What is the weight to weight ratio for using the 777 for charges opposed to BP?
I haven't tried to establish a weight or volume equivalency, I use them in different ways. My HP rockets get the normal charge cups, BP, and ematch. So far I've used Triple 7 in my mid power rockets only. I suppose I decided to stick with tradition in HP, but I like to experiment in mid power.
For what it is worth, here is my experience based on ground testing. If you just stick Triple 7 in a charge cup instead of BP, the results will be poor. It doesn't burn completely so you get smoldering bits that char things and unburned Triple 7 coating the insides of your rocket. My best and most consistent results have been with this:
It is a centrifuge container with less than 0.5 cc of Triple 7. I fill to the 0.5 cc line, but the ematch is taking up volume inside. I fill to the top with dog barf, glue the lid shut, and then tape over everything with Scotch 88. With this, I get complete combustion, it burns cleanly, and it is more than sufficient for my BT-55, BT-60 and BT-80 mid power rockets.
I've found that trying to use less than that volume of Triple 7 has been less consistent, some times the ejection is pretty anemic. When I use more, the charge is extremely energetic. Once I compared Triple 7 and BP in a HP rocket, and 1 cc of Triple 7 was way more energetic than 1.3 grams of BP. With the Triple 7 the rocket nearly catapulted itself across my back yard.
I think what is going on is that BP will just burn quickly when lit, so you get linear results that can be extrapolated and interpolated predictably. With Triple 7, my results seem to be more quadratic, I suspect that is due to the dramatic increase in combustion efficiency with increased pressure. But I haven't done testing any more scientific than putting it in my rockets and watching the results.
So I have one tested quantity and use of Triple 7 that has given me very consistent results, in a few dozen mid power launches. I can prep a couple of canisters in advance and get my rockets ready to fly quickly and easily at the field. Probably with more testing I could find a combination that works well for my HP rockets, but I'm content with BP for those, at least for now.