ironworkerwill
Member
This was posted in the " Are small bores inherently more accurate " thread.
Best Accuracy-Boat Tail vs Flat Base
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Edarnold
Member
If your barrel has appreciable wear, either at the throat or the length of the bore, a flat-base bullet will slug-up to fill the bore: boat-tail bullets will not. The flat-base bullet, for given weight, will also have a longer bearing surface than the comparable boat-tail, which helps the bullet get started straight in the bore. In an auto-loading .223 rifle with a conventional unlined bore, bore wear is liable to set in quickly due to high rates of fire, the accuracy will still be there but easier to achieve with a flat-base bullet.
If any bullet doesen't 'slug up' to fill the bore (grooves) and make a good gas seal, wouldn't their muzzle velocity be significantly lower and easily noticed?
If the bullet didn't seal the bore and not bear against the groove bottoms, then no copper wash would be visible at the groove diameter. And such bullets would probably have visible signs of gas cutting on their soft jacket material. I've never seen that on recovered boattail bullets .3075" in diameter when new fired in barrels with .3086" groove diameters; copper wash in the barrel was fully the width of the grooves.
If the bullet didn't seal the bore and not bear against the groove bottoms, then no copper wash would be visible at the groove diameter. And such bullets would probably have visible signs of gas cutting on their soft jacket material. I've never seen that on recovered boattail bullets .3075" in diameter when new fired in barrels with .3086" groove diameters; copper wash in the barrel was fully the width of the grooves.
NWcityguy2
Member
I think the whole benchrest shooting/ultra accuracy discussion is getting off track, unless that's specifically what the OP is asking about. I don't shoot accuracy competitions, but my best guess is that a lot more than flat base vs BT goes into the equation.
My personal experience is that anything out shoots a cheap BT bullet, like the 55gr and 62gr fmjbt. OTOH, a cheap flat base bullet can be very accurate. But I've also gotten good or better accuracy out of a match bullet in a non-match gun.
My personal experience is that anything out shoots a cheap BT bullet, like the 55gr and 62gr fmjbt. OTOH, a cheap flat base bullet can be very accurate. But I've also gotten good or better accuracy out of a match bullet in a non-match gun.
Regarding the specifics of the OP's query, Sierra Bullets published info in one of their earlier reloading manuals that factory sporter and arsenal barrels usually shot flat based bullets more accurate than boattail ones. Precision made match barrels typically shot boattails more accurate. But one may not tell the difference until both can be shot consistantly under 1/2 MOA at short ranges.
As far as bullets entering the throat imperfectly, many people get about 1/2 MOA accuracy with 30 caliber bullets at 300 to 600 yards with bullet runout up to .003" and bullets a couple thousandths off center in the throat when fired. Few bullets fired are perfectly aligned with and their axis parallel with the bore axis. So unless they're quite a bit crooked relative to the case axis, I doubt accuracy will suffer much at all.
I've read that the military rifle teams' M14NM's would shoot inside 4 inches at 600 yards with good lots of commercial .308 Win. match ammo with bullet runout up to .003". That's with 22" barrels so whatever irregularities the bullets had entering the bore didn't seem to cause much accuracy issues as they must have straightened out quite well. Expecially considering the number of parts those semiautos have to get back into the same place for each shot.
As far as bullets entering the throat imperfectly, many people get about 1/2 MOA accuracy with 30 caliber bullets at 300 to 600 yards with bullet runout up to .003" and bullets a couple thousandths off center in the throat when fired. Few bullets fired are perfectly aligned with and their axis parallel with the bore axis. So unless they're quite a bit crooked relative to the case axis, I doubt accuracy will suffer much at all.
I've read that the military rifle teams' M14NM's would shoot inside 4 inches at 600 yards with good lots of commercial .308 Win. match ammo with bullet runout up to .003". That's with 22" barrels so whatever irregularities the bullets had entering the bore didn't seem to cause much accuracy issues as they must have straightened out quite well. Expecially considering the number of parts those semiautos have to get back into the same place for each shot.
He asked about accuracy, which is what Benchrest is all about.
That's a good guess, yes, it takes more than an accurate bullet to shoot well enough to win.
Question remains, which is more accurate, flat base or boat tail. To discuss it we must assume the other factors are all equal. Barrel quality, action quality, load quality, etc, etc. In other words, not guessing about the other factors.
Will my Bench gun shoot a custom or semi custom boat tail bullet with the right load better than a lesser quality flat base bullet? Yes it will.
Yep, 55 gr and 62 Gr FMJ bullets tend not to be very accurate because of the poor bases. Yes, even cheap flat base bullets, or cheap boat tail bullets, will out shoot them. It has nothing to do with anything except that the FMJ design with an open base cannot compete accuracy wise with an open front bullet like HP, SP, etc, etc.
Now they are starting to make bullets that look like "FMJ" designs, but they are a closed tip design with a solid base, either flat or BT. They are doing this for accuracy, with the closed, or nearly closed, tip that feeds well in autos.
Weren't Lapua's .309" 185-gr. FMJ rebated (open) base boattail match bullets shot in 30 caliber target rifles with great accuracy?
There's a report that several were fired at 600 yards shooting many 10 shot groups well under 1.5 inches. Some were under .8 inch.
ironworkerwill
Member
I found a good read. It may raise as many questions as it answers but here it goes:
Page 2, Rebated Boattail Bases...
"The conventional boattail bullet does have three problems associated with it.
(1.) The angled boattail base tends to focus escaping muzzle gas like the nozzle of a hose, so that the
gas flows in a laminar manner over the boattail, along the parallel shank, and partly attaches or follows the
outline of the ogive until it separates at or near the tip, and breaks up into turbulence just ahead of the
bullet. This can add as much as 15% to the total dispersion pattern of a given bullet design. The boattail
bullet literally flys through its own muzzle blast because of the focusing effect of the streamlined base during
the moment of exit from the barrel.
(2.) Since gas pressure acts normal to all surfaces (at 90-degrees), the compressive force of chamber
and barrel pressure tends to compress the boattail section of the jacketed bullet inward, peeling it away
from the bore and allowing gas to channel its way into the rifling grooves, causing gas cutting of the rifling
edges and the edges of the rifling imposed on the bullet. Micro-droplets of melted jacket material can be
observed on most boattail bullet jackets along the rifling edges, especially toward the rear of the bullet
shank, some large enough to see without a magnifying aid. The flat based bullet tends to compress in
length so that the shank is expanded into the rifling, for a superior seal.
(3.) The boattail bullet is sensitive to slight manufacturing variations in the position and concentric
alignment of the boattail angle starting point. At the moment of exit from the bore, while the rifling is just
losing contact with the shank diameter, any difference in position of the junction of the shank and the
boattail gives a tremendous leverage to the escaping gas, which allows it to push the entire bullet in the
direction of the higher starting point. That is, if the boattail is even slightly higher on one side of the bullet,
the bullet will be deflected toward that side at the moment of exit by gas pressure escaping earlier from the
opposite side."
Source:http://www.swage.com/ftp/rbt.pdf The full 2 page report.
Page 2, Rebated Boattail Bases...
"The conventional boattail bullet does have three problems associated with it.
(1.) The angled boattail base tends to focus escaping muzzle gas like the nozzle of a hose, so that the
gas flows in a laminar manner over the boattail, along the parallel shank, and partly attaches or follows the
outline of the ogive until it separates at or near the tip, and breaks up into turbulence just ahead of the
bullet. This can add as much as 15% to the total dispersion pattern of a given bullet design. The boattail
bullet literally flys through its own muzzle blast because of the focusing effect of the streamlined base during
the moment of exit from the barrel.
(2.) Since gas pressure acts normal to all surfaces (at 90-degrees), the compressive force of chamber
and barrel pressure tends to compress the boattail section of the jacketed bullet inward, peeling it away
from the bore and allowing gas to channel its way into the rifling grooves, causing gas cutting of the rifling
edges and the edges of the rifling imposed on the bullet. Micro-droplets of melted jacket material can be
observed on most boattail bullet jackets along the rifling edges, especially toward the rear of the bullet
shank, some large enough to see without a magnifying aid. The flat based bullet tends to compress in
length so that the shank is expanded into the rifling, for a superior seal.
(3.) The boattail bullet is sensitive to slight manufacturing variations in the position and concentric
alignment of the boattail angle starting point. At the moment of exit from the bore, while the rifling is just
losing contact with the shank diameter, any difference in position of the junction of the shank and the
boattail gives a tremendous leverage to the escaping gas, which allows it to push the entire bullet in the
direction of the higher starting point. That is, if the boattail is even slightly higher on one side of the bullet,
the bullet will be deflected toward that side at the moment of exit by gas pressure escaping earlier from the
opposite side."
Source:http://www.swage.com/ftp/rbt.pdf The full 2 page report.
I have no idea, but I assume you are correct.
You can see from side pics that the design is entirely different than the typical FMJ bullet I talked about. It has a false base that would help the bullet remain stable/stabilize soon when leaving the muzzle, just like a regular base, then the rebated boat tail, and they rolled the jacket over that edge as well, making it nice and uniform. I see no reason it would not shoot well, and no reason to compare it to the typical open base FMJ I am speaking of.
You can see from side pics that the design is entirely different than the typical FMJ bullet I talked about. It has a false base that would help the bullet remain stable/stabilize soon when leaving the muzzle, just like a regular base, then the rebated boat tail, and they rolled the jacket over that edge as well, making it nice and uniform. I see no reason it would not shoot well, and no reason to compare it to the typical open base FMJ I am speaking of.
lysanderxiii
Member
aka stepped boat-tail
Hornady had a line of these back in the 1980s-90s. I still have a box of the 162 (?) stepped boat-tail 7mms, they also offered a 168 gr HP Match with a stepped boat-tail along side their conventional 168 gr match .308s.
I wonder why they stopped making them?
Hornady had a line of these back in the 1980s-90s. I still have a box of the 162 (?) stepped boat-tail 7mms, they also offered a 168 gr HP Match with a stepped boat-tail along side their conventional 168 gr match .308s.
I wonder why they stopped making them?
Hornady may have quit making them because of the issues with Lapua's rebated base match bullets. Some people had made careful measurements of Lapua's bullets and found 3 or 4 succinct ogive shapes; evidence that 3 or 4 lots of bullets from different sets of forming dies were blended into a retail lot. And oft times 2 or 3 bullets from a box of Lapuas shot far away from group centers with the rest of them which meant they were severely unbalanced for some reason such as a bad lead core.
The DCM bought several thousand 30 caliber Lapua 170 grain rebated base match bullets sometime in 1985 to 1990. I think Lake City arsenal loaded some in 7.62 NATO match ammo but they were no more accurate than the 172 grain FMJBT bullet used in M118 ammo. They were eventually given to state teams for distribution to their members as deemed necessary for reloading.
Or maybe Hornady couldn't make them consistently good enough from lot to lot to make it worthwhile.
The DCM bought several thousand 30 caliber Lapua 170 grain rebated base match bullets sometime in 1985 to 1990. I think Lake City arsenal loaded some in 7.62 NATO match ammo but they were no more accurate than the 172 grain FMJBT bullet used in M118 ammo. They were eventually given to state teams for distribution to their members as deemed necessary for reloading.
Or maybe Hornady couldn't make them consistently good enough from lot to lot to make it worthwhile.
lysanderxiii
Member
I think everything that has been about boat-tails, uniformity in uncorking, concentricity and length, all go double for stepped boat-tails because now you have two discontinuities in the base to control.
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