I have been experimenting with front ignition loading for years now. You can buy a book on E'Bay written by Roger Stowers called Gibbs Cartridges and Front Ignition Loading Techniques. There is also a small amount of info in the Cartridges of the world book. Rocky Gibbs, Elmer Keith and a few others did a lot of work here. The miltary 20 mm case I have has a flash tube as part of the primer, so somebody out there knows A LOT about this.
Duplex loading is obtained by putting a very small amount of faster burning powder at the bottom then the slower burning powder at the top. Sounds good in theory because as the pressure drops off from the slow powder the fast burning powder at the bottom boosts it back up. (remember you are using a flash tube so the ignition starts at the base of the bullet and burns down in the case. I have found it is very hard to get consistency with duplex loads.
I have found no gain in velocity, and it is a real lot of work to make cases tubes and install them.
The gain I see is, and I no longer mess with duplex loading, the powder burns in the case, not in the barrel so the barrel lasts longer b/4 burning out, there is much less muzzle flash, and since the combustion chamber is the same every time vs powder bouncing and burning down the barrel, my velocities are much closer together. It is not uncommon to have a v spread of 20 fps on 5 shots. The barrel stays cooler too.
The best thing is the reduced "Felt" recoil in front ignition. In a 30-378 Wby, I burn close to 120 gr powder and a 130 gr bullet. With regular loads you get the recoil of a 130 grain bullet and 120 gr of powder. Both get accelerated down the barrel at the same time and close to the same acceleration rate. With Front ignition the bullet leaves the barrel, but the powder burned backwards, and is not accelerated. True the gasses do go down the barrel, but it has a delay, however slight it may be, it spreads the kick out over time. The full foot lbs of energy are still the same, you just get more time to absorb it.
I have an FFL 07 license, and if I were to make these commercially I would have to charge $10 per cartridge just to break even. So is it worth it? For me and my own loads on the super power cases. Yes. Would I pay $10 each to buy them. No. I don't bother on my 270's or smaller, as I do not shoot competition. I just hunt. I believe ther is more gain from long cases, than the short fat ones.
Oh ya. Be careful picking up a freshly fired case. Since nearly all the powder burns in the case vs the barrel, they get hot enough to burn you.
I haven't made any new cases in over a year as I started making Lowers for ARs, but as soon as we get insurance for ammo and there is interest, I could be talked into modifying a few cases.