I have a springfield trapdoor and want to load some 300gr honady #4500's for it.
In the hornady load book it has pyrodex ctg but no other blackpowder load info so how much do you put in it if you want to use Goex FFFG or 777? which would be better for it or does it not matter?

1. for BP I would go with a cast bullet 1-20 1-30 tin lead with a BP lube such as SPG with a .30 or .60 over powder wad
2. figure the overall length of your load,then figgure how much powder it would take to reach the base of the bullet after dropping with a 24'' -30'' drop tube,this would be your startting load. increase a grain at a time after each increase of powder check to make shure the round chambers,if you put too much pressure on the bullet it will deform the bullet & the round wont chamber, if you reach that point STOP
if you have not found a good load out of the test rounds tothis point you will need a compression die to increase your ammount of powder,thiss would compress the powder before seating your bullet. If using a compression die check to see that the round chambers after each powder increase ,with a compression die you can bulge the brass out & the round wont chamber somewhere between the starting load and the point that the brass would deform should you get that far you should find a good load ,you can't put too much powder in your BP load too do so would take over compression of your powder & that woud deform your brass making it unchamberable
I would also recamend reading Shooting Buffaloe Rifles of the Old West by Mike Venturino
Also check post under Blackpowder Black powder cartridges also checkout www.bpcr.net

for the 300 gr Hornady I would use between 28.5 & 32.0 gr of Accurate Arms 5744

It would be best to follow the advice given by Jim above. Keep in mind the fact that the steel in original trapdoor rifles is softer than that found in modern production rifles. Smokeless powder and jacketed bullets will erode and wear the barrel out with extended use.

Assuming your rifle is in good condition, I would start at 58 grains of ffg powder. This will give you some bit of compression which is needed without being overly large capacity wise. Check out a Lyman Reloading Manual or do an online search for Cowboy Action loads. The 5744 load that was suggested is also a very good smokeless load, although I would start out at maybe 25-26 grains and work up.