I spend a lot of my free time exploring the design space for infantry rifle cartridges. I use a system of models based primarily around the JBM Ballistics Calculator, SolidWorks, and an online Powley computer, but with a lot of care and attention to the shortcomings of each program (especially the Powley computer, which can be downright wrong much of the time). 

We've hashed out discussions of the GPC before, and I don't think it's necessary to re-tread that ground, so in this thread it is just my intention to be transparent about my models and where I'm coming from. I'll update the thread from time to time with some of the stuff I'm working on, as I go along. Currently, I have no design goals on the table, as the conclusion I've reached is that near-final design configurations cannot be determined through this system of models, but I continue to model cartridges as they come out, and design new cartridges controlling for certain parameters.

The two rounds I worked on yesterday were variants of the .264 USA using an improved internal contour model* based on the internal contour of 7.62 NATO. One retains the .264 USA's case length, but uses 7.62 NATO case taper and shoulder angle (this has virtually no effect on the internal volume of the round, and was just a variant I produced while grafting the internal contour of 7.62 NATO to the .264 USA, and I ran with it). The second is the same thing, but with a case 0.160" shorter.

*Internal case contour is the big hurdle for accurately modeling a round in SolidWorks, as it can have major effects on case capacity and case weight, and therefore performance and cartridge weight figures. There is no sure way to accurately model this contour without having a factory drawing that gives it.

Internal contour of the above round. The below round is the same, just shorter:

The bottom round ended up producing velocities about 4.5% lower according to Powley, with a case weight about half a gram lighter. Judging based on parameters like 1,000 meter retained energy and overall cartridge weight with various bullets, I didn't really see either round having a major advantage over the other. What this did show, however, is that shortening the cartridge case gives pretty modest returns in weight reduction (something like 2-3% of cartridge weight, depending on exact load; you can also think of it as about 0.125 grams per millimeter of case reduction, for this case diameter - 11.3mm/.445"), with correspondingly modest reductions in cartridge performance.

Looks interesting.

Slightly OT, but I thought that this pic of the .264 USA, plus a hybrid polymer/brass case from MAC LLC, might be of interest.

Awesome job.

In short, there are diminishing returns in everything. What do you think would be the optimal weight x volume point with these parameters (rim diameter, taper, etc)? And what volume would that be?

renatohm said...

 

Awesome job.

In short, there are diminishing returns in everything. What do you think would be the optimal weight x volume point with these parameters (rim diameter, taper, etc)? And what volume would that be?

 

If you are just measuring the volume-to-weight ratio within one case head diameter, then you end up with a surprisingly long cartridge. This is because the case base itself is the heaviest part of the case. However, that additional volume isn't necessarily greatly improving your performance either.

I imagine there is also a diminishing return towards having much longer cases, as the action of the gun needs to be longer, and the working components of the gun are among it's heaviest too. So you really never should design a cartridge entirely separate from a gun.

Yeah, though the biggest factor here is additional magazine weight. One of the issues I have with Jim Schatz's article on future assault rifles is that the .264 USA AR-12 featured in it appears to just be a lengthened AR-15, so of course it will be closer in weight to an AR-15 than an AR-10 (there are other reasons to think the comparison in that article isn't exactly representative, either, such as the handguards). I don't think in the long run that configuration will work out very well, due to bolt strength concerns among other things. 

The point of me mentioning this is that, at least with the AR platform, increasing the OAL of the round doesn't have such dramatic effects on weapon weight, but it does increase the weight of your magazines quite a bit.

Here's what I mean about the handguards, BTW:

http://www.superiorweaponssystems.com/ar10_rifle_sniper_fftube_rev3.htm

http://geissele.com/mk4-keymod.html

If we compare the two, the SWS handguard with an AR-10 barrel nut and locking ring is about three-quarters of a pound heavier than the Geissele MK4 rail and nut.

NathanielF said...

Yeah, though the biggest factor here is additional magazine weight. One of the issues I have with Jim Schatz's article on future assault rifles is that the .264 USA AR-12 featured in it appears to just be a lengthened AR-15, so of course it will be closer in weight to an AR-15 than an AR-10...

Even going with the comparison as is, the AR12 weighs a full pound more than the M4.

Then there is the matter of having a bit more than half as many .264 rounds as 5.56mm: 130 rds of .264 (assuming 26-rd mags) vs 210 rds of 5.56mm (in 30-rd mags).

If the number of .264 mags and/or mag capacity is increased to get a better ability for sustained combat, then soldier load increases again.

BTW, not only was that piece of propaganda verrrrry long, it was full of errors.

So now we get to the lovely practice that is. For a given case volume do you get the lightest total package of loaded magazine and weapon. There are probably some optimal designs. And they all change when case and magazine materials change.

Isn't engineering grand?