[unclenick]

E.O.,

As Darkker commented and Bryan Litz has also commented, it is simply easier to make a perfectly symmetrical flat base bullet. Two separate dies form the base and the ogive and very small imperfections in alignment measurably degrade accuracy. The reason is illustrated below. I've actually seen some cheap foreign boattails that resemble a less dramatic, but clearly visible misalignment like the bottom one has. As it exits a muzzle, the gas stream will start pushing that side of the bullet off the bore axis, introducing one off-trajectory drift component. It will also have an off-axis center of gravity that will introduce another. Which direction it flies off the nominal trajectory will depend on which side of the bore the defect was located when it cleared the muzzle.

Even if a boattail is perfectly symmetrical, the base takes longer to clear the muzzle blast gas stream, exaggerating the influence of any crown imperfection you may have. It's not uncommon to recrown a rifle and improve boattail group size by 15%, where a flat base is less affected by it.

The main advantage boattails have is the higher ballistic coefficient means that, fired at the same velocity, they take less time to arrive at a long range target, giving any drift component less time to to move the bullet off the point of aim. So, at long range you can have a situation where the lower of-trajectory drift velocities of a flat base bullet at firing can start to catch up to and eventually surpase those of the boattail in terms of actual displacent of point of impact from point of aim.

 

[unclenick]

A simplistic way of looking at the boattail as a concept is the roughly 15% higher BC you get from having it means, for a given muzzle velocity, the bullet will arrive on the target sooner, giving various influences on group size less time to act. But, you have to have the same initial accuracy (short range accuracy) for the reduced drag to be a proportional advantage. I, personally, wouldn't hunt a four-legged beast at ranges or fire in wind conditions for which that difference matters, so I would just shoot whatever grouped best at modest ranges.

I am reminded of an article some years back (I've forgotten where it was) by a couple of guys who'd co-invested in one of Dave Manson's barrel crowning tools sets. They proceeded to recrown muzzles for about half their shooting club and found (IIRC) that it improved accuracy in half of them and had no effect on the rest. If that's typical, you have a 50:50 chance of improving a rifle by recrowning it. I have to wonder how many rifles might recrowning take from being guns that don't like boattails to being one that does?

 

[unclenick]

Darkker,

The numbers I get are:

80°F, 35% RH, 900 ft altitude (Barometric pressure 28.962 inHg) firing 1° off true North at Camp Perry Latitude and Longitude.

90 grain SMK, 2500 fps
G7 BC 0.257
10 mph from right (90°) wind correction 3.20 moa / from left (270°) 4.17 moa (the difference is due to spin drift for 8" twist barrel).
13.21 clicks up from 100 yards
TOF 0.7144 s

Nosler 40 Grain BT, 2500 fps
G7 BC 0.109
10 mph from right wind (90°) correction 11.46 moa / from left (270°) 12.59 moa (the difference is due to spin drift for 8" twist barrel).
21.78 clicks up from 100 yards
TOF 0.9577 s

So it's really quite a difference. This is the reason our soldiers in Afghanistan could not return effective fire on snipers 600 yards away with the M16. Put a little gusting into that wind and you're missing left and right with the light bullets.

I think maybe you used Nosler's G1 BC and forgot to change the drag function from G7 to G1 for your calcualtion. That would give you those close results, but, of course, they'd be off by about 2:1 with the results of drag effects.

 

[unclenick]

If you have Litz's big book of actual measured BC's, it shows Nosler's published G1 numbers are about right at around 3,000 fps. So I used the drag function converter in QuickTARGET Unlimited to convert their published G1 BC to a G7 BC at 3000 fps, then just assumed that the G7 BC from 3000 fps would not change enough over the total velocity range to need significant adjustment. Litz doesn't have the 40 grian BT in his database, but if you look at the 55 and 60 grain BT's, which he does have, the G7 BC drops only about 1% at 2500 fps and 2% at the terminal velocity. So I could have used .107 and been a little closer, maybe. I always keep in mind that Litz said he's found up to 3% bullet-to-bullet BC variation all in one box of match bullets, so I figure you are getting into the noise at that point.


RJ,

For varmint hunting you may be right. But when you're pinned down behind rocks and having to wait for air support to bail you out because you can't return effective fire on one guy presenting a clear profile only half a klick away, you just may start to wonder why nobody at the top reviewed all that BC stuff before they put you into that situation.