What is the minimum bearing surface length for something like a 5.56 (or really any militarily viable cartridge)? And what is the optimal length? Surely there is a point after which one gains nothing from increasing this length. So how short could you go without sacrificing whatever it is that is sacrificed with a short bearing surface. Concentricity in engraving, maybe?

Picrel is a 5 caliber long bullet I modeled with a 1.4 caliber long bearing surface. Its i7 form factor is 0.8. I wonder if it’s possible/sensible to shorten this even further and add the saved length to either the ogive or the boat tail

>>63854684 (OP)
why would you want a shorter bullet?

>>63854705
A Shorter bearing surface (the cylindrical part that engages with rifling), same bullet length. A shorter bearing surface allows a longer ogive.

>>63854684 (OP)
I feel like you'd eventually run into issues either centering the projectile or have too little space to get a good crimp with the case mouth. maybe reduce the bearing surface to a pair of driving bands instead of a single surface?

>>63854684 (OP)

You may want to look up the development history and ballistic data for the lazydog, believe it or not it was the first time anyone bothered putting a bullet or bullet like object in a wind tunnel. With basic data they managed to create a air dropped projectile that at terminal velocity could go through 20mm of steel.

The summary of the wind tunnel experiments should be of interest to you, given below in PDF form:


https://en.wikipedia.org/wiki/Lazy_Dog_(bomb)
https://web.archive.org/web/20190502012501/http://www.dtic.mil/dtic/tr/fulltext/u2/352807.pdf

I suggest you start with something that is close to an ideal projectile then work any compromises for your needs backwards from there.

>>63854684 (OP)
are you holster mk18 guy?

>>63854684 (OP)
1.5 caliber. You can look at all the vld and custom lathed bullet.
You can always go with Gerald bull's ERFB fins or triangular boat tail for ultimate ogive gainz.

>>63855072
Kek. Yes. I remember you. It’s like the 3rd time you’ve spotted me from across the internet.

>>63855050
Thanks. Will read now.

>work any compromises for your needs backwards.

I’m only LARPing. I’m trying to design a cartridge on paper, just trying to see what is possible within the dimensional confines of 5.56/STANAG mags. I can’t make bullets of varying bearing surface length and test them all out.

>>63855101
It occured to me to do that, but then I’m not sure if there is a difference in allowable bearing surface between ELR shooting and something that’s supposed to survive thousands of rounds in the trenches. Perhaps the state of the inside of the barrel informs what the correct bearing surface length is? I just don’t know what variables are involved.

>>63855182
LARP it until you make it. 25 years ago when i was in the Army i started bullshitting about North Korea until after a few years i suddenly realized i knew more about them than anyone else around me including at one point a Two Star General.

The mere fact that you are asking the question and actively trying to find the answer puts you in the one in a million category when it comes to humanity, go with it bro.

At one point a couple of bicycle repairmen fantasized about making a flying machine. There were people who saw it take off who saw us land on the fucking moon.

Be like that, i believe in you!

>>63855225
>I can’t make bullets of varying bearing surface length and test them all out.

Of course you can. There are a huge selection of chronometers and 3d modeling software available, i am sure you know them and how to use them more than me. All you need to test 5.56mm is a bolt action .22 mag and creativity. Everything else is just details given the nominally .22 bullet molds out there.

I spent a year and a half working as a range instructor for a US Government (state retracted) shooting range, we used to have a guy who did competitive shooting with a homemade chronometer, shooting rest and hand loaded .22 shorts. The guy made them, he figured out that .22 nail gun blanks had more consistent charges and primers than actual .22 so he cut them up and hand loaded .22 short.

Whatever your firearms related passion is, be like that guy.

>>63854684 (OP)
You will have less stability in internal, external and terminal ballistics. Bullets already wobble during the external ballistics phase. You will increase this wobble. In applications where you want reliability in penetration and/or expanding you will also struggle. For straight line penetration you would INCREASE not reduce bearing surface.
Short version you are just screwing yourself over trying to play the minimum game to gain elsewhere.

>>63855182
Most cartridge neck is about 1 caliber in length. Yes there is a hazard like breaking a wood pencil lead when fed from mag in autoloader. One can always crimp but groove on the land may or may not be a source of drag.
Pushing extreme will always be a specialty thing. At extreme range one would be using tripods and turn knobs to shoot their rifle, stability requirement will be no different than artillery piece.
I persume having the center of mass fat outside of the bearing surface would require a comparitively longer bearing length.

>>63855369
>>63854684 (OP)
For the record both of you are more competent to talk about this than i am and i shall leave this thread on that note.

>>63855369
>>63855182
With a triangular boat tail. You can make the boat tail as the bearing surface.

>>63856037

Watch this:

https://www.youtube.com/watch?v=iZ50nZU3oG8

>>63855225
>>63855305
Thank you for the kind words bro, but I think you overestimate who I am and my abilities. I’m just some guy with an iPad and internet access. I don’t have the resources for my firearms interests to bear any real fruit.

>>63855310
All this makes sense. But surely there is an optimal bearing surface length, after which you don’t gain any considerable amount of bullet stability. Otherwise all bullets would be perfect cylinders, lol.

>>63856037
So it looks like the triangular boat tail can keep the projectile properly oriented within the barrel, but I’m struggling to understand how a twist can be imparted onto the bullet in this way, with the “bearing surface” along the boat tail being a thin strip of material.

>>63854684 (OP)
Rule of thumb: you need 1.5 caliber bearing surface and center of gravity inside it.
But otherwise limits of bearing surface reduction are pure emperical min maxing. They depend on chamber and barrel (groves) geometry, bullet material and construction. Just make bullet and shoot it then observe launch angle of attack.

>>63856265

>I think you overestimate who I am and my abilities.

I think you underestimate yourself.

>>63854684 (OP)
Part of the issue you run into is that the bearing surface and sealing surface are the same thing. That one surface has to keep the round centered, impart spin from the rifling, and create a gas seal. You're not going to be able to appreciably reduce that ratio with out increasing bullet length. Increasing the length is much, much more doable, so your key dimensions are going to be magazine length, chamber length, and free space inside the case. There really isn't that much to be gained from min-maxing for this in a service rifle cartridge though.

If you were bigger, you could do Gerald Bull's ERFB concept, but that's just not financially do able at small arms scale.

>>63856265
>>63856484
Having a channel cutted out doesn't mean it won't engage the rifling. In fact a drill bit create and engage its own threading with disconnected bearing surface so shavings can leave.
In triangular boat tail, the channel is only half way through and sealing surface is like saw tooth on one side and straight flat line surrounding the cyclinder.
One can go to the extreme of more aggressive of replacing the ogive flatline with another sawtooth so that the sealing is a thin sawtooth wave. Half in the ogive and half in the boat tail.

>>63857681
Pic2

>>63857686
What if the bullet ends up situated in the barrel in such a way that the rifling engraves along the section(s) I highlighted in green? Wouldn’t this be tantamount to a bullet with only one driving band?

>>63855101
I think you’re right, this is what I’m noticing now as well. I went back and looked at the drawings for the 5.56 FABRL bullet and they used exactly 1.5 caliber long bearing surfaces. Whatever, that’s perfectly fine. An i7ff of 0.8 out of a 5 caliber long bullet is already amazing. Shaving a millimeter or two off of it will only serve to introduce problems, and all for a tiny improvement in form factor.

>>63855305
>.22 nail gun blanks had more consistent charges and primers than actual .22
Christ in heaven

>>63858245
shells has spun with less surface.
Rifling usually has more than 3 grooves
Lastly if it slips, it slips to near by triangles.