I have gotten 3 responce from 3 different engineers. Each one came within a couple HP of each other. It seems this all boils down to the "time " part of the equation. A very large amount of power over a very very short time.. Im sure the results from a .223 would be just as amazing. These results are way higher than I ever thought.
Given that you want to know horsepower (hp) these are like the proverbial apples and oranges. Power is the rate of change in the energy something posseses. Or mathematically (change in energy)/(change in time) or dE/dt where calculus is concerned. It can be estimated for the acceleration of a bullet given two not so great assumptions.
1. That the rate of acceleration of the bullet traveling down the bore is constant. Or in other words, lets take your example. If for a 31" barrell, the muzzle velocity is 2800 ft/s, then when the bullet is 15.5" (half way) down the bore it's velocity is 1400 ft/s, etc..... for estimation purposes this is not that bad of an assumption.
and 2. If we disregard the energy lost in the process of propelling the projectile down the bore due to friction, (which means that we will underestimate the power by at least 20% (Although I am totally guessing on that one).
So I believe you are asking "What is the power generated in propelling the projectile down the bore?"
It can be roughly estimated by dividing the change in energy by the amout of time the energy change occured. So here goes, the change in energy is 12000 ft-lbs (since it starts with zero kinetic energy) and if we use the constant rate of acceleration assumption I get 0.00185 seconds for the time required.
So 12000(ft-lbs)/0.00185(s) = 6,500,000 (ft-lbs)/s Which are odd units of power.
If we use the metric unit for energy (the joule 12000 ft-lbs equals 16,270 J) this value of energy divided by the same amount of time gives us 8,795,000 Joules/s
Which is the units of Watts. and 745.7 watts equals one horsepower (hp) so dividing our answer in watts by 745.7 gives us the astonishing value of about 11,825 Horsepower!!!!!!! And yes this is a very high power output, and if you consider the power output of the powder, It is underestimating the value due to the energy lost due to friction (as well as heat energy lost to the "cooler" sides of the bore from the hot propellant gasses.
But remember that this power is output only for a small amout of time. You can think of it this way, The total amout of ENERY released from approx. 225 grains of rifle powder is probably about the same as that released from burning about a pint of gasoline in your lawnmower mowing your yard. However since that enregy is released over a 30 to 60 minuite time frame the power output is MUCH less, say 4 to 5 hp (typical mower engine power).
Wow, I guess I turned this into an entire physics lecture or something, hope this helps
Bill