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?

The biggest thing I take away from this. Is the fact I am glad I am more interested in classic firearms. Modern firearm furniture is expensive!

Fooling around with a certain cartridge template, scaling it up and down. Figured I'd show how I measure case capacity - this and component weight is why using SolidWorks properly is important to get the best estimates:

It's a 6mm based on the SPC case with a 1.721" case length and a 2.42" OAL (not with bullet shown, which is one of my stock 6mm bullets). Internal contour was based on 7.62 NATO, but adjusted for the SPC case's wall thickness. Initial estimates put cartridge weight at almost exactly 15g with a 77gr bullet. Depending on the resultant ballistics, the cartridge when fired from 14.5" barrels could potentially meet the specific energy threshold of 7.62 NATO at 1,000m if very fine projectiles are used.

When comparing ballistic performance of cartridges, the barrel length should always be stated. The official NATO specs are currently taken from 20" barrel for 5.56mm, and 22" barrels for 7.62mm (although of course the GPMG/M240 does have a 24.5" barrel as standard). In fact, bearing in mind that MGs are always likely to have longer barrels than IWs (especially in vehicle mounts and in the tripod-mounted support role held at platoon level) it's probably a good idea to give two sets of figures for each round, reflecting different barrel lengths.

Taurevanime said...

 

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?

 

Even with the models I have, I refuse to propose a "solution" for next-generation small arms, for that very reason. I think there's a whole lot of modeling and testing of individual cartridges - let alone other kinds of research and experiments - that needs to be done before a configuration can be decided upon.

stancrist said...

 

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.

 

One notes they left off polycase 5.56mm in their comparison, despite the fact that 5.56mm polycase ammo has been ordered.