Here are my calculations for the 168gr Sierra BTHP when the speed of sound is 1116 fps (59F), and using mach 0.9 as the average subsonic speed.īC = W/7000 divided by caliber squared times iīC (G1 form) = 168/7000 divided by. 010 in your gun and see if the keyholing goes away. 165gr spire point flatbase bullets will be more stable than 168gr BTHP bullets in your gun. JDSquire, I get the award for hijacking your topic.ġ50 gr bullets will be more stable in your 1in12 twist than 168gr bullets. The picture above gives the coefficient of drag curve down to 0.5 mach. We can manually calculate accurate subsonic rifle BC's (all the necessary equations are in Bryan Litz's book), if we know the coefficient of drag. This is due to increased base drag from the flat base.Ģ. The G1 coefficient of drag (flat base bullets) doesn't drop as fast in the 0.8 to 0.98 mach speed range as does the G7 coefficient of drag (boattail bullets). So.the drag curve shows a couple of things.ġ. The dark curve is the G7 drag form and the light gray line is the G1 drag form. Notice the HUGE change in the coefficient of drag in that speed region. For subsonic bullet shooting we are interested in the range generally from 0.8 to. In the chart, the vertical axis is Coefficient of Drag, the horizontal axis is mach speed. The following one for the Sierra Matchking 168gr BTHP shows the problem for calculating rifle subsonic BC's. In his book, Bryan measured the G7 ballistic coefficients for 175 long range bullets and has a page of info for each. The new book by Bryan Litz, Applied Ballistics For Long Range Shooting, is focused on shooting long ranges, but he has all the tools there for subsonic rifle BC calculations. This is really the neglected area of ballistics, for rifle bullet subsonic shooting. While there is plenty of information on BC's for supersonic speeds, there is a dearth of it for subsonic shooting. An example is Clays, where I had some keyholing in 338 but never any with Trailboss. Also, some powders cause keyholing because they are position-dependent because of low fill rate. 015-.020, it may be stable because of reduced pitch and yaw. 070 might have keyholing, but when you reduce the jump to. So.unless you're shooting distance is short for subsonics, then loading a bullet backwards is not a good plan.īTW, my experience developing 308 and 338 subsonics show that reducing the jump distance to reduce pitch and yaw greatly decreases the possibility of keyholing. Bryan is also the Chief Ballistician for Berger bullets. They would have increased nose drag because of the flat nose and increased base drag like a flat base bullet.

They would act like a flatbase bullet and have increased base drag. What does this mean? Many boattail bullets loaded in backwards would not act like a boattail because of the greater than 20 degree angle. Here is a good article at Rifle Shooter talking about base drag and flat-base versus boattails.īryan Litz, many times world champion and author of: Applied Ballistics For Long Range Shooting, shows in his book that a boattail greater than 20 degree angle causes the laminar flow to dettach and act like a flat base bullet. However, Crosshair was right that BC matters at subsonic speeds if you want to shoot at distance at subsonic In flat-base bullets, they are shorter than a boattail bullet of the same weight and their center-of-gravity is farther back to the rear, so they (flat-base) have better stability at marginal twist rates. Other research shows that rebated boattails work very well at subsonic speeds (200gr subsonic bullet made by Lapua).

This results in high subsonic BC and less loss of speed with distance. Boattail bullets have their greatest benefit at subsonic speeds with up to half the base drag as flat-base bullets. At subsonic speeds, base drag becomes far more important. It is true that at supersonic speeds, the base drag is far less than air resistance and wind deflection.

Subsonic bullet backwards