Varying accuracy: why does it happen?

Q: My rifle is a Tikka T3 in .308 Winchester which gives varying accuracy with different bullet weights in my handloads.

The 150gn prints consistent ¾-minute groups, but the 165gn and 180gn loads open up to 1½  and 2-MOA at 100 yards.

Why is this happening? Surely accuracy shouldn’t vary this much.

Ben Brown

A: One reason you’re getting different sized groups with the three bullet weights is that each bullet weight causes different levels of vibration in the barrel.

This is the reason why varying bullet weights seldom print to the same point of aim when fired in the same rifle.

The vibrations established as the bullet passes through the bore interact with the bedding surfaces in the stock. This interaction, in turn, results in greater or lesser accuracy.

A light sporter-weight barrel will vibrate more than a heavy varmint barrel.

The rate of acceleration in the bore and the exit velocity of the bullet also affect barrel vibration.

This is one reason why reducing or increasing the powder charge by a grain or two can sometimes have a dramatic effect on accuracy.