Boat tail length

[DAVE A]

Hi all

I've been swaging my own >308s for a few years now with some success but I have been limited to flat base or rebated boat tailed bullets

Today I finally got round to making a punch that can make a true boat tail, I was trying to get something similar to the Scenar profile and although I got the angle right the length is about 1mm longer than the Scenar boat tail.

Overall I am quite pleased with the results and I am still going to shoot them but I was wondering if anyone can tell what the ideal angle and length of boat tail should be ,my bullets are 1.2" long with a 6 ogive and weigh 175.5 grains ,

Thanks for any help

Dave.

 

[marty]

I can't help with the angles but have to ask are you using a hydraulic press and where do you get the jackets ? Years ago I had a Corbin die set for .357 bullets and getting jackets was always a problem.

[Cheetor]

Dave,

If you haven't seen this before, it may be useful?

 

www.geoffrey-kolbe.com/drag.htm

 

Maybe by altering the angle and length of the boat tail you may be able to arrive at the most efficient boat tail for your bullets, may be worth a look?

 

Good luck

 

Alan

[Laurie]

The ideal angle is 7-deg plus another 1 or 2. You don't want to get up into the 11, 12-deg or steeper angles as not only do they not work, they actually potentially increase drag and can induce turbulence leading to instability under some conditions and speeds. (Why the 168 and 180gn Sierra MKs are poor long-range bullets.)

 

In terms of boat tail length, the variations between successful models are enormous. The emphasis in bullet design these days is heavily skewed to the nose section with ever-longer lower radius designs (up to 17R) as the nose section generates the majority of drag at supersonic speeds so optimising its shape pays dividends in improving the 'form factor' (drag generation as a value in relation to the 'reference projectile') and hence the BC. To provide an adequate central bearing section and retain overall dynamic balance, the tail often has to be made relatively short as can be seen in the superb .30 155.5gn Berger Target BT Fullbore and similar lighter 30s from Berger with tail sections ~0.140 - 0.145". On the longer overall heavier models, another 40 thou' can be added, amd on the latest heavy Hybrids of 200-230gn they increase to 0.200-0.220".

 

A very much older L-R design is that of the 190 / 200 / 220 / 240gn Sierra MKs with relatively short blunt front ends with nose radius values ~7 calibres, and 9-deg boat-tails all longer than 0.2". These bullets have relatively high drag / form factors mostly well above 1.000 (unity with the reference bullet when the G7 model is used) compared to recent Bergers in the mid to high 0.9s so their BCs are relatively poor for bullets of their weights, BUT they do have a superb reputation for transonic stability and likewise being relatively unpertubed by passing through the sound barrier. Until the modern long-nose / short tail 210 SMK and the Berger Hybids came along, these bullets were much favoured by 'Match Rifle' shooters for whom transonic speeds are a fact of life. These older bullets seem to have acquired their very long tail sections from the American military 173gn FMJBT bullet which in turn borrowed heavily from the old Swiss 173 used in the 7.5X55mm GP11 cartridge and which predated WW1.

 

Looking at your bullets, the 6R front end means the form factor will be a bit poorer than that of the 175gn Sierra MK with its 7R nose. The central parallel section looks rather long which (if this is the case) increases in-barrel friction and chamber pressure limiting MVs all other things being equal (diameter, jacket material and thickness, core allow mix and hardness). The trend in recent designs is to have as short a bearing area as you can get away with, so the equivalent Berger 175s are 0.324" for the LR BT, 0.328" for the OTM Tactical and the much older VLD is odd man out at 0.421", a hangover from its design age.

 

Still they look good .... and if they work well, that's all that matters.

[DAVE A]

Thanks guys some very useful information there,

Marty I use a Corbin S press its manual and can be converted to a reloading press , I had the same problem as you getting jackets so now I make my own,I use a hydraulic press for that

I will try to post a link to a video of me making the first stage of a jacket .

https://youtu.be/pfV0WZxT94Y

Dave.

[andybrock]

Thanks guys some very useful information there,

Marty I use a Corbin S press its manual and can be converted to a reloading press , I had the same problem as you getting jackets so now I make my own,I use a hydraulic press for that

I will try to post a link to a video of me making the first stage of a jacket .

Dave.

That's interesting thanks for posting. Have you got a video of how you make your bullets once you've produced your jackets?

[DAVE A]

That's interesting thanks for posting. Have you got a video of how you make your bullets once you've produced your jackets?

Not yet Andy but maybe soon.

[gunner]

This would be really good to see !