BT vrs FB bullets... where / how fast is a difference noticed ???

[Magnum Wheel Man]

Any generalizations here???

my range only goes to 300 yards... in the past, I've used both boat tail & flat based bullets with seemingly similar results, though I really didn't stretch my legs to 300 that much in the past...

I've heard that a boat tail takes "a bit" to settle down... if this is true, is there a general distance at which this takes place ??? does it vary by velocity magnum vrs efficient AR based cartridges ???

your thoughts & or experience's welcome...

[Jim Watson]

My thought is that flatbase bullets are usual in benchrest shooting 100-300 yards.

I have never seen a credible explanation for bullets "settling down" or "going to sleep" but think from the experience of the benchrest shooters that GOOD flatbase bullets might be a bit more inherently accurate. While the higher ballistic coefficient of the boattail becomes an advantage at longer ranges where the wind has longer to work.

[Mike / Tx]

Years ago before Al Gore invented the internet, and when I thought I knew all I needed to know about loading bullets, I shot mostly boat tails in most of my loads. Why because they were the best of the best. Most of my loads then were shot at 100yds and while I shot many awesome groups, (in my mind) there was a whole new world opened up to me once I started shooting out 3-500yds and further.

Yes some BT's will shoot better in some rifles than FB's, but the contrary is also even more true. I have had loads at 500 that groups almost within a 2" or less circle, with flat based bullets while the same could hardly be said with the groups of the BT's.

I have also had one rifle that really didn't like either, but it loved RN FB bullets. I guess the bearing surface wa what did it the most good.

Even so I found that while some BT's did actually shoot better, on game performance lacked sadly, where FB's usually held together and did what I wanted done.

Even out to the longer ranges I shoot nowadays, I find I use FB's mostly in just about everything. Not that I dislike the BT's but I find that I am simply more accurate with a standard load that I shoot all the time, over something I only load to do a special thing with.

[Bart B.]

While flat based bullets remain the choice of benchresters up through 300 yards of target range, boattails do better at the longer ranges. While the difference at short ranges is very difficult for most to observe, if you shoot your stuff no worse than 1/3 MOA through 300 yards, the difference will be seen.

Sierra Bullets' ballistics man observed decades ago that in factory sporter barrels, they typically shot flat based hunting bullets more accurate than boattails in their 100 yard test range. In custom match grade barrels, they shot their boattail hunting bullets more accurate. Sierra also says their bullets stabilize well in the first hundred yards.

[Wallyl]

Do boat tail bullets wear out a barrel sooner?

[Magnum Wheel Man]

not to my knowledge...

[Wallyl]

I had heard they are hard on the throat of a barrel as the \ / shape angled hot powder gases under very high pressure/concentrated will erode it much faster than a plain base bullet.


I have 500 55 FMJ-BT bullets that I wanted to use in my .22-250 and have been afraid to due to the fear of eroding the barrel.

[Magnum Wheel Man]

my experience is the 22-250 is hard on chamber throats even with flat based bullets... ( I have a Remy 700 I'm re-barreling / re-chambering to 6.8 SPC, as the throat was shot out, & I can pretty much bet it never had a BT bullet down the barrel )

what you are saying likely makes sense, though I don't know of any studies done specifically on that cartridge

[Bart B.]

Regarding boattail bullets:

they are hard on the throat of a barrel as the \ / shape angled hot powder gases under very high pressure/concentrated will erode it much faster than a plain base bullet.

Another myth about barrel erosion. There's the same amount of pressure concentration at the smaller radiused base-body edge of flat-based bullets. If that were true, then the muzzle of barrels shooting boattail bullets would wear out faster, too, 'cause the angled base of the bullet has pressure on it just like flat based ones have. Never heard of any bullet type wearing out the muzzle crown faster than some other type.

Folks shooting 22 and 24 caliber bullets, flat based ones in benchrest matches up to 300 yards and boattail based ones in shoulder fired competition up to 1000 yards got the same barrel life. About 2000 rounds of super accurate life and another 1000 with pretty good accuracy.

[Rangefinder]

MOST shooters will never shoot at distances where BT's will actually show a significant difference. The largest effect of a BT is better stability of the bullet as it passes from supersonic velocities to subsonic. Now look at a ballistic chart of what you shoot to see how far you'll have to stretch in order to see that occur with your bullet.

[Bart B.]

Rangefinder, all bullets have some stability issues as they slow down transitioning the speed of sound in flight. But boattail's on bullets mean they lose velocity slower than flat based ones. They have higher BC's and lower drags than the flat based ones. Therefore, boattail bullets will have less vertical shot stringing from muzzle velocity spread at long range compared to flat based ones leaving at the same muzzle velocity and spread.

[Rangefinder]

Bart, we can discuss all the technicalities all you want, but I was simplifying the matter to terms that won't need 5 pages of discussion for someone who won't likely need it. Yes, I shoot long range--upward of 2000 yards, so I've gone through the fine details of what a BT is or isn't. A vast majority of shooters won't attempt beyond 4-500 (or aren't developed enough to hold competitive groups beyond that with reliable consistency), which in simplified terms means they aren't going to see significant difference between a BT or FB---THAT was the point.

[JD0x0]

Boat tails reduce the aerodynamic drag on a bullet. This generally creates a longer projectile, for a given weight, which could require a faster twist rate in some applications.

I think the only significant advantage FB bullets have over BT bullets, is that generally it's easier for a factory to produce more perfectly shaped FB, because the BT adds more complexity to the bullet shape. I believe this is why FB bullets are said to have the potential for slightly higher accuracy at shorter ranges.

[Rangefinder]

In terms of the average reloader, a BT is much easier to seat than a FB--especially in smaller calibers. That little detail alone helps their popularity.

[Bart B.]

JD0X0, bullets of the same weight and shape except for boattail shape need a different muzzle velocity to attain the same stability factor for a given twist.

Using JBM's site for calculations comparing Sierra Bullets' 30 caliber 180-gr. spitzer with a 9 degree boattail to their 30 caliber 180-gr. 13 degree boattailed HPMK, the spitzer bullet is .008" longer than the HPMK's 1.28" length. If both bullets are fired in a 1:12 twist barrel and wanting a 1.6 stability factor, the HPMK needs to leave at 2600 fps but the spitzer boattail hunting bullet has to leave at 2750 fps; an extra 150 fps velocity. Or shoot the hunting bullet at 2600 fps from a 1:11.9 twist barrel

In comparison Sierra's shorter 180-gr. spitzer flat base 30 caliber hunting bullet at 1.226" long fired at 2600 fps, needs a 1:13 twist barrel for the same stability factor of 1.6

All those bullets will shoot accurately enough leaving 2600 fps in the same 1:12 barrel that very few folks will be able to tell the difference.

A bit of history regarding Sierra's 30 caliber HPMK 180-gr. match bullets.... They originally had a long, 9 degree boattail (like the HPMK 190's and 200's) and a BC of about .520. That bullet was a favorite for M14 rifle competitors in 1000 yard matches; they won a lot and set a few records with it. In the 1980's Sierra decided to use the same base forming dies as their 168-gr. HPMK bullets and put a shorter 13 degree boattail on those 180's as a cost cutting effort. That lowered the 180 HPMK's BC to .496. When the military teams got that new 180 HPMK bullet, it didn't stay supersonic through 1000 yards. So they contracted Sierra to make the old style, long-tailed 180 HPMK's for a few years. When Sierra's 175-gr. HPMK came out, the 180's were no longer needed.

[Brian Pfleuger]

I had heard they are hard on the throat of a barrel as the \ / shape angled hot powder gases under very high pressure/concentrated will erode it much faster than a plain base bullet.

This would be true if the bullet were sitting there and was "sprayed" with gases like they were coming out of a hose. The shape of the bullet would redirect the gases toward the barrel.

That's not what's happening in a fired cartridge though. There's already gases against the bullet and barrel and there's simply pressure building evenly (more or less) from every direction. There is no unbalanced force redirecting gases toward the barrel.

[Bart B.]

Magnum Wheel Man asked....

Regarding when boattailed bullets settle down, they're much like flat based one according to Sierra Bullets. Both fly true by about 100 yards. It depends on how well they're balanced to start out with, how little additional unbalance happens when they're fired and if they're spun by the rifling twist and muzzle velocity the optimum rpm rate to spin them perfectly on their long axis.

[Unclenick]

What happens with the shorter bullet is it has a higher gyroscopic stability factor than a longer one does when fired at the same velocity and with the same rifling pitch. That causes it to settle out yaw in fewer coning cycles. But because it's shorter, it is also has more gyroscopic rigidity so that each coning cycle takes a greater distance to complete. Within normal ranges of gyroscopic stability factors used in shooting, that tends to neutralize the advantage of the fewer cycles required to damp out yaw. Net stabilization distance difference: often not significant.

You can look at 6 DOF program results on damping of yaw for different bullets at different stability factors in Harold Vaughn's book, Rifle Accuracy Facts, and in Brian Litz's book, Applied Ballistics for Long Range Shooting. Assuming you aren't talking about a bullet whose stability factor is barely over 1.0 and that is almost unable to ever stop coning, they all seem to take about 200 yards to damp about 80% of the yaw out. The yaw is not great, so they may appear to be fine at 100 yards. It depends how small a group you are shooting.

Litz confirms that flat bases are usually easier to get tight groups with at shorter ranges, but bear in mind this is tight the way a benchrest shooter sees it. If you are interested in position shooting, chances are you won't see a difference unless your muzzle crown is off. Litz puts the reason to what JD0x0 mentioned: It's relatively simple to make a square base symmetrical, just like putting a 90° crown on a rifle is easy to get right. But as you create an angle, getting perfect symmetry where the conical angle meets a straight cylindrical surface is more difficult. The steeper the angle a boattail has that meets the bearing surface, or that a crown has that meets the bore, the greater the longitudinal error the same amount of offset creates. It's proportional to the cotangent of the included angle or twice the cotangent of the half angle. A 0.0001" offset of the cone axis of a 9° boattail will produce 0.0013" of longitudinal unevenness where it meets the bearing surface.

I will add that a boattail takes longer to finish clearing the edge of the muzzle crown than a flat base does, so the boattail dwells in the highest speed region of the muzzle blast jet longer than a flat base does as it exits the bore. This means any small imperfection in the barrel crown or any unevenness in the distribution of unburned particles moving forward in the bore and deflecting off the bullet base will tend to have their influences on initial yaw and any lateral drift component they impart exaggerated by that extra dwell. The boattail should, therefore, do best with a load that burns up completely in the bore, produces the least muzzle pressure, and is fired from a barrel with a perfect crown. A long barrel with a target crown looks good from these standpoints.

Another muzzle blast factor is that blast deflects off a square bullet base with good perpendicularity, translating momentum mainly forward. But with a boattail it also deflects off the tapered sides, tending to push inward on the bullet base. If there is any imperfection of coaxially of the boattail with the bearing surface of the bullet, this results in net greater push on the longer side, pushing the bullet to the side and instilling a lateral drift component that stays with the bullet all the way to the target. I have actually seen this unevenness in vendor photos of some cheap import BT "match" and FMJ bullets. If you desire a bullet with the accuracy of military ball ammo, this is one way to get it.

All that said, the modern quality name brand match bullets all seem to be awfully good these days. I'll repeat that it's hard for anyone not shooting consistent cloverleafs to really see a difference even off the bench. Getting the right stability factor seems to matter more.

[Bart B.]

Nick,

Thanks for posting the info you're familiar with; much deeper and broader than mine.

However, I've seen some of Sierra Bullets' 10-shot test groups of their 30 caliber Match King bullets shot from .308 Win. cases in the 100-yard indoor range at their California plant. Some of them were in the low ones; darned near 1/10th inch. Others in the high ones. Not too shabby at all with full length sized unprepped cases with metered powder charges and bullet runout up to near .003" I think the vast majority of all the gyroscopic settling down had happened by the time they went through the paper.

When Sierra's 30 caliber 180 and 200 grain FMJBT match bullets were "the" thing to shoot in big bore matches, their accuracy in the 100-yard test range seldom got below 1/4 inch at 100 yards in their rail guns. Some high power competitors claimed better accuracy with good lots of 180-gr. spitzer boattail hunting bullets; hollow pointed ones as far as the jacket was concerned but had a pointed lead tip exposed. They decided their first 30 caliber light weight match bullet would have a hollow point; the 168-gr. International was born and 10-shot sub 2/10 inch groups were frequent with good lots of jacket material. Dimensionally, its heel was more perfect than the FMJBT bullets.

So they quit using FMJBT match bullets and made them hollow point versions in 190 grain and 200 grain weights; later ones even heavier. Test groups finally went down below 2/10 inch with these heavies enough times to prove the benefits of hollowpoint bullets so the heel (body-boattail junction) diameter was more perfect. Gas ejected very uniformly around their bases as they exited perfectly crowned/faced barrels and they settled down quickly in flight.

[Unclenick]

Bart,

One of the things I remember from Kevin Thomas's articles in P.S. in '98 or '99, IIRC, is he mentioned that Sierra had a set aside lot of 168's that had proved exceptionally accurate that they used as accuracy reference bullets. He was using them in the tests he wrote up for the magazine. It may be any bragging groups you saw were fired with these. It may be they've also set aside extra accurate examples of the other MatchKing weights, but I don't know.

The only way I can think of to verify the flat base theory via Sierra would be to look at accuracy standard deviations from different lots, including rejects. Assuming the flat base is easier to manufacture accurately, I'd expect to see lower SD's with correspondingly fewer rejected lots. Once you get past the rejects it's harder to tell, as the rejection criteria, assuming it's equal, biases them all toward sameness.

And the stats would cover only the bullet half of the equation, as Thomas disposed of test barrels as soon as any evidence of them being shot out appeared. I assume that's S.O.P. there, and not just for his tests. So, to the extent using a flat base confers any better immunity to minor crown imperfections and the like, you wouldn't see it there. That would have to tried with an intentionally damaged crown for hard evidence.

[Bart B.]

Unclenick, as far as I know, Sierra has always kept a batch of "super accurate" bullets of each caliber to qualify their test barrels for accuracy. Match bullets were selected for this if possible and called "standards" as they shot in the ones (under 2/10ths MOA) when tested. The accuracy standard for 168's has been 1/4 MOA average for decades. Up until when they moved from California to Missouri, they used to package some 30 caliber Match King standards of all weights in plain brown boxes with labels hand written on them; 1000 or 900 bullets each. All They still had the lanolin sizing lube on them from the jacket forming, coring, tailing and pointing dies making them. With a good batch of jacket material and forming operations, the machines would pump them out at 90 per minute. Mid Tompkins and Bob Jensen would bring a few dozen boxes of them to the bigger matches selling them a bit under retail prices. They shot 30%+ more accurate than anything sold in green boxes. Most of the matches won and records set from the '60's through the '80's with 30 caliber match rifles happened with these greasy "standards." I've still got a few boxes of them I'd sell if the offer was right.

Both the original ballistics man, Martin Hull, as well as the later one, Kevin Thomas, have told me they replace their .308 Win. barrels testing 180's and lighter weight ones at about 3000 rounds. .300 Win. Mag. test barrels used for 30 caliber bullets 190 grains and heavier got replaced after about 1/3rd that many rounds.

[Unclenick]

Bart,

That fills in some information I was missing. Thanks.

[Bart B.]

One other thing regarding the quality of bullets; boattails versus flat based ones.

It's been pretty well established that J4 jackets hand cored and pointed in dies fit in single stage stiff presses making flat based ones will typically shoot a bit more accurate at the shorter ranges. Benchresters are deep into such methods. They can feel the tactile feedback from the press for every bullet cored and pointed; anything that feels different than the norm means that bullet gets tossed in the fishing and paper weight box. High speed mass production machines don't do that.

Same thing for handmade boattail bullets. But the micro dimensions of their rear end ain't quite the equal of what flat based ones can be. Benchresters get very, very close to the accuracy of flat based ones with handmade boattails, but most of the best scores and groups fire at 300 yards or less are done with the flat based ones. It takes stuff that shoots 'em into about 1/10th MOA in perfect conditions to see the difference; and that itself is small. A friend made some 30 caliber 168-gr. HPBT match bullets using J4 jackets formed in Rorschach dies held in his Rockchucker press then tested them against Sierra 168 HPMK's. No difference in accuracy at 200 yards as fired from his machine rested rifle; they all shot about 3/8" groups. AT 600, they both shot into about 3 inches.

Beyond 300 yards, the reverse is the norm and then some. Few, if any, benchrest folks shooting at 600 yards and further use flat based bullets. I think the higher BC's of the boattail ones produce less vertical and horizontal shot stringing caused by velocity and cross wind spreads. And that's more important than what small accuracy difference there may be