Concentricity?

[rookie101]

Why does a bullet have to be concentric with a case and the chamber and the bore?
Or; why does a straight bore that a bullet fits properly into need the bullet and case to be concentric with the bore with little or no runout for accuracy?

 

[Supersubes]

The simple answer is they dont. Tapered throats, friendly ogives, and tight fitting freebore can mask issues with non-concentric ammo. Many people don’t or can’t measure their concentricity, but still have good results on target. Factory ammo shoots, and it’s usually not very straight. In a worst case scenario though, a bullet entering a crooked bore or entering the bore crooked, is going to deform the bullet. When it leaves the bore it’s no longer the shape that it started, and it will rotating around its new and off axis center of mass. Not a recipe for accuracy.

 

[Country]

Because of yaw . If a bullet goes into the rifling at an angle the sides of the jacket are immediatly made parallel but the base and ogive stay at the angle of entry . Result is in bore yaw and loss of accuracy to some degree depending on how much yaw .

 

[longgunJR]

When the bullet is launched out of the case you want it to engage the rifling grooves as consistent as possible. Bullet run out can give high SDs, higher pressure, and can cause your groups too open up mainly due to the bullet being cocked at a different angle every time. In theory that’s why it’s supposed to matter

 

[kthomas]

I personally don't measure concentricity, never saw the need with my down range results, and I don't want to add another step to my reloading process.

Over the 3 cartridges I reload for (6BRA, 6.5 Creedmoor, .300NM), my ammo easily shoots 1/3-1/2 MOA (not too uncommon to get 1/4 MOA groups either) with SD's of 5 or less.

In my opinion there's other reloading steps that are more important to focus on.

 

[OLD308]

Ok here goes.

This only works if you have quality brass, quality dies, a trued action, and outstanding reloading methods. Lastly, you have to be able to shoot to a degree that few can.

On a concentricity gauge, measure virgin brass at 2 points on the body and one on the neck. FL size and measure again. Fire and repeat 3 more times. If your action is correctly trued, reloading tools and methods top notch, your casings should have effectively 0 run out. Some cases do it sooner than the 3rd firing.

Neck turning is another issue to long to cover here.

If your cases still have a measurable low and high side, mark them on the base. Either the high or low does not matter just mark them all the same, high or low.

Also if your cases still have run out after 3 firings, you have other issues. Possible non trued action, poor chambering, crappy dies, or poor reloading methods.

Hand load 5 rounds into your rifle with the marking pointed up. Shoot a group. Repeat with 5 pointing down. Repeat with them randomly thrown in the chamber.

If you can’t measure the difference on paper, rejoice and go forth with out worrying about concentricity. If you can sign up for a match and mortgage your house to cover your bet on 1st place.

I did all this crazy crap years ago Bench-rest shooting. When I switched to more functional shooting I never saw the difference on paper.

Hope this helps.

 

[rookie101]

Thanks all!