Boat Tail Gas Checks

[Zombie Whisperer]

I have been working with the NOE 311 247 BLK/WISP boolit. Since its designed for subsonic it drops like a rock, and range estimation needs to be almost exact to hit something. Because of this I was thinking that a boat tail would help. What do you guys think of casting a boat tail gas check out of zinc that crimps onto the base of the boolit? I know that there have been boolits that are cast in multiple parts then assembled, and I know of zinc gas checks, but have never heard of a combination of the two. Let me know what you think.

[Ben]

I don't know how far you're planning on shooting your bullets, but I'd be interested in seeing " a side by side " comparison test of their drop. The BT vs. plain based.

I'd think it might be a very small difference when the two different style were actually shot at long distances. It is my opinion that since both are subsonic , that both are going to drop like a rock.

[Zombie Whisperer]

Ben,
The difference between a 240 GRN SMK (sierra match king)(.658 BC) and the cast 247grn (.324 BC) at 1050fps is about 4 inches at 100 YDS, and its much greater at 300 YDS. When a projectile is traveling slower than the speed of sound the base of the boolit creates more drag then nose, this is backward for supersonic flight. I should have added that I am most worried about not seating them perfectly square and throwing off the boolit, because if they are inaccurate then it don't matter how far they fly.

[Ben]

UUmmm, shows what I know about projectiles flying subsonic, I guess................

Getting a boat - tailed cast bullet to take a gas check
and it be put on squarely , I have no earthly idea ? ?

[Hardcast416taylor]

If I remember the statement concerning when a BT shaped bullet starts being worthwhile, it is when both a BT and a flat based bullet (jacketed in this case) being fired at the same velocity and same componets will fly nearly identically out to about 200 yds. It is only once past that distance that the BT design begins to be worthwhile using at farther distances. Since my long distance shooting days are long past, I restrict my shooting to flat based bullets.Robert

[onondaga]

You could do a practical comparison by entering the BC of the boolit with and with out the boat tail. You could even guess at the optimized BC for your idea and get a reasonable comparison without firing a shot by running the free to use online software at:

http://www.handloads.com/calc/index.html


Gary

[Zombie Whisperer]

Hardcast416taylor,
I got ya, but what I am doing is kind of a specialty thing, and a boat tail is perfered in this application. It would also help if someone wanted to push the range on a cat boolit further.

Gary,
That is a good calculator. I have been using the Hornaday one, but I like that one better.


NOE 311 247 .324 BC @1050FPS

Range Velocity Impact Drop ToF Energy Drift
0 1050 -0.5 0 0 612 0
10 1042 1.04 0.22 0.03 603 0.45
20 1035 2.27 0.76 0.06 595 0.5
30 1028 3.17 1.62 0.09 587 0.59
40 1021 3.75 2.81 0.12 579 0.71
50 1015 3.98 4.34 0.15 572 0.86
60 1008 3.88 6.21 0.18 564 1.05
70 1002 3.43 8.42 0.21 557 1.26
80 996 2.64 10.98 0.24 551 1.51
90 990 1.49 13.89 0.27 544 1.79
100 985 0 17.15 0.3 539 2.09
110 979 -1.86 20.77 0.33 532 2.43
120 974 -4.08 24.76 0.36 527 2.8
130 969 -6.67 29.11 0.39 521 3.19
140 964 -9.62 33.83 0.42 516 3.61
150 959 -12.96 38.93 0.45 511 4.06
160 954 -16.66 44.4 0.48 505 4.54
170 949 -20.76 50.26 0.52 500 5.04
180 945 -25.23 56.5 0.55 496 5.57
190 940 -30.1 63.13 0.58 491 6.12
200 936 -35.35 70.15 0.61 486 6.7
210 931 -41.01 77.57 0.64 481 7.31
220 927 -47.05 85.38 0.68 477 7.94
230 923 -53.51 93.6 0.71 473 8.6
240 919 -60.36 102.22 0.74 469 9.28
250 914 -67.64 111.26 0.77 464 9.99
260 910 -75.32 120.71 0.81 460 10.72
270 906 -83.42 130.57 0.84 456 11.48
280 902 -91.93 140.85 0.87 452 12.27
290 898 -100.88 151.56 0.91 448 13.07
300 895 -110.25 162.7 0.94 445 13.9

240 SMK .585BC @ 1050fps
Range Velocity Impact Drop ToF Energy Drift
0 1050 -0.5 0 0 612 0
10 1046 1.01 0.22 0.03 607 0.44
20 1042 2.21 0.75 0.06 603 0.46
30 1038 3.08 1.61 0.09 598 0.51
40 1034 3.63 2.79 0.12 594 0.58
50 1031 3.85 4.3 0.15 590 0.66
60 1027 3.75 6.13 0.18 586 0.76
70 1024 3.31 8.3 0.21 582 0.87
80 1020 2.54 10.8 0.23 578 1.01
90 1017 1.44 13.63 0.26 574 1.16
100 1013 0 16.8 0.29 570 1.32
110 1010 -1.78 20.31 0.32 566 1.51
120 1007 -3.9 24.16 0.35 563 1.71
130 1004 -6.36 28.35 0.38 560 1.92
140 1001 -9.17 32.89 0.41 556 2.15
150 998 -12.33 37.78 0.44 553 2.4
160 995 -15.83 43.01 0.47 550 2.66
170 992 -19.69 48.6 0.5 546 2.94
180 989 -23.9 54.54 0.53 543 3.24
190 986 -28.46 60.83 0.56 540 3.55
200 983 -33.38 67.48 0.59 536 3.87
210 980 -38.66 74.49 0.62 533 4.21
220 977 -44.3 81.86 0.66 530 4.57
230 974 -50.3 89.59 0.69 527 4.94
240 972 -56.67 97.69 0.72 524 5.32
250 969 -63.4 106.15 0.75 521 5.72
260 967 -70.5 114.98 0.78 519 6.13
270 964 -77.97 124.18 0.81 516 6.56
280 961 -85.82 133.76 0.84 513 7
290 959 -94.04 143.71 0.87 511 7.45
300 956 -102.63 154.03 0.9 507 7.92

The improvment seems worth it to me, mainly in retained energy. I just don't want to buy expensive J words.....any ideas on the design?

[onondaga]

That Calculator is fun. There is a way to retain the columns when copying but it is for the wizards. Once and a while I get it to keep the columns in line but can never remember how I did it!!!!! I do something when I drop it into Microsoft Word and it comes out framed with columns intact.

If you figure that out, I congratulate you!

Gary

[HollowPoint]

Hey Zombie Whisperer:

I posted a similar question a while back and got similar answers.

My reason for posting the questions was to see if anyone had made an effort to actually make Boat-Tailed Gas-Checks. And if so, what results did they get with them.

The answers I got were not first-hand or hands-on answers, they were just answers similar to the ones here. Not saying they were good, bad or anything else. If anything they were "Common-Sense" answers based on computations and extrapolation.

My reason for asking, at the time, was because I had come up with an idea on how to make a set of Boat-Tail Gas-Check dies. In fact I made up some CAD drawings and it looks like the dies themselves will work but, I have yet to attempt to machine them into existence.

The design I had in mind are for dies that make Rebated Boat-Tailed Gas-Checks as apposed to Boat-Tailed Gas-Check per se.

Right now that project has about seven other projects ahead of it. The main reason for this is that I'll also have to make the Cherry to make the Bullet Mold that will accomodate these Specialized Gas-Checks. That will be like killing three birds with one project.

The bend-angles needed to form a Rebated-Boat-Tailed Gas-Check calls for a three-stage die. The first stage holds the outer edges of your unformed Gas-Check disk so that it doesn't crimp inward like a pop-bottle cap.

It has just enough pressure to keep the disk from crimping but not enough to hold it fast. The second stage of the die stretches and starts to form the bevel of the Gas-Check just as the edges of the disk slip out from under the "First Stage" of the die.

The last stage of the forming die does basically what a conventional Gas-Check die does. It forms the disk into its final Rebated Boat-Tail shape.

Describing it in writing makes it sound more complicated than it really is. Including the forming cup, different sized cylindrical stages, the springs, and the Bullet Sizing push rod, it's a seven part die setup.

In theory, it should work easy enough. My theory is based on the drawings I've made. As of right now, it's all the first-hand experience I have to go on. Wether these Rebated-Boat-Tail Gas-Checks will improve the exterior ballistics of your bullets is yet to be seen.

Unless you try, the best we can do is run the numbers and extrapolate. In many cases that's enough to get the answers we're looking for. For the bullet casters that have the know how to push their cast bullets above subsonic speeds, Boat-Tailed Gas-Checks may very well be worth looking into.

If I get a chance, I'll post the drawings. It may take a while cause I'm in the process of converting over to new CAD software. The software I initially made my drawings with shows only static drawings. This new software allows me to post animations showing how the parts actually work.

HollowPoint

[badbob454]

I have been working with the NOE 311 247 BLK/WISP boolit. Since its designed for subsonic it drops like a rock, and range estimation needs to be almost exact to hit something. Because of this I was thinking that a boat tail would help. What do you guys think of casting a boat tail gas check out of zinc that crimps onto the base of the boolit? I know that there have been boolits that are cast in multiple parts then assembled, and I know of zinc gas checks, but have never heard of a combination of the two. Let me know what you think.

what about a bevel base cutter it would give that boat tail base you want or try a pencil sharpener? if it works then worry about a tapered gas check or perhaps a half jacket ?, jest throwing out ideas ,

[Zombie Whisperer]

Hollow point,
Thanks for the info. My thoughts were to cast a zinc base that crimped onto a standard boolit's gas check shank. I too will post some pics when I can get some time to draw them up. I am looking forward to your drawings, it is a lot easier to understand an idea from a picture.

[flounderman]

I have a question. why woud you use a gas check on a boat tail. it would never touch the barrel unless you are going the half jacket route. subsonic, you don't need a gas check.

[HollowPoint]

Hi flouderman:

I'm not clear on what you're asking.

A Boat-Tailed Gas-Check would touch the inside of the barrel the same way a conventional Gas-Check touches the inside of the barrel to form a gas seal. Only the tapered end of the Boat-Tail wouldn't touch the barrel.

The reason I decided on a Rebated Boat-Tailed Gas-Check was because I feared that the angle of the Boat-Tail might channel the hot gasses toward the outside edges of the bullet base and possibly increase the chance of gas-cutting.

The "Rebated" part of the Rebated Boat-Tailed Gas-Check forms a perpendicular edge at the base of the bullet similar to that of a conventional Gas-Check.

If the shape of a Boat-Tailed Gas-Check does in fact channel those hot gasses to the outer edge of the bullet, in theory, the Rebated section of the Rebated Boat-Tailed shape will block the gasses from working their way around the sides of the bullets to cause gas-cutting.

HollowPoint

[longbow]

No personal experience here but some info that might be of interest:

http://www.bpcr.net/site_docs-results_schedules/documents/Dan_Theodore_Boattails_for_BPCR.htm

http://castboolits.gunloads.com/showthread.php?t=75756

http://www.public.iastate.edu/~jessie/PPB/PPB_files/Page1187.htm

http://members.shaw.ca/cronhelm/762ProjectRifle.html

One bit of first hand info I can add is that I have loaded GC boolits without GC over COW filler and recovered boolits showed a modified GC shank looking more like a boattail than a GC shank. So, you may get distortion at the joint between any add on boattail "gas check" and boolit unless fit is perfect. Distortion will ruin accuracy unless it is identical every time.

It would be interesting to see your results if you pursue this but I think it would be easier to PP a boattail boolit than to make a zinc add on boattail.

Also, based on the last link to the Thumper project I have to wonder if a boolit like say the 311365 ( http://castboolits.gunloads.com/showthread.php?t=108231) loaded backwards might be more effective at reducing drag.

FWIW

Longbow

[357Mag]

Zombie -

Howdy !

If sub-sonic is the way it's gonna' be, why not put the bullet " improvement(s) " at the OTHER end ?

Example:
I've been looking into potential use of a .357" cal lead SPBT w/ the .35 Rem case.
In this instance, the whole bullet is lead, included the boat tail.

The bullet manufacturer has added a plactic tip to the bullet ( which is basically then, a "capped hollow point".

Pointy bullets have less drop than more-blunt bullets styles, when comparing same cal bullet,
same bullet weight, same MV; et al.

Such a bullet would not have to be supersonic, to realize some of the aerodynamic bennies
provided by the boat tail and the pointy nose.

The .357" cal bulleet mentioned, comes in 3 different weights, and is supplied w/ a sabot for an intended muzzle loader application.

I DK if something like that is workable in .311 cal >

Regards,
357Mag

[Zombie Whisperer]

357mag,
At subsonic velocities the base creates way more drag then the nose, this is why I want to create a boat tail. The reason a gas check is being used in this application is because it is used in gas operated firearm, and the gas check reduces the lead in the gas system, and the boat tail needs to be hard enough to not distort when fired.

[Zombie Whisperer]

longbow,
Just when I thought that I had read everything on the internet pertaining to subsonic ammo, someone throws me a few more links. Thanks.

[357Mag]

Zohmbie -

Howdy !

I once had the pleasure of getting to talk with Walt Berger @ his factory in AZ.
I wanted in-particular to ask him about WHEN ( how far out ) a boat tail actually begins to make sense to use.

Uncle Walt left me with the notion that aound 450 yd or so, the boat tails are comin' on strong . Perhaps, his comments were couched in my querries about .224" cal and 6mm cal bullets, and those..... at super-sonic speeds.

For an example (perhaps not the best ) of what a pointy FB can do vs a similar-weight & cal boat tail ; a review of the J-word Berger 6mm 88gr FB w/ a #15 OGIVE stats might prove interesting. These aquit themselves pretty well, for not having a boat tail; and can fill a "niche slot " when applied prudently.

With regards,
357Mag

[greywuuf]

357MAg, I am not sure if supersonic results are similar enough to subsonic to draw any conclusions from, From what I have read, studies have shown that the BASE drag on subsonic projectiles is dominant. meaning it contributes to more slowing than the nose shape. When one starts subsonic, you allready have horrible drop because it takes so long for the bullet to get there, anything that can be done to decrease that drop is worthwhile in the subsonic world. The subsonic shooters of Castboolits , have developed a maximized bullet for a particular cartridge, and are very interested in shooting it subsonic, So the OP is exploring options to put a more efficient base the an existing projectile.

[Zombie Whisperer]

Okay, I finally had the time to draw it up...I know its crudely drawn, I was in a hurry, but I think that you can get the idea. If it was cast from zinc I think that it wouldn't deform, and could be added to any existing boolit without the need for a specialty mold.

http://castboolits.gunloads.com/imagehosting/189804e7e8a1b3904b.jpg

[Zombie Whisperer]

357MAG,
I would love to have that tour! Sounds like Walt was talking about supersonic (were most shooting is done). Compare the shape of a 747 to a Concord, where the 747 is the subsonic projectile, and the Concord is designed for supersonic flight. The link below gives a good over view of subsonic Projectiles.
http://www.corbins.com/subsonic.htm

Greywuuf,
Roger that.

Hollowpoint,
The reason that I was thinking cast zinc is because I would be using it for subsonic loads (which are low pressure), and it would be one operation, instead of multiple forming operations.....makes me wish that I was still working at a die shop.

[Zombie Whisperer]

Here are some rough drawings of the proposed mold.
http://castboolits.gunloads.com/imagehosting/189804e7f37d557fc8.jpg
http://castboolits.gunloads.com/imagehosting/189804e7f37d5b2503.jpg

[longbow]

Before going to too much trouble and/or expense on getting a multicavity mould made (unless you can make it yourself) I think you should test the castability of zinc... or at least readily available zinc alloys.

Die cast zinc parts can be very intricate but die casting is done under pressure and you are planning to pour into a mould like in boolit casting. I am not sure that you will get the fillout to produce sharp edges and more importantly the thin band required to crimp onto a gas check shank.

Longbow

[Zombie Whisperer]

longbow,
Good point on the cast-ability of zinc. I was going to use zinc carburetor bodies as the alloy. Another option, that would be expensive, would be to turn them on a lathe. Turning them was how I was planning on making a prototype.

[turbo1889]

Is this what you guys are suggesting?

http://farm7.static.flickr.com/6173/6181540460_258b259bbd.jpg (http://www.flickr.com/photos/54455625@N04/6181540460/)

If so I think you could make a handful of prototypes from brass stock on a lathe to see if they worked as advertised before proceeding further.

[longbow]

Now there's a good idea!

[Zombie Whisperer]

Turbo 1889
Exactly what I was thinking. Now all I need is a lathe.

[HollowPoint]

That's an excellent idea:

It would certainly cut down on the labor involved in milling the die setup I had in mind.

From the looks of the renderings that zombie posted, and those of the gas checks that turbo posted, it looks like such a mold would almost require it have some sort of integral plunger system in order to eject you newly cast gas checks. As it looks in the rendering, trying to pry off the freshly made checks would most certainly ruin the finish.

Have you considered any of the "Pros and Con" of such a design? With the design I had in mind one of the Pros are the availability of free aluminum cans to make them out of. As I stated already, one of the Cons is the complexity of putting the dies together.

I'm still going to pursue my design after I've completed all the projects I have ahead of it but, I really like the simplicity of Zombie's design. I think it will work.

Now I'm kind of mift that I threw out all the zinc wheel weights I had mixed in with my lead weights.

HollowPoint