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Old January 3, 2011, 01:19 PM   #1
Doodlebugger45
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Pressure differences between flat-base bullets vs. boattails?

I was noticing that on Alliant's site, they list completely different powders for the recipes for boattail bullets compared to flat base bullets. Both bullets were Speer and both were 150 gr SP. But one load recipe (for plain base I guess) listed RL 19, 22, and 25. But for the BTSP, they give data only for for RL-17.

Yeah, I know RL-17 is fairly new stuff, so perhaps they just listed what they actually measured so far. But it got me to wondering if the pressure characteristics were much different for a plain base bullet compared to a boattail bullet if everything else was equal?
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Old January 3, 2011, 02:33 PM   #2
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Just based on physics, I would think boat tails have less pressure because there's less surface area for the gas to contact directly.
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Old January 3, 2011, 03:28 PM   #3
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My thought would be the surface area that the gas presses against is the same with both bullets because once they are locked into the lands and grooves the diameter of the bore is the same for each bullet.

As I have read, the only advantage to boat tail bullets is stability out beyond 400 yards is better than with flat base. But you really need to try both in your particular rifle to find out. I have seen one do better than the other in various calibers in my collection without any predictability.
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Old January 3, 2011, 10:42 PM   #4
700cdl
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I load using the same data for both and have for many, many years. The data difference you are seeing is most likely related to a manufacturer's bullet/powder. kind of like a marketing tool so as to say this is what works for this bullet/ powder so they can associate the two as a mated pair. But that is onyl my opinion based on hand loading the two.
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Old January 3, 2011, 10:54 PM   #5
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Agree. That's Alliants test schedule showing.

Pressure against the bases of the two types turns out to be the same. When you analyze the force vectors along the taper of a boattail, they add up to equaling the flat base force at any given pressure. The main difference is the same weight, same ogive shape boattail is a little longer and winds up seated slightly deeper in the case. That raises pressure a little, but in many cartridges the difference is too small a percentage of the total to make you adjust the load.

Flat base bullets usually are easier to make accuracy loads with at shorter ranges. The longer bearing surface helps them self-align in the bore better and the flat base clears the muzzle faster so it spends less time subjected to being tipped by the muzzle blast that follows.

All else being equal, the boattails will have higher ballistic coefficients, and that can tell in as little as 300 yards or not show up until 500 yards, depending on the wind conditions. The higher BC means they will be going faster at any point down range from a same weight, same MV flat base, so the farther you go the harder they hit, by comparison.
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Old January 3, 2011, 11:15 PM   #6
Win_94
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Quote:
The longer bearing surface...
... of the flat based bullet, causes a bit more friction against the bore.
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Old January 4, 2011, 07:57 AM   #7
FlyFish
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Quote:
All else being equal, the boattails will have lower ballistic coefficients, . . .
You meant higher, right?

Quote:
Flat base bullets usually are easier to make accuracy loads with at shorter ranges. The longer bearing surface helps them self-align in the bore better and the flat base clears the muzzle faster so it spends less time subjected to being tipped by the muzzle blast that follows.
So wouldn't that be an advantage for the flat-base bullet at any [known] range? Does the boattail somehow "recover" from the muzzle-blase effect at very long ranges and then become more accurate? I can see how the boattail's higher BC translates into a flatter trajectory, which is an advantage in a hunting situation where the range is only estimated, but for target work where the range is fixed and known, is the potential advantage of the boattail only in having less wind drift due to the higher overall velocity? Thanks for your insights.
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Old January 4, 2011, 11:23 AM   #8
Doodlebugger45
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Unclenick is saying the same thing as I read over at the 6mmBR forum where there are some mighty serious benchrest shooters. It seems that a whole lot of them prefer the flat base bullets for their work. That kind of surprised me when I first got into loading rifles, since I hear so much about boat tail bullets. I kind of figured the pressures would be the same basically, but it's just one of those things that got me thinking.

As for my own experiments with loading different rifles and powders and bullets, I can't honestly say whether a boat tail or a flat base works better. For some combinations, one is better, but for other combinations, the other works a bit better. I doubt if the base is the determining factor in my case, but then I'm not a serious benchrest shooter. I do like the boat tails simply because they seat a bit easier on the case mouth.
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Old January 4, 2011, 12:31 PM   #9
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Quote:
Just based on physics, I would think boat tails have less pressure because there's less surface area for the gas to contact directly.
Nope.

The total cross section is all that matters and it is the same.

Flat base bullets of the same type (ogive, weight, etc.) are going to have a longer bearing area and increased friction.
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Old January 4, 2011, 01:21 PM   #10
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Fly Fish,

Right. Higher. My next sentence had it right. I changed the sentence from one speaking of the flat base and did an incomplete edit. I've fixed it now.

Last night I finished Bryan Litz's book, Applied Ballistics for Long Range Shooting. Near the end it he covers this very topic, flat base verses boattail. Litz includes what I said about greater muzzle blast deflection on the base of a boattail contributing to pitch and yaw. This is why Harry Pope said over a century ago that the base steers the bullet. The more perfect it is, the better. Litz adds that a flat base is easier to manufacture precisely than a boattail, so it also less prone to having axial mass symmetry errors. Axial mass symmetry error causes bullet jump at muzzle exit and contributes to pitch and yaw and wobble that cause an epicyclic (helical) trajectory. It takes distance to recover from that. It is obvious that even for a flawed bullet, the slower you spin it, the less of that kind of trouble it can cause. Because a shorter bullet requires less spin to stabilize, you can use a slower twist barrel with it, and that contributes even less effect from that kind of error. This is why shorter range benchrest is all shot with short shape, flat base bullets in minimum twist barrels.

Litz divides sources of error into determinate and indeterminate ones. The determinate ones are your range and sight elevation, spin drift, windage zero at a known side winds, the range of the target and elevation adjustment for that range, shooter position and control over rifle cant, and so on. Basically, these are things that the shooter can and should measure and control by determining them and making appropriate adjustments. The indeterminate influences include individual bullet imprecision or muzzle velocity variation in any particular round, the target moving unexpectedly (in hunting) and, of course, all the stuff with the wind that might change direction or temperature and density part way downrange.

The indeterminate atmospheric influences are all proportional to the time air drag adds to the flight of the bullet from the muzzle to the target. In other words, if there were no air, moa of error would be in straight lines and proportional to distance. But because the bullet is going slower and slower down range, it takes longer to cover the last hundred yards than the first. Though a constant side-wind's influence on moa tends to be less further down range because there is less total time for the bullet to drift before impact, that longer total time per unit distance increases the absolute number of inches drift over that unit distance.

The result of the above is shot dispersion tends to be shaped like a horn, flaring wider as the targets are further away. Litz has a simple formula to account for this in constant wind. Use a ballistic program to determine the time to the target. Divide that by the time for the bullet to go the first 100 yards. Multiply your group size at 100 yards by the resulting ratio to get expected group size at long range.

Example:

150 grain flat base with a BC of 0.350 and a 150 grain boattail version with a BC of 0.400. 15% is about the kind of difference a boattail can make if the nose is the same shape for both.

For a muzzle velocity of

Flat Base Times of Flight
100 yards 0.1126 s
1000 yards 1.9187 s

Ratio 17.0:1

Boattail Times of Flight
100 yards 0.1118 s
1000 yards 1.7822 s

Ratio 15.9 : 1

Thus, if both loads group 1.0" at 100 yards in steady wind or still air, you can expect them to group 17" and 15.9" at 1000 yards in still air, respectively. An advantage, but not a huge one. Once you introduce variable wind, though, it is proportional to the difference in the transit time above that in a vacuum. At 2800 fps, vacuum transit time would be .1071 s at 100 yards and 1.071 s at 1000 yards. The differences for the flat base are 0.055 s at 100 yards and 0.8473 s at 1000 yards. For the boattail they are .0047 s and 0.7108 for 1000 yards. So, at 100 yards the flat base is blown 0.0055/0.0047, or 17% more, and at 1000 yards it is blow 0.8473/0.7108, or 19% more. In a 10 MPH cross wind that works out to be only 0.13" of wind drift difference at 100 yards, but 24" of drift difference (149" vs. 125") at 1000 yards.

So, yes, it's mainly wind drift that gets the boattail the nod for long range.


Win_94,

If you push a bullet slowly through a bore with a ram, you are correct that the added bearing surface length adds friction. If a bullet needed, say, 600 lbs of force to push through a bore, then, yes the flat base version might add, say 150 lbs. However, that friction is small compared to the friction that results from the pressure difference between the bullet base and the nose and due to the base pressure acting against the bullet's inertia during firing. The pressure difference and mass inertia acting in reaction to the accelerating force swells the bullet radially outward enough to amounts to several thousand pounds of frictional reaction against forward motion, the amount varying with the pressure at the base at the moment. You can see this because, in a smooth. uniform bore (and even in many that aren't so perfect, the copper fouling builds up thickest in the first couple of inches beyond the throat, which is where the bullet was during the pressure peak. Friction there was also highest.

Though the simple friction of a few hundred pounds due to bullet elasticity pressing against the bore varies with surface area, the higher pressure-induced friction does not. In a longer bullet that swelling is spread out over the longer contact surface, so it is less per square inch of contact surface, and the total friction remains about the same. The other way to look at it is that pressure-induced friction is due to net reaction forces to the pressure so it is determined by the pressure alone.

So, as a percent of total friction in firing the greater length is a pretty small effect. The one thing you may get, though, with the pressure-induced friction being spread over a longer area in the flat base bullet, is less copper fouling per shot.
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Old January 5, 2011, 07:50 AM   #11
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Unclenick - Thanks, as always, for the continuing education.
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