Q: Can you give me the formula for calculating how much backthrust there is against the bolt of a centrefire rifle? Also, I’ve heard mention of a Greenhill Formula for determining the rate of barrel twist needed to stabilize different lengths of bullets. How does it work?
A: I think you’re trying to have me on, but here goes. To calculate backthrust: square the internal base diameter of the bolt and multiply it by .7854 to get the backthrust area. The backthrust area multiplied by the breech pressure will give you the backthrust in psi (pounds square inch). Using the popular .308 Winchester as an example: .400x.400 x .7854 = .126 x 50,000 psi = 6,283 psi. The Greenhill formula was developed by Sir Alfred Greenhill to determine the correct amount of rifling twist for proper stability in the early 1900s. As applied to modern sharp-pointed spitzer bullets it considers a bullet length to diameter ratio in the formula and can be stated as: Divide the length of the bullet by its diameter in hundredths of an inch. Then divide the constant number 150 by the product of the first step and and multiply the result by the diameter of the bullet to get the required rate of twist. Again take a .30 calibre bullet of .308 inch diameter, divide that into its length, say 1.125 to obtain 3.65 calibres long. Divide the 3.65 into 150 to obtain 41.09 or one turn in 41.09 calibres. To convert to inches, multiply 41.09 by the diameter of the bullet in inches (.308) and you get 12.655 or one turn in 12-5/8 inches -(.625 = 5/8 inch). Considering its age the old Greenhill formula still works very well.