This has been beaten to death, but to be charitable, for the season, I will repeat: This is a formula that the Late Charles Davenport, an Early and long time member of the NMLRA. Charlie was the Chief of Naval Ordinance during WWI and WWII, in the 20th century. They used and still use black powder to propel large shells in those guns. He would walk down the firing line at Friendship, I am told, asking men the length and caliber of their guns( many still shooting originals) and tell them their maximum powder charge, and then suggest reducing the max. by 10% to find a highly accurate load, and " Sweet spot " for their particular gun. Over the years, men came to rely on him and his advice. This information came from the Late Nathan Merrill, one of the original Founders of the NMLRA, by way of Phil Quaglino, long time barrel maker, and gun builder, and holder of several national rifle and pistol records.
The formula is: 11.5 grains per cubic inch of bore. With any cylinder, you first find the area of the bore, then multiply it times the length of the cylinder. Area is PiR2, where R is the radius, half the diameter of your bore. Squared means you multiply the radius by itself. Pi is 3.1416. You multiply that times the length of your barrel in inches to get the maximum charge your barrel will burn EFFICIENTLY.
This formula has nothing to do with the MAXIMUM charge you can stuff in a barrel. You should already know that most guns can be filled to the muzzle, have a ball pressed down on the powder and fired, without blowing up.
If you will look at loading data in a manual, you will find that for each 10 grains added there is an increasing gain of velocity. However, at the maximum efficient point, the RATE of gain begins to drop off, so you still get some increase in velocity, but the increase becomes less and less the more powder you put in the gun.
In effect, when you shoot more powder than your barrel can efficiently burn, you are wasting money. Not much, but its still a waste. You aren't getting more bang for the buck. You do get more recoil, and more noise, and often more muzzle flash. At some point you will even find you are pushing unburned powder out the muzzle which is falling to the ground in front of you.
Now, do you understand what I have been talking about, compared what it sounds like you are talking about?
If you want to stuff 90 grains of powder in a barrel that will only efficiently burn 75 grains, Have At IT! If you think you are getting a more accurate load, GO FOR IT.
But here's a tip that will save you money: Don't get into a target shooting contest with someone who uses a load that involves a smaller charge of powder: The odds are heavily in his favor that he will win every time.
I hear all the time, here, that someone is increasing their powder charge to " Flatten the trajectory". This, shooting iron sights.
If you compare the trajectory of the lighter load to the heavier powder charge load, the difference in the mid-range arc is measured in tenths of an inch.
I have yet to meet any off-hand shooter using iron sights who can even hold the difference at 100 yards. If you can, by all means, please come to Friendship, and become a National Champion, and then teach all the rest of us how you do it. The Patched Round Ball is just not going to deliver the kind of " Flat " trajectory" you are used to seeing shooting modern guns, with conical bullets, no matter how much more powder you stuff down the barrel.
