JBM stability calc

[spitballer]

I've been fooling with the JBM bullet stability calculator (http://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi) and I especially like the feature which allows you to enter the length of a plastic tip. But what about hollow points? Can the plastic tip feature be made to accommodate them?

For example, a .224 cal Berger 64 grain HP bullet with an OAL of .815". Some of that length is empty jacket. Can a short length of plastic tip be typed in to estimate the effect of this empty section? Or, perhaps a slightly shorter length instead of the full .815"?

 

[Haxby]

They show the length of the plastic tip for some of the bullets they list. They don't mention anything about hp's. I'd use the number as is.
Have you tried it? How much difference does it make?

 

[jwrowland77]

Just use the bullet lengths as is, because that's the length of jacket whether hollow or not.

 

[spitballer]

They show the length of the plastic tip for some of the bullets they list. They don't mention anything about hp's. I'd use the number as is.
Have you tried it? How much difference does it make?

Using my 12"twist I typed in the .224 70 VLD to see what would happen. I didn't expect it to stabilize, and it didn't. But with a typical .125" plastic tip it would at least be marginally stabilized. Surely the hollow point isn't going to be nearly as light as a plastic tip?

I wouldn't expect to use anything as heavy as a 70gr out past 300 yards anyway (and only on paper targets), but it would be good experience to find out whether even marginal projectiles can be pampered along with this longer (30")barrel I'm using. Anyone have any experience with anything as heavy as 70 grains using a 12" twist? Just curious and thanks.

Also interested in other ballistics software that might be useful here. - TH

 

[Haxby]

Berger has a stability calculator online, with a list of their bullets.

 

[spitballer]

Berger has a stability calculator online, with a list of their bullets.

Very good! Thank you, I'm surprised at myself for not having thoroughly checked this (http://www.bergerbullets.com/twist-rate-calculator/) earlier! Their notation about the flat-based bullets explains the outstanding performance I've had with their 64 grainers using a 12" twist, and at the ranges I shoot I don't need a boat tail anyway, not with something as heavy as a 64. Thanks again - I've added the link to my favorites.

 

[ironworkerwill]

Tips are a factor as they add length to be stabilized. It doesn't really matter if its metal or lightweight polymer. Is there a box for poly tips?

 

[ironworkerwill]

I'm sorry SB. I totally misunderstood what your saying. MR. Rowland is spot on. I think I was guess reading?

 

[spitballer]

Tips are a factor as they add length to be stabilized. It doesn't really matter if its metal or lightweight polymer. Is there a box for poly tips?

Yes, there's a box for the poly tips on the JBM calc (http://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi). The Berger calc, (http://www.bergerbullets.com/twist-rate-calculator/), doesn't have the box for the tips but it does include some valuable information about flat-based bullet calculations. The Berger calc is obviously the one to go to if you're using their bullets.

Essentially we're talking RPM here, but none of the manufacturers provide RPM recommendations that I'm aware of. They simply recommend a twist rate and sometimes a velocity as well. Adding an RPM rating would be a big help because a barrel designer could easily juggle different twist and velocity combinations to come up with an efficient barrel design.

 

[ironworkerwill]

MV x (12/twist in inches) x 60= RPM

I think the JBM has the recomendation built in. If the length, pressure, weight, Mv,...ect are all plugged in. The Miller formula JBM uses even adjusts for increased pressure at the tip of the bullet at various velocity.

 

[spitballer]

MV x (12/twist in inches) x 60= RPM

I think the JBM has the recomendation built in. If the length, pressure, weight, Mv,...ect are all plugged in. The Miller formula JBM uses even adjusts for increased pressure at the tip of the bullet at various velocity.

Ironworker, man, you're throwing wiffle balls - increased pressure at the tip as a result of hollow point structure, you mean? I realize I'm probably making a big deal about nothing and I should just be grateful for the calculators that are available, but it sure would be nice if there were a device available that could calculate a stabilizing RPM from a bullet itself. But spinning a bullet gyroscopically at 3000-4000 RPS and measuring the pressure needed to destabilize it would be no small feat. It might involve a simulated bullet with internal magnets, spun by a magnetic field within a wind tunnel of sorts. Not the kind of thing your average reloader could build in a weekend.

 

[ironworkerwill]

hollow point structure, you mean?

No, all bullets will suffer increased resistance as velocity goes up. Ever noice BC changes with velocity?

A simple example would be: the force applied on your car's window at 50mph doubles at 100mph right? No it would more than double.

You are correct in saying its not that big of a deal. The wind blowing across the range will make a much bigger impact than anything missing in the calculation.

 

[spitballer]

Yes but wind resistance that increases exponentially would actually work to the advantage of the tester. First you'd need to make some oversize mock-ups of a few popular bullets on a 3D printer, about the size of a tank round. Magnets could be mounted peripherally insisde the bearing surface of the mock-up and it would be suspended in a field coil, not unlike an armature in a DC motor. A potentiometer would control the rotational speed of the simulated bullet, and a strong wind could be concentrated in a large, transparent pipe with the mock-up suspended inside. Since the density of the simulated projectile can be considerably less than a jacketed bullet, you wouldn't need a 2100 MPH wind blowing through the pipe.

Actually it might be easier to just see what the military does to test simulated artillery shells and go from there. For the time being I'll just be thankful for the stability calculators that are available.

 

[Berger.Fan222]

I've been fooling with the JBM bullet stability calculator (http://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi) and I especially like the feature which allows you to enter the length of a plastic tip. But what about hollow points? Can the plastic tip feature be made to accommodate them?

For example, a .224 cal Berger 64 grain HP bullet with an OAL of .815". Some of that length is empty jacket. Can a short length of plastic tip be typed in to estimate the effect of this empty section? Or, perhaps a slightly shorter length instead of the full .815"?

http://forum.accurateshooter.com/index.php?action=dlattach;topic=3834705.0;attach=916992

Here's a stability calculator that can account for open space in the tip. I've found it to be useful and accurate.