hollow. I don't recall seeing Buffalo Bill's plastic blow tube at the museum at Cody.

Harry Pope wasn't the only fellow who invented a duplex powder measure. Ideal offered at least two different models of duplex powder measures and many of their #6 measures are still in use by clandestine BP-duplex shooters.

Larry's explanation of increasing the smokeless component until the fouling is gone - then stop - is right on. Old timers would lay out old white bedsheets in front of the firing line when working up the smokeless amount. After each shot they would examine the bore and count granules of burnt black powder on the sheets to gauge if an increase or decrease of smokeless was in order.

I'd like to see pressure data comparisons on some duplex loads w/10% smokeless priming charge vs. 100% BP. Increases to much more that 10% by volume might be treading onto dangerous ground for some antique guns and certain action-types. This may be the main factor for many of the objections to these loads in BPCR matches.

SR-4759 seems to be the standard gunpowder for priming charges. I've heard of other smokeless powders being used, but I'd be afraid to experiment. Limey, I've never heard of any of those Nobel powders being used in duplex loads with BP.

405
03-28-2008

I too have heard and read of others using all manner of smokeless types in their duplex loads. The pressure unknowns may get a little out of my comfort zone venturing too far off published/ pressure tested loads.

Limey,
I too don't have a reference for your powders but the two very commonly used smokeless powders for duplex loads are SR4759 and Accurate5744. They are both mid-fast powders that are very bulky. On a powder burn chart like that shown in the VihtaVuori manual these two powders are in the burn rate range of VhtV. N110, Norma 200 and RWS P806 and RWS R910... if that helps. Matthews stated that the pressure equivalent between smokeless and black in these type applications is about 1:3... meaning the pressure generated by each grain weight of smokeless is about the pressure equivalent of 3 grain weight of black. How accurate that is I don't know but something to think about with duplexing.

Proceed with caution.

Larry Gibson
03-28-2008

Sorry I can't help with Nobel powders as I don't use them. I've tried several different types of powders but pretty much prefer 4759. Keep in mind I use the duplex in trapdoor M1873s and there replica's. Some out there duplex to increase the power, I don't I only duplex to reduce the fouling and it does that nicely. In the 45-70 I reduce the BP 2.6 gr for every grain of 4759. My service load with a 500 gr bullet is 7 gr 4759 under 52 gr GOEX Cartridge. the velocity is equal to a straight 70 gr charge of COEX Cartridge with that same bullet. Some seem to like RL7 also but i've not tried it.

Larry Gibson
03-28-2008

xtimberman
"I'd like to see pressure data comparisons on some duplex loads w/10% smokeless priming charge vs. 100% BP. Increases to much more that 10% by volume might be treading onto dangerous ground for some antique guns and certain action-types. This may be the main factor for many of the objections to these loads in BPCR matches."

As I mentioned in my post to Limey I reduce the amount of BP a a proportional amount to the 4759 I use. I have one of my trapdoors and a Siamese Mauser that is a 45-70 with strain guages on them so I may measure pressure using an Oehler M43 Personal Ballistics Laboratory. I jsut have gotten around to testing anything in those two rifles yet. I will be testing my own BP and duplex loads in the TD. I suppose I could load some of the 10% vs 100% duplex charges you mention and test them in the Siamese mauser. It will handlle the pressure. Any specific powder combinations?

w30wcf
03-28-2008

I'd like to see pressure data comparisons on some duplex loads w/10% smokeless priming charge vs. 100% BP. Increases to much more that 10% by volume might be treading onto dangerous ground for some antique guns and certain action-types. This may be the main factor for many of the objections to these loads in BPCR matches.

The Lyman Cast Bullet Handbook shows this data:
.45-70 - 420 gr. bullet
7 grs. 4759 + 63 grs. FFG = 1,388 f.p.s. / 18,700 C.U.P.
...70 grs. FFG = 1,268 f.p.s. / 16,400 C.U.P.

Personally, when adding 10% 4759, I like to reduce the b.p. charge by 20% to equal the velocity of a straight b.p. charge.

xtimberman
03-28-2008

I don't have a .45-70, but my SR-4759/BP proportions are nearly the same for my 40-70 Ballard. The 2300CUP pressure increase from pure BP to the Duplex load are very comforting. I knew that the duplex load was almost certain to be of higher pressure - and I had hoped that the probable pressure increase wouldn't be excessive.

My Lyman Cast Bullet Handbooks have no duplex loads published in them, but they are old editions. Does the new edition have pressure data for duplex loads in other cartridges?

Larry, when you get around to doing your pressure testing, you might try to duplicate the Lyman pressure data that w30wcf noted - 7gr. SR4759+63gr. FFG and 70gr. FFG. It couldn't hurt to doublecheck Lyman's credibility.

WickedGoodOutdoors
03-29-2008

Duplex? This is the first that I have ever heard of Duplex. I have been shooting my TC .50 Cal Hawkins for over 30 yrs just using regular old FF Black Powder. Recently I have been using Pyrodex becuse finding Black Power is getting difficult.

How would you rig up to use smokeless? Maybe make a tissue paper cartidge like they used in Cival War rifles? with yhe two powders and the shot?

SharpsShooter
03-29-2008

Quote Originally Posted by NEWENGLANDCHARTERS
This is the first that I have ever heard of Duplex. I have been shooting my TC .50 Cal Hawkins for over 30 yrs just using regular old FF Black Powder. Recently I have been using Pyrodex becuse finding Black Power is getting difficult.

How would you rig up to use smokeless? Maybe make a tissue paper cartidge like they used in Cival War rifles? with yhe two powders and the shot?

This is for cartridge firearms loading, not front stuffers. I cannot recall ever hearing of duplexing in the muzzleloader.

xtimberman
03-29-2008

Amazing. Sometimes, I think that there's too much information floating around out there.

I'm glad SharpShooter straightened all that out.

Every so often, I read or hear about shooters duplexing loads in their C&B revolvers and MLs - and it almost always ends badly. A casual acquaintance was doing this very thing ~25 years ago with his TC Seneca to try to reduce fouling. I wasn't present when it happened, but his duplex charge blew the nipple out of the breech, bent the nose of the hammer and dug a deep 1/4" groove in his scalp from the eyebrow to the part in his hair. He was very fortunate. I saw some fellows looking at his blood all over the shooting bench the next day.

montana_charlie
03-29-2008

Quote Originally Posted by Limey So anybody like to make some suggestions of which of these two to experiment duplexing with?

Limey,
Here is some data from the DuPont site to start your thinking process...

SR 4759
This bulky handgun powder works great in the magnums, but really shines as a reduced load propellant for rifle cartridges. It's large grain size gives good loading density for reduced loads, enhancing velocity uniformity.

Taking that as a starting point, it appears that your Nobel Ba 9 is similar to SR-4759.

I used to keep some 4759 around (years ago), but I think I used it in shotgun shells... maybe.

xtimberman
03-29-2008

SR-4756 is a great shotgun powder. I can't recall any shotgun applications for SR-4759. Totally different.

Larry Gibson
03-29-2008

Quote Originally Posted by xtimberman
Larry, when you get around to doing your pressure testing, you might try to duplicate the Lyman pressure data that w30wcf noted - 7gr. SR4759+63gr. FFG and 70gr. FFG. It couldn't hurt to doublecheck Lyman's credibility.

Can definately give that a try. I 'm thinking of trying my load of 7/52 gr, wcfs mentioned 7/63 and then 7/70 as that would actually be a 10/100% load. I can shoot a string of 70 gr GOEX Cartridge to set a "baseline" first. What do you think?

Ricochet
03-29-2008

The only duplexing in muzzleloaders I've seen "officially" recommended is for flintlocks using modern "replica blackpowders" to use a small priming charge of real BP with the newfangled stuff on top. I've never tried it.

montana_charlie
03-29-2008

Quote Originally Posted by xtimberman
SR-4756 is a great shotgun powder. I can't recall any shotgun applications for SR-4759. Totally different.

As I said, it was long ago.

But the SR-4759 information from the DuPont site is valid.

Irascible
03-29-2008

Identifying Duplexing or BP substitutes

Hmm, If our club puts on a BPCR match, how would one know whether a shooter was duplexing or not? For that matter how would one tell whether it was Triple 7 or American Pioneer?

montana_charlie
03-29-2008

Quote Originally Posted by Irascible
Hmm, If our club puts on a BPCR match, how would one know whether a shooter was duplexing or not?

For a match where ammunition components are restricted to specified choices, there is usually a provision for disassembling a randomly selected loaded round.

Don McDowell
03-29-2008

Quote Originally Posted by Irascible
Hmm, If our club puts on a BPCR match, how would one know whether a shooter was duplexing or not? For that matter how would one tell whether it was Triple 7 or American Pioneer?

777 and app both put out a huge amount of white smoke, but no smell. So if somebodies smokin the place out, but there's no burnt bp smell.

Also they'll likely be the ones with the lowest scores.

Ricochet
03-29-2008

Only "replica blackpowder" I've tried is Pyrodex. It makes a bit less smoke and smells WAY different from BP.

Dale53
03-30-2008

The Canadian long range single shot competitions allow up to 25% duplex. Before the local BPCR matches went to NRA (Silhouette requires pure black or approved black substitute) we ALL duplexed. Most of us used 10% duplex. My favorite powder was RL-7 for duplexing (measured MUCH better than 4759 which measures poorly) and RL-7 is also some slower than 4759. I have shot thousands of these loads and they are a pleasure to shoot. Shoot all day without that DERN blow tube without fouling problems. For the record, I shot 10% smokeless.

Pat and Spence Wolfe's book on the trapdoor stated that each grain of smokeless was worth about 3 grs of black powder. So, you will get a small increase in pressure and velocity but according to Lyman's pressure testing information, well within the limits of a Trapdoor Springfield.

Boz330
03-30-2008

Quote Originally Posted by Larry Gibson
Some seem to like RL7 also but i've not tried it.

I use to use the RL7 in my 40-65 when I first got it and it worked well. I don't remember the exact load but the load came with the gun as a suggested load. It did give about 100fps extra velocity over a straight BP load.

iowa
04-01-2008

NRA produced a "white paper" on duplexing some years back. I'm sure it's out there somewhere, and I'd like to re-read it again if someone can find it. From what I recall, and don't count on this as gospel, I think the bottom line was stay at 10% or less for safety and reduce your BP volume an equivelent amount.