Homemade BTHP .308?

[BulletFactory]

I just got into reloading for the rifle, and I was wondering if there was a way to make a copy of the 168 gr SMK BTHP that I have been shooting. I really dont want to cast for this gun.

[Reload3006]

yep I have dies to make them rebated boat tails though.

[Mountain Prepper]

Like 30-06 I also have a .308 RBBT set of dies - mine are 8.5s VLD/ULD with a very long sloped RBBT.

It will make bullets from 110gr to 250gr.

The RBBT is a superior design to the standard BT because of the inherent accuracy gained from smoother barrel exit.

[BulletFactory]

This is all new terrortory for me guys, pardon my ignorance. What equipment does a person need to do this? Are there videos readily available?

[Slimjim33]

I would be very interested in the above too.

[Mountain Prepper]

Youtube

http://youtu.be/SN_fYyl8pEw

[Mountain Prepper]

http://www.swage.com/ftp/rbt.pdf

Rebated Boattail Bullet Design

Based on the observable: "flat base outperforms boat tail at short range” The Rebated Boattail combines the best features of the flat base bullet design with the best features of the Boattail design.

The unusual phenomenon that cannot be added into the typical BC mathematics:

Kline–Fogleman airfoil or a stepped shape trapped vortex, a somewhat “lubricating” phenomenon that seems to be most pronounced in smaller sizes (RC airplanes and of course our subject - bullets).

Lapua had a rebated boat-tail D-46 bullet starting in the 1930’s The RBBT bullet is actually even older.

Several of the long range records have been fired using RBBT bullets (popular with the 50 cal guys).

You will of course see more on this as the RBBT riles up several of the internet math ninjas and traditionalist (and that faux-traditionalism is another subject). RBBT does not fit into the desired mathematical calculations for BC easily and so it gets falsely and with prejudice derided (because of ignorance and stubbornness). The development of the RBBT was not because of tooling limitations as stated by some internet ninjas but deliberate and after the "standard" boat-tail was already developed.

Often it comes down to “it (RBBT) is not that much better than “standard” BT bullets and therefore all the hype on RBBT bullets is just ********” this of course is convenient for a self-important internet ninja but it is not taking everything into consideration.

First we know that BT bullets will not preform as well as flat-based bullets at shorter ranges (think bench-rest at 100) - why?

The answer - bullet wobble induced by bore exit inconsistencies - this is very small but significant, enough to see BR bullets as predominantly flat-based.

Second, we know that there is not more than a 2% improvement in the flight of a RBBT over a “standard” BT at ranges past 1000 - I have seen listed that the RBBT drops just less than 2 inches less at 1000 meters. So is there reason to shoot a RBBT over a “standard” BT?

Possibly no, at least at longer distance, but I would dare say the elimination of wobble and the very slight increase at distance creates a bullet with wider usability. Does that actually make it “superior” again this is problematic because we are not taking into account the cost of manufacture - RBBT bullets are small lot runs and more expensive, ROI takes enough in number (per bullet) to question the cost of dies and personal time, or the expense of small lot run production costs (the custom bullet market).

A very low third - boat tail bullets increase throat erosion, not an important consideration unless you are using hot combinations in high volume (like the 220 swift or RUM/AI cartridges).

The one thing that will hold no water is the idea that somehow the general market and popularity of “standard” BT bullets somehow equates to “better” general consensus is alway mediocre not excellence.

Sometimes this gets at tedious as the steel v. carbide BS, at least with this side-path we have some real evidence to work with (sadly just not enough to make me happy).

[BulletFactory]

Seems odd that a bullet design that was more accurate at 1,000 meters would not also be more accurate at 100 meters.

???


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[Reload3006]

the benefits of a boat tail bullet aren't realized until the projectile slows from sonic to trans sonic flight. its that buffeting that a boat tail bullet bucks better than a flat base that is why the flat base is more accurate or equally as accurate at close ranges.

[Mountain Prepper]

Seems odd that a bullet design that was more accurate at 1,000 meters would not also be more accurate at 100 meters.

???

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Exit pressure can cause a wobble to develop in the rear of the bullet.

Flat base bullets exit with a clean sharp edge to the end of the rifling and cause a nice even “doughnut” shape of propellent gasses - at this point the gasses are still forcing the bullet forward and pushing.

The standard boat-tail has the chance of forming jets of gas that escape more from one side or the other - inconsistently - this inconsistently exiting gasses cause the wobble that cannot be compensated for due to the fact that the direction of exit is not the same every time.

The wobble causes larger elongated tears in the paper giving lower scores - that is why BR is dominated by flat-based bullets.

At longer ranges this wobble evens out and the shape of the boat-tail helps this by the shape.

Here are some photos that may help.

http://i1143.photobucket.com/albums/n636/MountainPrepper/Handloading/startbild2.jpg

http://i1143.photobucket.com/albums/n636/MountainPrepper/Handloading/bullet-rifle-22-1a.jpg

http://i1143.photobucket.com/albums/n636/MountainPrepper/Handloading/high_speed_photography_11.jpg

http://i1143.photobucket.com/albums/n636/MountainPrepper/Handloading/incredible_highspeed_photographs_640_02.jpg

http://i1143.photobucket.com/albums/n636/MountainPrepper/Handloading/H6300108-High_speed_photograph_of_bullet_leaving_gun-SPL.jpg

http://i1143.photobucket.com/albums/n636/MountainPrepper/Handloading/H6300104-High_speed_photograph_of_bullet_leaving_gun-SPL.jpg

[BulletFactory]

Awesome pics.
I guess what was throwing me, is how could a bullet be accurate at 1,000 yards, if the blast imparts a wobble from the launch. I understand how a BT would enter the transonic speed more smoothly, but it seems that it would be off aim by the time it got there.

It makes sense that a flat base would be accurate, but Im missing something here.

[BulletFactory]

Quote:


The wobble causes larger elongated tears in the paper giving lower scores - that is why BR is dominated by flat-based bullets.

At longer ranges this wobble evens out and the shape of the boat-tail helps this by the shape.


I think this is what I was missing here. Am I on the right track?

[MIBULLETS]

Awesome pics.
I guess what was throwing me, is how could a bullet be accurate at 1,000 yards, if the blast imparts a wobble from the launch. I understand how a BT would enter the transonic speed more smoothly, but it seems that it would be off aim by the time it got there.

It makes sense that a flat base would be accurate, but Im missing something here.

The twist given to the bullet eventually allows the bullet to re-stabilize or "go to sleep" as the bench resters say. Until that time, there can be be a slight wobble in the bullets flight. Flat base bullets do this much more quickly than a standard boat-tail, but the higher ballistic coeficient of the boat-tail bullets win out at long ranges. The rebated edge of the RBBT acts like the flat base to some degree and you get the benefit of a boat-tail for longer ranges.

[Slimjim33]

The twist given to the bullet eventually allows the bullet to re-stabilize or "go to sleep" as the bench resters say. Until that time, there can be be a slight wobble in the bullets flight. Flat base bullets do this much more quickly than a standard boat-tail, but the higher ballistic coeficient of the boat-tail bullets win out at long ranges. The rebated edge of the RBBT acts like the flat base to some degree and you get the benefit of a boat-tail for longer ranges.


Sounds to me like a great compromise almost the best of both worlds!

[BulletFactory]

What is a good weight for a 16" barrel, shooting 200 yards?

[Reload3006]

actually in terms of BC the rebated boat tail is about equal to the boat tail and is actually "Discounting Dave Corbin's snake oil sales approach" not enough different to make any note. It is more in terms of making more robust tooling than any shooting benefit. tools to make a boat tail bullet would have a knife edge and would be easily broken. Where as a Rebated Boat tail punch has a blunt edge and is not as susceptible to breakage.

[BulletFactory]

I talked to a tech at Sierra, and was told that for the 1-12 twist at those ranges I should be looking at a 135 to 150 grain bullet.

Trouble is, where do I get one at an affordable rate? The other problem Im having, is trying to find something that has a flat base.

Where do I find such a bullet, and if I can't, how would I just make my own without casting?

[Reload3006]

well buying jacketed bullets there are plenty available from Sierra Nosler Hornady Speer. to make them at least with my set you would have to buy the shortest J4 Jackets Berger sells and then cut them off. then just swage them
commercial there are lots of options in jacketed in that weight range
Sierra
2121
2120
2123
2190
2125
2130
2155
2156
Hornady
30312
30314
3033
3031
30302
Nosler
53152
53155