Little Moe: In a word, Yes, you are wasting powder. What you get is more pressure because of the added mass, more recoil, and perhaps a more efficient burning of some of the powder in the barrel because of the slight pressure increase. You may also get a few extra fps in velocity, but not enough to be worth the extra cost of the powder you use.
55 grains of FFFg powder in a short barreled .50 is a good charge, and will certainly take a deer with any broadside hit out to 100 yds. So don't be apologizing to anyone for using that load. Many of the members here talk about using 50 grains to take deer all the time. I shot 50 grains in a 45 cal. rifle with a 26 inch barrel, and was at the maximum amount that could possibly be burned in that short barrel. In fact, FFg powder did not shoot as well, simply because it did not all burn inside the barrel using 55 grains. I used strictly FFFg powder in the short .45 cal. rifle.
SNAKEBITE: YOU USED A DIAMETER SQUARED, instead of the radius.
For a .54 cal rifle with a 34 inch barrel, it is .27 x .27 = .0729. times 3.1416 = .2290; times 11.5 = 2.6337: times 34 = 89.54 grains.
You can shoot 90 grains max out of that barrel of yours, not 358 ! Again, you can stuff more powder in the barrel, but you are burning it outside the barrel, and simply adding more recoil and smoke without getting anything appreciable in velocity. That 230 grain ball does not have to be going a " Gee-whiz " velocity to kill game. Its weight does the penetration, and the killing, if you hit where you aim. If ou can find some tables on the .54 I think you will be delighted at the velocity you can obtain with only 80 grains of powder, and what kind of killing power that round ball has at 100 yds. Using data from Hornady on shooting round ball in a .54 cal. percussion rifle, 80 grains of FFg powder will give you a little more than 1500 fps at the muzzle.
Killing Power( KP) is dtermined by multiplying velocity( 1500) times bullet weight ( 230 grains) times caliber ( .54) divided by 100 . Or a KP of 1863 at the muzzle. Assume that at 100 yds, that ball is now only going 1000 fps. The KP at 100 yds. will be 1242. NOT BAD at all when compared to a 180 grain .30 caliber bullet at 2700 fps, having a KP of 1480, or .30-30 cal. 150 grain bullet going 2300 fps. having a KP of 1035.
Remeber that a lead ball kills because even at very low velocity, it still expands its diameter, and makes a much larger primary wound channel than do modern firearms.