Duplexing for Dummies

[redpep]

Could someone put together a post (maybe eventually a sticky) on the basics of Duplexing-

It might be very helpful for those thinking about this area but aren't quite sure of a couple things

Why you would want to, the benefits of doing so, powder used, Barrel size, bullet size, sabot used, bullet speed....

thanks

[Dave W]

I type too slow so here are a couple links that cover duplex very well.
dougva.proboards.com/index.cgi?board=Savage&action=display&thread=3887
dougva.proboards.com/index.cgi?board=Savage&action=display&thread=4481
Basically it is stacking two powders, a fast burning powder(I prefer N110 as the booster) is poured down the bore first. I settle the booster by tapping on the barrel with a short starter but others feel this is unnecessary. Then the primary powder (H322 is my preference) is poured in on top of the booster creating a stack of two different powders.

Why: Some use the booster as an insurance policy for ignition when shooting slow burning powders. It also allows the use of slow burning powders with light bullets where if you shoot some of these powders with light bullets the load will be inefficient. The booster allows the use of these powders since it raises the pressure quickly to a more stable level. Since the bullet is not forced into the lands we don't have enough back pressure and resistance to shoot shoot some of these powders efficiently.

Benefit: My reason is consistency, accuracy and speed. No single powder I have shot performs as well with a 250gr bullet in the .50 or a 200gr bullet in the .45.

Powder: the booster usually consists of one of the three book powders- N110, IMR SR4759 or AA5744. The primary will depend on the caliber, bullet used and speed desired.

Duplex works in any of the 3 calibers .40-.45-.50 used here with a multitude of bullets, saboted or sabotless.

Now the experts can weigh in.

[redpep]

Thanks this is a great start and I hope others will add

[Richard]

I have gotten some of my lowest Extreme-Spreads in velocity using duplexes and best accuracy. As Dave mentioned, reliability in low temperatures.
Richard

[dougedwards]

As I understand it, duplex allows you to use slower burning powders that provide "push" for the bullet not just at ignition but also as the bullet proceeds down the barrel towards the muzzle. This is the reason that greater speeds can usually be achieved with longer barrels that can fully utilize the benefit of slower burning powders.

However these powders usually require some assistance to get going especially in large bore rifle barrels. The primary faster burning powder provides the initial pressure needed and the secondary powder takes over from there. Someone can correct me if I am off base a bit here.

Doug

[rbinar]

8-)Duplex was not my invention, I’m a long way from being the first to use it successfully. However I guess you could say I brought it to smokeless muzzle loading. My motive was to supply some alternative to Lil Gun powder which I thought was a danger as it was being used at that time.

From a realistic point of view it takes little effort to see that powder selection for a 250 grain bullet in a 50 caliber bore is very limited. The book powders work well at 45 grains or less but blow sabots with any type of warm spell when that level is exceeded. The next slower powders in line (4198 or RL7) need in excess of 65 grains to reach even reasonable pressure. Powders that allowed varied load selection in the 50 to 65 grain level were nonexistent.

Of course that wasn’t going to last long. Many shooters tried alternative methods. That was all fine and good to me except I saw they were doing so with no concept of the pressure. I then started my own trials using a self imposed pressure limit of 40,000psi. I took the concept from the Lyman reloading manual for Black Powder cartridges. There dozens of 10% duplex loads are listed using Imr-4759 as a booster.

So when people asked me why I’d even try to do such a thing, I think they were surprised when I answered: I know it’s not dangerous! I think the Lyman data that was produced for rifles with very limited pressure maximums was sufficient to prove my point. Still all was not right in Mudville as I was barraged with critics claiming I would surly kill us all.

With all that happening I was surprised that some would actually try it and honestly report it was working. It worked so well it began to have a following, small at first, then more extensive. Now it’s common place. But I have some concerns. This post is long so I’ll type those concerns in another reply.