Range Report Bullet design question

[sharac]

Hi,

i wonder if someone with technical knowledge could help me out. I've been cooking up a custom made .308 solid (bronze or copper) bullet in solidworks however my math or physics is rather weak and i'd need some help in evaluating it.

I've found some stability calculations which would discount the speed ( klik ) and i wonder if that would be enough for starters (also the greenhill formula of course however the URL before has different approach to measuring bullet stability).
Also i've calculated dynamic pressure to be at 4.9bars at 900m/s which seems quite a lot however i don't have enough knowledge in supersonic airflow to be certain.
I'd like to do as much calc. as possible before creating a few samples however i'm not sure what else to try or ifs even needed - i have an itch to just go and try them

.

Bullet:

Length = 1.3"
Diameter = .310 (drive bands, body is .3)
Weight = 130gr (copper - Cuzn5)

Moment of inertia = 416 g/mm2
SD = .205
BC(g7) = .215 (estimated shape around .95 to standard projectile)
Twist required (assuming speed of 2900fps which should be easily achieved) 1:11 will work with 1:12 perhaps or should work >3000fps.

pic:

 

[flounderv2]

Re: Bullet design question

Calling Bohem in 3....2.....1.....

 

[damoncali]

Re: Bullet design question

The math on that is horrific. Just turn a few and try them.

 

[bohem]

Re: Bullet design question

Here's a free Miller calculator that's much more robust for this type of bullet than the old Greenhill formulae.

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

The Greenhill method breaks down when the bullets are not pretty close to a cylindrical shape (RN flat based, pistol bullets, etc) and begins to introduce large amounts of error.

Based upon the information you posted I'm getting the following results:

Stability
Input Data
Caliber: 0.310 in Bullet Weight: 130.0 gr
Bullet Length: 1.300 in
Muzzle Velocity: 2850.0 ft/s Barrel Twist: <span style="font-weight: bold">11.0 in</span>
Temperature: 25.0 °F Pressure: 29.92 in Hg
Output Data
Stability: <span style="color: #009900"><span style="font-weight: bold">1.254</span></span>



Stability
Input Data
Caliber: 0.310 in Bullet Weight: 130.0 gr
Bullet Length: 1.300 in
Muzzle Velocity: 2850.0 ft/s Barrel Twist: 12.0 in
Temperature: 25.0 °F Pressure: 29.92 in Hg
Output Data
Stability:
<span style="color: #009900"><span style="font-weight: bold">1.054</span></span>




Now, because it's a driving band bullet and the bands will make a difference, here's the same thing run with an OD stipulated at 0.300" instead of 0.310"


Stability
Input Data
Caliber: 0.300 in Bullet Weight: 130.0 gr
Bullet Length: 1.300 in
Muzzle Velocity: 2850.0 ft/s Barrel Twist: 11.0 in
Temperature: 25.0 °F Pressure: 29.92 in Hg
Output Data
Stability: <span style="color: #009900"><span style="font-weight: bold">1.178</span></span>




Stability
Input Data
Caliber: 0.300 in Bullet Weight: 130.0 gr
Bullet Length: 1.300 in
Muzzle Velocity: 2850.0 ft/s Barrel Twist: 12.0 in
Temperature: 25.0 °F Pressure: 29.92 in Hg
Output Data
Stability: <span style="color: #FF0000"><span style="font-weight: bold">0.990</span></span>


These conditions that I ran are pretty harsh in terms of atmosphere. Under a lot of conditions the formula is going to predict stability and if you speed that bullet up over 3000fps even under these conditions the calculator predicts it will be on the ragged edge of stability. Bore transit speed is not the most effective way to promote stability but when your hands are tied by an existing platform, pushing it harder can sometimes get the edge needed to remain stable.

A piece of advice having done a little investigation into monolithic solids myself: Push that OD back to nominal groove diameter.

IE: If you're planning to shoot them in a 0.308 bore, make them 0.308 -0 + 0.0005" on the groove diameter. It will help keep the engraving pressures and friction levels down.

 

[sharac]

Re: Bullet design question

Much obliged sir. I was a bit afraid of gas seal with solids so i went a bit over with drive bands.
Have you by any chance had any experience with bronze bullets would they be more aggressive on the bore and would they influence barrel life significantly? Also .308 is pretty limited and i guess this playing of mine is more me wasting time than actually getting some extra profit from it

.

Still its kind of fun to design your own bullet i hope it will also fly true we'll see at least i got a stash of scenars as a backup

.

 

[bohem]

Re: Bullet design question

Do you have the machine directly available for making these? I don't know what the laws are in your country but if I had one of these machines I'd probably be going broke making mono's for myself. You're right, there is something intensely satisfying about seeing your own design work.

I can't comment on the bronze at this time.

 

[sharac]

Re: Bullet design question

Yeah i have an option with a friend (turned some for handgun

). No restrictions regarding bullets and brass. Only problem is material (costs wise i was considering bronze (CuZn 36Pb3) 8mm stick - would be very cheap compared to copper) as i want 8mm "wire" to keep it simple and waste low. We'll see i'll do some experimenting perhaps i "struck gold"

.

 

[ply1951guy]

Re: Bullet design question

Out of curiosity, how did you get 4.9 bars of dynamic pressure? With knowing more about the geometry, I can not check your math. But I am assuming that calculation is based on oblique shock theory.

Please report back here if you decided to make some.

 

[bohem]

Re: Bullet design question

My trouble with Bronze in the US is I can't find the stuff under 1/2" (12.7mm) bar

By the time you hog off the material from a 12.7mm down to a "7mm" bullet the waste account for about 98% of the material cost savings, the machine time accounts for a little more than the rest.

 

[sharac]

Re: Bullet design question

Don't laugh but i used simple Pressure = 1/2*specific heat (1.4)*static pressure*(mach squared)
Solidworks has some surface plots which can estimate dispersion of pressure on the surface of the object so i will also play with it.

I will try them as for reporting if i stay quiet it didn't work and i'll move on in shame

.

Yeah getting proper source material is a pain and with raw materials prices going to the moon it will only get worse. Lapua is increasing prices on 1st of July quite a bit and i guess the rest will soon follow.

 

[pcschwenke]

Re: Bullet design question

The design looks very neat. I've considered creating my own as well. Having access to Solid works and a entire CNC machine shop helps!

bohem, checked here for small bronze. This is not the cheapest source but they have everything. http://www.mcmaster.com/#red-metals/=c9ygjm

Another source would be Alro Steel or Copper & Brass Sales.

Paul

 

[jbailey]

Re: Bullet design question

How did you estimate the form factor of 0.95? It seems the form factor has to be better than that to justify the development - a 0.215 G7 BC is not impressive.

Why not increase the density a bit with longer bottails and more drive bands, i.e. 155gr, which if your form factor is good, yields a SD of ~.25 and a BC of ~.26

Yes, you will need more twist in the barrel, but will not have problem moving it close to 3000fps, which will help stability.

 

[sharac]

Re: Bullet design question

I went on a gut feeling (which in my case could be a shot in the dark - probably is

) as shape is near g7 standard with more aggressive cone and no(very small) meplat so that should reduce drag a bit.
But bear in mind this is just a "paper" version and before firing i'd also have to check nose cone hardness (if i manage to get it bent when tumbling or chipped i will have to thicken it - i coat with hBN) and also how the seal is working, what tolerances will be in final product etc...
I'm aware of poor BC and one of the main problems with solids is getting dense enough material which would enable to get a good weight in still acceptable shape (bronze with 8.4g/cm3 or copper with 8.9g/cm3 are just not dense enough to be able to get weight up there and special alloys with Pb which would be better in density department are too weak in friction or hardness departments - i think this is main problem some big solids manufacturers like GS Custom had to solve and if i'm not mistaken they all use custom alloys for their work). I can make 155 or 180gr bullet just fine but i'd need a twist in the 1:8 or less region to stabilize it as bullet would be a long beast. A tradeoff is a more tangent nose cone which i will also try. A longer boat tail is possible but to a certain point as you want center of mass a bit back.

I kinda look at it as a learning project for me and in the process i'll sure waste a lot of my time, go up several dead ends probably waste a few € and in the end maybe not even get the projectile to fly true. But if i learn stuff in the process brush up on stuff long forgotten (math and physics) learn a few tricks in Solidworks and CNC it will be well worth it. Reading a book on ballistics is all fine and dandy but getting a hands on is a lot better i think

.

 

[tobias]

Re: Bullet design question

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: bohem</div><div class="ubbcode-body">Here's a free Miller calculator that's much more robust for this type of bullet than the old Greenhill formulae.

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

This formula only calculates the gyroscopic stability factor. Gyroscopic stability doesn't guarantee a stable flight, you have to consider the dynamic stability as well.

Regards

 

[bohem]

Re: Bullet design question

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: tob</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: bohem</div><div class="ubbcode-body">Here's a free Miller calculator that's much more robust for this type of bullet than the old Greenhill formulae.

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

This formula only calculates the gyroscopic stability factor. Gyroscopic stability doesn't guarantee a stable flight, you have to consider the dynamic stability as well.

Regards </div></div>

Without the aeroballistic deep dive to find out the aerodynamic coefficients he's not going to calculate dynamic stability. Miller gets a lot closer than Greenhill, that's the point, and that's what I said.

 

[sharac]

Re: Bullet design question

I think the link i provided in first post deals with dynamic stability as i understand it you want bullet to align with parabola (bullet path) and bullet which is under (and over) stabilized will either yaw wildly (unstable) or fight the trajectory and fly with nose off the path (overspun bullet).

I still need to chew upon a few books regarding external ballistics but still thank you all for you help.

PS: I would imagine some serious equipment is needed to really evaluate projectiles if one wants to do accurate calculations so this basic stuff is all i need.

 

[tobias]

Re: Bullet design question

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: bohem</div><div class="ubbcode-body">
Without the aeroballistic deep dive to find out the aerodynamic coefficients he's not going to calculate dynamic stability. Miller gets a lot closer than Greenhill, that's the point, and that's what I said. </div></div>

Yes, but maybe it's worth noting, that the gyroscopic stability condition only is a necessary condition to guarantee a stable flight, but is by no means sufficient. That's the point and that's what I said.

 

[tobias]

Re: Bullet design question

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Sharac</div><div class="ubbcode-body">I think the link i provided in first post deals with dynamic stability...
</div></div>

I didn't read everything, but I think the link deals with the static (gyroscopic) stability.
There's an interesting paper by Beat Kneubuehl (Defense Technology Agency, Ballistics Division, Switzerland):

What is the Maximum Length of a Spin Stabilized Projectile?, Proceedings of the 9th Int. Symposium on Ballistics (1986)

Kneubuehl presents formulas and figures to determine the gyroscopic stability factor as a function of the length and slenderness of the projectile using Munk's formula for the overturning moment. The results could be useful for your plans.

 

[sharac]

Re: Bullet design question

Thanks i'll try to find it and go through. Much obliged.

 

[sharac]

Re: Bullet design question

After a long long long time there has been some progress with my design to a point it's actually thing with mass and feel to it (aka manufactured

). I've redesigned the bullet totally and final result is as follows:

Length: 37mm
Weight: 158mm
Nose cone based on LD-Haack curve (von Karman nose cone from rocketry)
Estimated BC based on form factor derived from computer simulation (comparison of force of drag between this model and standard g7 projectile @800m/s)

BC<span style="font-size: 8pt">g7</span>= .250 lb/in^2

Computer:

Real life:

How they shoot? Dunno will check soon at least they look pretty

so not all is wasted...

 

[sharac]

Re: Bullet design question

Shot them last Saturday and my friend shot few from his Savage 1:10"


In my Tikka i'm unable to reach lands so i've loaded them to 77mm COAL. Used charges from 42-45 with 44 and 45 being 3 shot. I've still 10 round left and this was initial setup just to get velocities and i must say i think bullet is a bit smaller (drive bands @ 7.83mm and body at 7.62mm) than bore and probably not sealing correctly since i get only 2640fps with 45grains of N140.
As you can see results are pretty shitty (inside of black circle is 70mm) with fliers (no shooter fault). Bullet still appear stable but i will try to confirm this with the last 10 as i will try them @ 300m to see if they remain stable out of 1:11" (we've fired a few from 1:12" and they still made round holes @100m though this was just to see if they will blow up the rifle)

 

[MitchAlsup]

Re: Bullet design question

A) I think you would do well with a bit more of a boat tail on them (i.e. longer BT section) This should reduce the integrated drag at the back end of the bullet.

B) weight in 158 grains not mm.

C) I guess I am surprised at the slowness. You might work up to 2900-2950 where the 155s are shot (Varget 47.8 gr in Win cases, 47.4gr in Lapua cases, with CCI primers.)

Good luck.

 

[sharac]

Re: Bullet design question

a) definitely longer BT and shorter nose

b) typo yes

c) by all accounts with 45gr of N140 they should've ran faster i'm worried seal is not such as it should be and next round of testing will be in 47gr area and 2800+ speed.

After doing some thinking perhaps the problem is secant nose and distance to lands but anyways i will redesign the nose as i don't want them loaded into lands as VLDs must be...