frontier gander, Knows and he is right any thing over 90 gr with PB is asking for several kinds of problems,
As for the 777 expect to clean between shot no matter what bullet you shoot it is the nature of the beast.
If you want to shoot with out cleaning try thr asorbic acid based powders [ you do still have to clean when you get home] Black mag if you can get it, Pinicle if your gun likes it, APP will shoot in most guns but it takes about 10 per cent more to get the same velicity.
There are some guns that the harmonics just don't match up with a certain powder, I have an Omega that shoots good with most other powder but shoots all over the place with 777 or pinicle so it can not hurt any thing to try some Pyrodex or APP.
One other thing do not expect you gun to dial in with the same amout of powder as any other one, barrel harmonics can prevent that. Try evry 10 gr from 90 to the upper limit of your gun but I will tell you a head of time most guns shoot best some where between 90 and 120 and that enough if you have the pin point accuracy necessary for long range. The only load that is normally an exception to that is the 50/40 2oogr Shock Wave it seems to always have more than one load it will shoot good with and one of them is usually a mag load.
I hope this helps. Lee