As many have told you, real black powder is what you want to use in a flintlock. All of the imitation BP's have a flash point about 400° higher than black powder. That's why it's so hard to get them to light in a flintlock.
The duplex loads will work fine in the main charge because the black powder explodes with enough heat to light the imitation stuff. If you can at all do it, avoid the duplex loads. Honestly, flintlocks are tricky enough to get dialed in without the added complexity of varying load pressures by mixing BP and sythetics.
Also, if you are looking at Triple 7, that's expensive stuff. The thing that runs the real BP into that price range is paying the Hazmat charge to have it shipped to you. But the Hazmat charge isn't per pound, it's per shipment.
For instance if you order BP from Powder Inc (one of the online companies that will deliver it to you), it currently costs $25.50 per lb ordered in a 5-lb lot. If you order it in a 10-lb lot, the price per pound goes down to 20.05 and if you order in a 25-lb lot, price is down to just $16.20 per pound. It's a chunk of dough at once, but ordering a 10-lb lot will save you a bit and you won't run out for quite a while. Buying it in a 25-lb lot is much less expensive.
A lot of folks will go in together to place an order, pay for it upfront to the person placing the order, and then split it up between them when it arrives. That way no one has to pay for all 25-lbs at once and everyone gets the low price on the powder.
So, lots of ways to do it, but save yourself the headache of dealing with duplex loads and unpredictable misfires. Just use real Black Powder.
Twisted_1in66 :thumbsup:
Dan