Tuning process

The graph at the left intend to explain how the bullet seating depth setting can be used to tune the ammo for a specific weapon. In my opinion this tuning process brings the the most gain in precision to a bolt action rifle. For explanation purpose I am using a clean sinusoidal function representing the angular vibration of the barrel while the barrel vibration is actually more complex. Also the bullet will slightly affect the vibration point as it travels through the barrel.

Also for this explanation is considered that the rifle has a good and stable configuration for barrel mounting so the vibration point of the barrel does not change from shot to shot.

Dt = shot to shot variation of total time from ignition to the time the bullet exits from the barrel. This variation is related to all error factors related to loading process. Round to round variation of: powder chamber volume, powder burning speed, powder quantity, round and chamber temperature, bullet concentricity to center of barrel, bullet weight, bullet diameter......

t = time ( x axis)

< = angular amplitude (y axis)

D< = angula variation

Then, assuming that there is a Dt from shot to shot then it is very important at what point of vibration the bullet is exiting the barrel. For example, in red color, if the barrel exits in the middle of the vibration center of the angular variation then the angular variation will be the maximum.

If the bullet exit point is centered close to the outside of the vibration, see the example in green color on the graph, the angular variation or error from shot to shot is minimum giving you the best precision.

Obviously the bullet seating depth can be used to control the timing of the bullet exiting to match the barrel vibration. The process is relatively empiric consisting on shooting 5 or 10 rounds groups each group with different seating depths. I usually start at -0.005" from the lands in 0.003" to 0.005" increments depending the diameter/stiffness and length of the barrel (larger diameter / stiffer and shorter 0.003"). These experiments for determining ideal bullet seating depth should be done when outdoor conditions are the best for accurate shooting, minimum wind and wind variation conditions.... etc. Ideal is to do the shooting at 300 y or over 250y since the bullet flight stabilizes around 250 y.

Well, you will have to repeat the experiment with each different powder you may want to use and also to determine the best powder since burning rate variation from powder to powder may affect the amplitude of the barrel vibration.

For example I did this process with a 7mm Rem Mag Sendero; First best group from the lands was at -0.012. The largest group at 185 y was 6" at using ammo purchased in a gun store and 1" at -0.012. The rifle is shooting consistently in within 1.5" groups at 300 y.

I am in process of accurizing my .338 Lapua Magnum, 110 BA made by Savage Arms and mi biggest problem is that I can not locate a bullet seating depth micrometer. I do have a micrometer bullet seating depth die, however, I will not trust the experiment without having the means to verify accurately the bullet seating depth. Yes, there are other tools using a caliper ... etc. . If anyone knows about where to buy one for this specific calibre please let me know!

Bullet exit timing set in reference to barrel vibration.