[Magnum Wheel Man]

have you considered loading soft lead balls ??? they can often be found in that just right diameter... & being soft lead if they are a bit bigger they squeeze down nicely

personally I think you'll be fine with the 300 grain bullet ( so long as the gun is in good condition )... I load many old black powder era cartridges from 32 S&W (short) to 44 Bulldog using various loads of Trailboss... you may see a slight bit more leading, but since you are not dealing with a rifle length barrel, or I assume not planning on shooting 100's of rounds in a session, I think you'll be OK

if you measure your case capacity, to a line that the base of the bullet will seat to in the case ( in grains of powder ) of Trailboss, & multiply that by 0.7, you will get a safe starting load... the powder is so bulky, that I have in the past ( on weaker action guns ) I've used 0.5 as a multiplier, with good / safe results... so if you wanted to be more conservative with the 300 grain bullet, you could use a 1/2 case of powder

if you have any questions about this, feel free to PM me & we can discuss this further