Why are flat based bullets so accurate?

[Canadian Bushman]

I know BT bullets are accurate as well but the inherent accuracy of flat based bullets seems amazing to me. Why is this?

 

[Hunter2678]

IIRC its due in part to FB bullets being easier to make more uniform vs BT and its also said that the FB bullets exit the muzzle "cleaner"due to a more concentric gas seal..of course the tradeoff on them is the reduced bc hence they are typically used as "shorter range" bullets.

 

[Canadian Bushman]

That makes a lot of sense, i was concentrating on the internal ballistics comparison between FB and BT. I didnt even consider the mfg process.

 

[Mikecr]

Cleaner release, and inherently more stable(and not just due to length).

With this, you can run with a shorter stiffer barrel even while muzzle pressures go way upward with this, and still have a relatively clean release.
You can run lower twist than with ANY boat tails, and then the gun torques less in recoil.

But there are prices for everything.

 

[J E Custom]

In addition to the other comments, flat based bullets normally have more bearing surface and tend to seal faster when fired.

Some boat tailed bullets have a very short bearing surface and some barrels/twist don't like them.

J E CUSTOM

 

[Canadian Bushman]

When you guys say cleaner release, you mean the base of the bullet physically stops touching the bore of the rifling at the same time around the entire circumference of the bullet?

Or do you mean, the way the hot expanding gases push the bullet after it has stopped contacting the rifling?

Cleaner release, and inherently more stable(and not just due to length).

Mike
Could you elaborate on the other inherently stable properties of FB bullets besides their shorter lengths?

 

[Canadian Bushman]

In addition to the other comments, flat based bullets normally have more bearing surface and tend to seal faster when fired.

Some boat tailed bullets have a very short bearing surface and some barrels/twist don't like them.

J E CUSTOM

Does the larger bearing surface contribute to accuracy in other ways beside aiding the flat base to seal to the bore?

 

[Mikecr]

I'm referring to muzzle blast across the back of the bullet. A flat base presents smaller area than boat tail area, and it's perpendicular to centerline(rather than more parallel).
Flat base bullets generate considerably more base drag, which pulls back center of pressure to shorten the Cg to Cp moment arm.

 

[J E Custom]

Does the larger bearing surface contribute to accuracy in other ways beside aiding the flat base to seal to the bore?

Larger bearing surfaces normally mean better engraving of the bullet by the rifling improving stability.

Bench rest shooters can probably answer this question better than long range shooters that
traditionally use boat tailed bullets.

J E CUSTOM

 

[Canadian Bushman]

I'm referring to muzzle blast across the back of the bullet. A flat base presents smaller area than boat tail area, and it's perpendicular to centerline(rather than more parallel).
Flat base bullets generate considerably more base drag, which pulls back center of pressure to shorten the Cg to Cp moment arm.

This makes a lot of sense.

Studying and designing my own suppressors ive found blast baffle designs that can significantly improve velocity (50-100fps) by keeping the muzzle blast behind the bullet. All my research with rifles was done with BT bullets.

Thinking about the FB bullets. The gases would not only act on them the way you describe, but also have a harder time over coming the bullet immediately after exit, allowing its path to remain less disturbed.

I should load up some light FB bullets and shoot them unsuppressed and then suppressed and compare the velocity gains with those of the BT bullets with the same equipment. Im betting the gains would be much less.

 

[Lefty7mmstw]

The fb bullet designs are also usually more tolerant to slightly out of spec barrel diameters, which is why I suspect some of my factory bbl.'s only tolerate them and not any bt's. I've killed to 700 yards with fb bullets; the bt's aren't all that necessary until you are well past the 1/4 mile mark if you are shooting a fast cartridge. That said, I am working my rifles that will shoot bt's over to them for the most part.

 

[Canadian Bushman]

The fb bullet designs are also usually more tolerant to slightly out of spec barrel diameters, which is why I suspect some of my factory bbl.'s only tolerate them and not any bt's.

Again, this is because of FB bullets large bearing surface and its flat bases ability to easily obturate to the rifles uneven bore?

Do bullet manufacturers purposely use a concave base instead of a truly flat base to enhance this property?

 

[HunterGreen]

This explains why berger bullets prefer jammed into lands.. better gas deal across the board

 

[TracySes23]

Again, this is because of FB bullets large bearing surface and its flat bases ability to easily obturate to the rifles uneven bore?

Do bullet manufacturers purposely use a concave base instead of a truly flat base to enhance this property?

I have only my own logic to explain a concave base with a jacketed bullet. The only bullets I've seen with a concave base have a .063" radius on the point. The radius is not tangent to the ogive. It isn't a hollow point. I would identify it as a FMJ bullet.The only example I have in my possession is a Remington 22 caliber 50 grain Power-Lokt Match bullet. It appears as though the lead core was pushed in through the base of the bullet and then sealed with a copper (gilding metal) plug. I suspect it's concave simply because extreme pressure was applied to the base with a convex punch in order to seal the base well. It appears as though the bottom of jacket was swaged inward and over the copper plug. All of this had to be done while it was still encased in the die that formed the nose, ogive & diameter of the bullet. These are nearly 40 years old. It looks very similar to Remington's 55 grain FMJ bullet, except is has no cannular (sp).
I still have about half a box of these bullets. I can't remember much about them except I didn't seem to have much luck working up a good load at the time. Likely my fault.
Searching the Internet, I can't find this bullet.

Perhaps someone else has a concave based bullet from another manufacturer. Who manufactured the one(s) you've seen?