Estimating Recoil

Well, just how hard does your pea-shooter wack your shoulder?

That’s question shooters have asked since firearms were first invented. We have seen a fair number of recoil calculators that are pretty good emerge over the last century.

What’s different about this recoil calculator?

To begin, this one is used in our very own Velocity estimator, so getting recoil for a particular load and barrel length is straightforward.

Further, most recoil calculators use a simple multiplier of 1.25, 1.5, or 1.75 times the bullet velocity to estimate
average gas velocity. Some employ a fixed gas velocity of 4000 ft/sec or some other value. This works well enough for most cartridge and gun combinations, but there are times when this technique significantly underestimates the powder gas contribution to recoil. Very large case volumes filled to the gills with powder behind light weight bullets are where this becomes most evident. One example is the the .30-378 Weatherby where one can find loads where there’s more powder than bullet! Low expansion ratio and LOTS of powder mean that the gun will kick a tad harder than a “standard” recoil calculator would indicate.

The “secret sauce” in the calculator? The ShootersNotesTM recoil calculator does not use the canonical velocity multiplier or fixed powder gas velocity. More advanced internal ballistics codes like QuickLoadTM employ a stepwise calculation of bullet, powder burn, and pressures that would be difficult for a high-volume on-line application. Instead ShootersNotes uses an extension of the adiabatic expansion used in the Powley calculator, coupled with a modification of a modification of a very mature tool explosives design tool, to estimate the average gas velocity. Resulting momentum statements are remarkably close to what one would get with QuickLoad!

NOTE: The input form asks for two items not normally seen in recoil calculators: Case Volume and Barrel Length. These are used on estimating the gas velocity.