.72 calibre question

[.72man]

does anyone know of a formula to estimate the length of a lead bullet by using the weight of the bullet and the diameter? I have to have the length to have a barrel made with the proper twist and to have a mould made, the maker requires a dimentional drawing. caliber (diameter) is .729 and the weight is 875 grains (2 oz) and would be shaped like a thompson center maxi-ball.
anyone know of any good barrel or mould makers working in .729 calibre?

 

[arcticap]

The only online company program that I know of for designing your own bullet mold and which provides the estimated weight has a maximum diameter of
.640 inches.

http://mountainmolds.com/

 

[robhof]

Lee used to make custom molds and I don't know his size limit but Veral Smith of LBT makes custom molds also. I have a few of Veral's molds and they are exceptional.

 

[Busyhands94]

track of the wolf sells ball molds, i have one for my .50 rifle. it works well, although i needed to tweak mine a little to get it to work well. now it casts balls that are just fine. you can also leave the sprue on if i ever need a fishing weight. here's a link: http://www.trackofthewolf.com/categ...18&subId=126&styleId=408&partNum=MOLD-BAG-710

 

[Cosmoline]

anyone know of any good barrel or mould makers working in .729 calibre?

For roundball Track and others have plenty. For slugs and custom elongated bullets, it's going to be tough. But the Oregon Barrel Company makes some big honking barrels and I'm sure they could do that end of it. They sell through these fine folks in Springfield:

http://www.thegunworks.com/

They may be able to suggest who can make a custom mold for you.

 

[Loyalist Dave]

Too bad it has to be a conical type bullet, for Lee and Lyman make roundball molds, as Cosmoline mentioned, in .690 and in .735 (could you go to .75 caliber?)

Lyman .690

Lee also makes a 12 gauge slug mold that is made to work in rifled shotgun barrels, and your .72 should be right there. It's only 438 grains though.

Lee 1 oz. slug mold

Lyman makes a Foster slug mold for 12 gauge, .705, which unfortunately is not made to work in rifled barrels.

Lyman Foster Slug mold

I hope this helps. Do you absolutely need a 2 oz. projectile, or could you use a lighter one made of an alloy to get penetration, if that's your concern?

LD

 

[Plastikosmd]

my Bresien .69 caliber conical shoots a slug that is about twice that 4oz (1700 grains). not sure of the twist of that barrel, will check for u.

 

[Busyhands94]

do you think a deer slug would work? Lee sells molds for those, i think they have hollow based ones.

 

[whosyrdaddy]

If you're thinking of the 2 groove maxi-ball, it won't work for you. Scaling up a maxi for a .729 bore would put you over your 875gr criteria by +/- 180gr.

If you're talking about the single groove maxi like the 320gr .50 cal, then your overall length would be +/- 1.048 inches long at 875gr.

As far as twist rate goes, you would need to determine an acceptable range for muzzle velocity, temperature, barometric pressure, relative humidity, etc.

Some quick examples using the Miller Stability Formula show that for for temps between 0 and 100 deg. F, and pressure between 28.44 and 29.44 a 1:60 twist should stabilize this projectile with a +/- MV of 725 fps to 1300 fps.

A MV range of 880 - 1600 fps would require a 1:62 twist and a MV range of 1065 - 1930 fps would require a twist of 1:64.

I hope this helps

 

[.72man]

your info

the information you supplied was exactly what I was looking for. I thank you very much for taking the time and effort to reply. thanks again.
steve

 

[.72man]

I tried them and found the same thing, but thanks for taking the time to reply it is much appreciated.
steve

 

[.72man]

wasn't aware of either but will give them a try. thank you much for taking the time to reply.
steve

 

[.72man]

I have their catalog but they don't have much for the 72 cal. thanks much for taking the time to reply it is much appreciated.
steve

 

[.72man]

will give them a try. thank you much for taking the time to reply. it is much appreciated.
steve

 

[.72man]

thanks for taking the time to reply it is much appreciated. I don't necessarily need a 2 0z conical it can be lighter. I came to that by trying to figure the correct sectional desity between the diameter and the length that would allow good stability. the shotgun slugs are so short that they would begin to tumble I think and actually weigh less than a round ball due to their hollow bases. I do intend to try a shotgun slug as I am getting a mold from lee to use for my 12 ga. my figures may be totally off, as i am relying on what i was told by someone. whozyrdaddy sent me some very specific calculations and information so will try what he sent and see what that give me. as he sounds quite knowledgeable. thanks again for your help.
steve

 

[.72man]

thank you much. that would be greatly appreciated. I've never engineered or built a gun before from scratch. have built kits, but this will be my first attempt to build a custom rifle of my own. information just does not seem to be out there. thanks again
steve

 

[.72man]

i'm going to try the lee slug when the rifle is built, but i hhave to figure out the correct length of bullet to give the info to the barrel maker so he can impart the correct twist. a 1 oz slug is actually only 437 grains which is almost 100 grains less than the round ball. i am looking for a bullet of sufficient leength to give good stability to the projectile. my figures, which are probably wrong, indicate a 2 oz or more. and the information sent by whosyrdaddy tells me I was incorrect, and he seems to know what I need. thank you much for your time and effort of your reply it is much appreciated, beleive me! I can handle all the advice i can get.
steve

 

[.72man]

would that 1.048" maxi-hunter type provide enough length for good stability? out to 150 yards or would I need no go with more weight to make it longer?

 

[Cosmoline]

Doesn't your stability depend on the relationship of bullet length with twist rate?

 

[whosyrdaddy]

Doesn't your stability depend on the relationship of bullet length with twist rate?

More accurately, any given bullet's stability depends upon its velocity and rate of spin in a given set of atmospheric conditions. This spin rate (rpm or rps) is determined by the twist rate and muzzle velocity.

A bullet's spin rate decreases much less than its velocity during its flight. A bullet fired from a 1:24 twist would need twice the velocity to achieve the same rate of spin as the same bullet fired from a 1:12 twist.

would that 1.048" maxi-hunter type provide enough length for good stability? out to 150 yards or would I need no go with more weight to make it longer?

Yes, the Miller formula applies to the entire flight of the bullet. In your case that would be a little over 3000 yards at MV of 1050 fps. Bear in mind that keeping your velocity subsonic will eliminate the unpredictability that the transonic range has on a projectile.

 

[.72man]

yes and atmospheric conditions and even elevation. I will have to average all of those out. I am not trying to bikd a tack driver, just a fairly accurate hunting rifle. I live in the desert but most of my hunting will be done above 4000 ft. I don't know if you can see the responses I get, but if you can there is a reply from whosyrdaddy that pretty much covers your question. he has been real helpful with the info I need.

 

[.72man]

once again you come through for me. thank you much. do you know where I might find that miller formula? I have looked online but hav had no success. too many millers around I guess. I would like to get between 1300 and 1500 fps, but that may be impractical for the bullet weight. I just want a bullet long enough for that calibre to give it good sectional density and good stability. i was told by the barrel maker that I had to have the bullet first before he could figure the twist needed.

 

[arcticap]

Any increase in weight over the modern 1 oz. foster slugs may adversely affect the velocity and flatness of the trajectory of any heavier .72 BP conicals that could be custom designed. The efficiency of the round ball design may be why some of the 12 bore guns seem to be designed for shooting large patched round balls.
Sizing them exactly may also be required for optimum performance. Some folks have found that sized & paper patched conicals work the best. See post #2 in the following thread for some of the problems associated with casting, loading & accurately shooting cast conicals:

http://www.thehighroad.org/showthread.php?t=623237

Below are some velocity figures for the 1 oz. 12 gauge foster type slugs from the various length modern shotgun shells:

Conventional Foster type rifled slugs generally weigh 1 ounce in 12 gauge....The 12 gauge slug has an advertised muzzle velocity (MV) of 1560 fps from a 2 3/4" high-brass shell, 1680 fps from a 2 3/4" Magnum shell, or 1760 fps from a 3" Magnum shell. These are Remington figures from their 2004 catalog. The MV's of the other gauges are similar.

The catalog energy figures for the common high-brass ("maximum") 12 gauge slug load are an impressive 2361 ft. lbs. at the muzzle, but only 926 ft. lbs. at 100 yards. This is due to the very poor BC of the slug. Sighted to hit dead on at 50 yards, that slug is 4.8" low at 100 yards. The more powerful 12 gauge slugs are only marginally better, and kick noticeably harder. No matter what, rifled slugs remain a short range proposition.

http://www.chuckhawks.com/shotgun_slugs.htm

Below is a chart that reflects the range and performance limitations of conventional 1 oz. slugs fired from 20 inch barrels, but loaded with smokeless powder.

Most people think that the trajectory of the 12 gauge rifled slug is close to that of a mortar, and since they don't think they could hit anything past 25 or 50 yards (which is probably true if they don't have a set of sights on their shotgun) they zero for slugs at 25 yards. Unfortunately, this short zero severely limits the effectiveness of the slug firing shotgun. Surprisingly, a slug's trajectory is quite flat out to about 125 yards (assuming the proper zeroing range). The biggest limitation of the shotgun slug is that penetration and trajectory drop off drastically beyond 125 yards due to velocity loss, so its maximum effective range is probably about 125 yards. (I still wouldn't want to be hit by a slug at 200 yards though!)

12ga Foster Type Rifled Slug (G1 = .109)
(20" barreled riotgun with ghostring sights)

Range Velocity Zero = 75 Zero = 100
0 1440 -1.0 -1.0
25 1320 0.7 1.4
50 1200 1.1 2.5
75 1120 ± 2.1
100 1050 -2.8 ±
125 1000 -7.6 -4.1
150 950 -14.6 -10.4
200 880 -36 -30
250 820 -69 -62
300 770 -114 -105


A problem with slugs at the longer ranges is their reduced hard target penetration due to velocity drop and generally soft lead construction.

http://www.frfrogspad.com/shotgun.htm

 

[whosyrdaddy]

The link at the bottom of this post will give you a good place to start. It has multiple calculators including the Miller Stability calculator. Elevation really does not factor into the twist equation beyond its effect on atmospheric pressure, so it is best to research the climate for the area you will be hunting i.e. highest and lowest recorded pressure and extreme temp range for the seasons you wish to hunt/shoot. Once you have these ranges you will need to determine the muzzle velocity (mv) range you wish to shoot.

Check the intended twist for under stabilization by using:

lowest mv
lowest temp
highest pressure

The resulting stability factor should be no less than 1.3

Check the intended twist for over stabilization by using:

highest mv
highest temps
lowest pressure

The resulting stability factor should be no greater than 2.0

Now that you have settled on a given twist increasing/decreasing bullet length and weight while keeping the stability factor within this range will give you the design parameters for your projectile.

bear in mind that at practical muzzle loading hunting ranges, over stabilization is not an issue of concern.

You will need to know your bullets ballistic coefficient in order to use most on line calculators. Since your hypothetical bullet doesn't fit any of the standard drag models, my best guesstimate would put it somewhere between .07 and .3.


Other info you may need to know about your hypothetical bullet are:

o.a. length = 1.048
nose length = .324
nose dia. =.7425 ( T/C Maxis are designed to engrave the rifling at loading so this number will change depending on the depth of you rifling. )
meplat dia. = .216
base dia. = .729
length fbs = .155 ( fore bearing surface )
length abs = .202 ( aft bearing surface )
g.g. dia. = .554 ( grease groove )
g.g. length = .366 ( major )
g.g. length = .346 ( minor )
ogive style = conical
weight = 875+ gr ( my original calc. didn't include the .010 chamfer shown on both ends of the grease groove. This will add slightly to the o.a. weight, but would probably be offset by the addition of 1-2% tin to improve cast-ability.)



http://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi