Bob's Boattails in Slower Twists

[rsterne]

There has been anecdotal evidence since I introduced the idea of my Rebated Boattail bullet designs that the minimum twist rate calculated by the Kolbe Twist Calculator was conservative…. ie shorter/faster than required for stability.... The first hard evidence of this has come to light by two different shooters successfully using my .308 cal 154 gr. BBT in a 14" twist.... Here is the Kolbe chart of twist rate vs. velocity for the 154 gr. BBT for a Stability Factor of 1.5 (recommended)....



As you can see, an 11" twist is optimum at 1000 fps (the velocity I usually use for airgun bullets)…. The Kolbe calculator allows you to input your twist rate, and generate a chart showing what the SF is at various velocities.... Here it is for the 154 gr. BBT in a 14" twist....



According to that chart, that bullet would be unstable at between 950 fps and Mach 1.... and the SF would be just over 1.1 (barely stable) at 800 fps.... Ricky B (sixshootertexan) has many targets at 100 yards showing good groups with this bullet (and nice round holes in the target) at a muzzle velocity of about 815 fps from a 14" twist TJ's barrel.... Another shooter has been using it in a Texan (also a 14" twist) and reporting good accuracy as well, this time at 935 fps.... One of two things appears to be happening here.... Either the bullet works OK when the stability is very marginal (SF just over 1.0)…. OR the Kolbe calculator is underestimating the stability because of the rebated boattail (it assumes a smooth, conventional boattail)…. Either way, the bullet is usable in a slower twist rate than the Kolbe calculator comes up with.... How much slower, we don't yet know.... We also don't know what would happen if they were pushed closer to Mach 1.... If anyone can test them at 1050-1100 fps, that would be very valuable data to have....

I have several other BBTs of exactly the same proportions as the .308 cal 154 gr. (which is 3.0 calibers in length).... Since the twist rate (in inches) is proportional to the caliber, they should also be usable in a slower twist rate than what Kolbe says is optimum.... The bullets in question, and the equivalent twist rate to a 14" with the .308 cal 154 gr. are:

7mm (.284 cal) 121 gr.... 13" twist instead of 10"
.257 cal 90 gr.... 12" twist instead of 9"
.224 cal 60 gr.... 10" twist instead of 8.5"

While I can't draw firm conclusions for BBTs of other proportions, it certainly points the way to being able to use them in slower twist rates than the Kolbe Twist Calculator predicts as well.... As always, data trumps calculations.... so if any of you have experience using BBTs in slower twist rates than we "thought" they should use.... please post in this thread....

Bob

[bluethunder]

Hi Bob, It's been awhile. I have been shooting your 30 cal. NOE 170 gr. rebated boattails for years. I tried a 1-14" twist TJ's and it seemed good up to about 135 gr bullets. I tried a 1-10" inch twist Shilen and it would shoot the NOE 145 gr rebated boattail pretty good but not the 170 gr bullet. So I rebarreled a 3rd time to a 1-7" twist Shilen. My logic was that I was basically shooting a subsonic 300 Blackout only 200 fps slower. All of the 300 Blackouts use either 1-7" twist or 1-8" inch twist. So back for round 3. I set new all time group records at 300 yds with the 170 gr. bullet. The 145 shot slightly worse. But I have to qualify this a little, the groups still showed some yaw on paper. The groups by the way,  were 3 consecutive 10 shot groups at 300 yds. They averaged 6.27" inches. I also remove the worst shot in each group and record the 9 out of 10 shot average and that was 5.09" inches.
      A few weeks ago I went out to Briar Rabbit Shooting Sports and they have a 540 yd range. So I did some 300, 400 and some 540 yd. shooting. Here's where it really gets interesting- my groups at 300 yds had their usual small amount of yaw but at 400 and 540 yds the holes in paper were perfectly round. So somewhere between 300 and 400 yds the bullets become perfectly stable. I had a 16" steel target at the 540 yd line and was having little trouble staying on it. The wind would shift a little and take me off of it and I'd make a wind correction and be right back on steel. My steel was hanging by a 2"X4" and one of my rounds went through the support board at 540 yds. You could see the exact angle that the bullet went through the board and it was noticeably angled downward. Not sure what all of this means, I'm no mathematician, but to me the 1 in 7" twist works just like a 300 Blackout should. By the way, the rifle is an Extreme Big Bore Hunter. It took 123 MOA dialed into my Cold Shot to reach 537 yards. This distance was verified via laser. Ballistic Coefficient is about .400!!

[rsterne]

Interesting results for sure.... I'm a bit surprised that the 10" didn't work for the 170 gr. as that was predicted by the Kolbe calculator, and on the shorter bullets a somewhat slower twist than predicted by Kolbe seems to work.... I guess NOE's predicted BC of 0.393 is not that far off, that is welcome news....

Bob

[dyotat100]

I'm trying to figure out how you come up with a BC of .4?

With my .338 shooting 165 gr and BC of .15 it takes 116 moa to go 540 yds. I have shot clay pigeons at 500 m(546 y).

[rsterne]

Drop is mostly a function of velocity, the BC plays only a minor role.... Neither post states the MV, or what the basic zero was that the MOA drop was measured from.... The 175 gr. BBT in .308 cal has about a 30% higher SD than a lighter bullet in .338 cal, so even with a flat base should have about that much higher BC.... Presumably the boattail should add something, but a BC of 0.4 does seem rather high.... I'm actually shocked that a 165 gr. in .338 cal has such a low BC for an SD of 0.206.... I would have expected a BC of around 0.2....

Bob

[dyotat100]

I'm shooting at 965 fps. 1-16 twist with gas check base.

The BC number is what worked in streloc so everything  lines up when using the chart.

[rsterne]

 

Bob