Bearing Surface

[Westerner]

Why do some Rifles like a Bullet with a lot of Bearing Surface?And some like vary little? How do you figure how much a Bullet has?

Just Thinking

Westerner

[Rocky Raab]

I couldn't answer why, except to observe that just possibly a barrel with fewer grooves or a rougher finish or a smaller diameter might perform better if the bullet has a shorter bearing surface. My reasoning is that there is less bullet to either engrave or force into the grooves.

As to measuring, the bearing surface is simply the length of the shank that is at full diameter. That's not too difficult to measure.

[iwerk2hard]

Wouldn't the twist rate have a lot to do with it?
Heavier bullets (of the same caliber) would typicaly have more bearing surface than lighter bullets because they are generaly longer. The longer heavier bullets seem to perform better at a higher velocity with a faster twist at greater distance.

[Rocky Raab]

That's certainly true, but there is a lot of bearing surface variation even in same-caliber bullets of the same weight.

Boattails reduce bearing length, as do long ogives. Flat-base roundnoses have the longest bearing surface, as a rule.

Which brings up an interesting observation: I've heard some shooters say that the .223 is fine, but its neck is too short. Doesn't hold the bullet straight or something.

But dig out a caliper and measure a standard 55FMJ military bullet and you find something interesting. The bearing length between the ogive and the boattail is SHORTER than the .223's neck length! The cartridge's neck could be a foot long, and it wouldn't hold that bullet any straighter!

[Evan03]

whats the term ogive all about.


i think you just said the 55fmj has a very short bearing surface regardless of lenght. that bullet should be fairly long.

its probly a flat base with the bearing surface at the end of the bullet. im just guessing hear hmmmmm or its in the middle of the bullet and youve gota stick a lot of tail in the case to get a good bite on it with the kneck?

[Rocky Raab]

The ogive is the "nose" of the bullet. Technically, it's the radius of the curve of the nose. It starts at the very tip of the bullet (which is called the meplat) and reaches to the point on the shank where the bullet becomes full diameter. Nothing ahead of that point touches the bore.

So the bullet bearing surface is the length of the shank between the ogive and the base.

The 55 FMJ bullet as originally used by the military has a long ogive AND a boattail, both of which have the effect of reducing the bearing length. On samples I've measured, that was a total pf .190"...WAY less than one caliber! Most case necks on the .223 run .210 to .215" in length.