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Bullet weight, barrel vibration and point of impact


Q: For quite a while now I’ve been using a 180gn bullet in my .30-06 for sambar hunting, but with the bush being a lot thicker after the bushfires, I set out to develop a load using the Woodleigh 220gn RN SN.

When I went to sight my rifle in on the range I expected the heavy bullet would shoot quite a bit lower and I’d have to raise the zero. But I got an unexpected surprise; the heavier bullet shot four inches higher than the 180gn and I had to lower the elevation.

What gives? How does this happen?

Francis Angwin

A: It’s a fallacy that a heavy bullet always gives a lower point of impact at any range than does a light one. Nothing could be farther from the truth.

Yes, if both bullets are zeroed to have the same point of impact at 100 yards, the heavy bullet will land progressively lower as the range increases, assuming both bullets are driven to their full velocity potential and have the same shape, but this has nothing to do with where they will impact at 100 metres with the same sight setting.

Some .30-06 rifles, such as yours, will land several inches higher at 100 yards with the 220gn bullet than a 180gn when sighted in for the 180gn. However, in other .30-06 rifles the 220gn may land the same amount low, or off to one side or the other.

The stress caused by a bullet travelling through the barrel makes it to vibrate in pretty much a circle at the muzzle, and how much a bullet departs from the line of sight depends on the point of vibration at the time the bullet leaves the muzzle.

Where it impacts is due to the amount of vibration of the barrel created by the bullet and the time the bullet takes to reach the muzzle. If it exits while the muzzle is at the top of its vibration pattern, it will land high; if the muzzle is at the bottom of its cycle, the bullet will land low.

Bullet weight has nothing to do with it.

 

 

 


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Nick Harvey

The late Nick Harvey (1931-2024) was one of the world's most experienced and knowledgeable gun writers, a true legend of the business. He wrote about firearms and hunting for about 70 years, published many books and uncounted articles, and travelled the world to hunt and shoot. His reloading manuals are highly sought after, and his knowledge of the subject was unmatched. He was Sporting Shooter's Technical Editor for almost 50 years. His work lives on here as part of his legacy to us all.

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