The Firing Line Forums - View Single Post - Bullet Trajectory different than computer model

**Unclenick** · September 4, 2005, 12:43 PM

Ethernectar,

Putting your external ballistics program at 105°F will only show the effect of the change of air density on the bullet path. It will not account for the internal ballistic effect of temperature change on powder burning rate, which in turn affects pressure peak timing and muzzle velocity. A load that puts the gun in the wrong part of its muzzle vibration transition can cause small increases in velocity to throw the muzzle disproportionately up or down, depending on the point in the curve (Vaughn, H., 2000, Rifle Accuracy Facts, 2nd Ed., Preceision Shooting Pub., pp. 85-89). Even so, it shouldn't be as great as you are stating.

External ballistics programs basically interpolate for the BC's given, and won't be more precise than these figures are. I have always found my come-up from 300 to 600 yards with a service rifle with iron sights is about 11.5 minutes, while the programs call for more like 13 minutes (168 grain Sierra MatchKing). I have always put this off to the change in sight picture and aim point that distance imposes on the naked eye, but have not verified the error with a scope. It should not be an issue with a scope. This 1.5 minutes difference would be about 6 clicks on your scope.

Soooo... A dumb question. My external ballistics program (QuickTARGET) let's me sepcify the number of inches per click at 100 yards before generating tables. Any chance your's does too, and that you got the wrong number in?

1.000 m.o.a. = 1.0472 inches/hundred yards
0.500 m.o.a. = 0.5236 inches/hundred yards
0.250 m.o.a. = 0.2618 inches/hundred yards
0.125 m.o.a. = 0.1309 inches/hundred yards

Assuming you had that covered, let's get down to nuts and bolts. Please post your load. If you know the water capacity of your cases, post that. Provide your barrel length. Let us calculatory nit-pickers have a go at it!

Also: What kind of chronograph did you use and what is the screen spacing? My dad's Chrony, with its short spacing, consistantly disagrees with my Oehler 35P (with 4 foot spacing and check screen) by about 240 fps in this velocity range.

Nick

September 4, 2005, 12:43 PM	#25
Unclenick Staff Join Date: March 4, 2005 Location: Ohio Posts: 21,063	Ethernectar, Putting your external ballistics program at 105°F will only show the effect of the change of air density on the bullet path. It will not account for the internal ballistic effect of temperature change on powder burning rate, which in turn affects pressure peak timing and muzzle velocity. A load that puts the gun in the wrong part of its muzzle vibration transition can cause small increases in velocity to throw the muzzle disproportionately up or down, depending on the point in the curve (Vaughn, H., 2000, Rifle Accuracy Facts, 2nd Ed., Preceision Shooting Pub., pp. 85-89). Even so, it shouldn't be as great as you are stating. External ballistics programs basically interpolate for the BC's given, and won't be more precise than these figures are. I have always found my come-up from 300 to 600 yards with a service rifle with iron sights is about 11.5 minutes, while the programs call for more like 13 minutes (168 grain Sierra MatchKing). I have always put this off to the change in sight picture and aim point that distance imposes on the naked eye, but have not verified the error with a scope. It should not be an issue with a scope. This 1.5 minutes difference would be about 6 clicks on your scope. Soooo... A dumb question. My external ballistics program (QuickTARGET) let's me sepcify the number of inches per click at 100 yards before generating tables. Any chance your's does too, and that you got the wrong number in? 1.000 m.o.a. = 1.0472 inches/hundred yards 0.500 m.o.a. = 0.5236 inches/hundred yards 0.250 m.o.a. = 0.2618 inches/hundred yards 0.125 m.o.a. = 0.1309 inches/hundred yards Assuming you had that covered, let's get down to nuts and bolts. Please post your load. If you know the water capacity of your cases, post that. Provide your barrel length. Let us calculatory nit-pickers have a go at it! Also: What kind of chronograph did you use and what is the screen spacing? My dad's Chrony, with its short spacing, consistantly disagrees with my Oehler 35P (with 4 foot spacing and check screen) by about 240 fps in this velocity range. Nick