Bullet Velocity and Twist Calculator for Random Barrel Lengths.

[AFJuvat]

All reload information, as well as published ammunition specifications are normally fired out of a test barrel or a test rifle which may not be the same length or twist as your rifle. Many of us do not own chronographs, or if you shoot at a public range, setting one up can be a pain.

This program will calculate:
1. Approximate velocity for a load using your actual barrel length
2. Optimal twist rate for the given bullet/load
3. Stability reference - A stability value of < 1.0 is not likely to be accurate in your rifle. Conversely, a stability value > 5.0 is likely to tear itself apart.
4. Indicate sub/supersonic.
5. "Gee Whiz" information such as MPH, Mach, RPM, and ft/lbs of energy at the muzzle.

Limitations: The formulas are optimized for modern ammunition in modern firearms, where most, if not all of the powder is consumed in the barrel. The values will not be accurate if you are shooting a SBR chambered in 45-70.

A few Screenshots

Values entered, user inputs are at the upper left. In this example, I used Hodgdon Reloading data for a 5.56 M855 (green tip) projectile. The published load data was developed using a rifle that has a 24" barrel with a 1:12 twist, which gives a test velocity of 2974 fps. Note the corrected the barrel length to 16" and altered the twist rate to 1:7.


After hitting the "Calculate" button. The computed actual velocity is 2752 fps. The stability values are based off of an offshoot of the Miller Twist Rule. Essentially, you want a value greater than 1.0, and preferably less than 5.0. Notice that the calculated stability for the test rifle is < 1.0. The "Gee Whiz" information in the lower right gives a visual indicator if the load will be subsonic or supersonic.


Same load again, but in a 9" barrel. Velocity is down to 2382 fps. Stability dropped from 1.92 to 1.71



New load from Hodgdon for 300 AAC Blackout using a 208 grain bullet. Test barrel was 16", so I corrected for a 9" SBR. Green boxes in the "Gee Whiz" section to denote subsonic.

Program runs on Windows 7, 8, and 10. Should work on Windows XP provided you have the latest updates.

Free to use and share. Does NOT need an internet connection to work.