stancrist said...

autogun said...

...bearing in mind that MGs are always likely to have longer barrels than IWs...

Two words that should never be used -- "never" and "always"   ;^)

Basic English comprehension: "always likely to have" does not mean the same as "always have" - it's describing a probability.

There is already one current thread concerning (just for a change) the merits of a GPC. 

One at a time is enough. This thread is reserved for its original purpose - anything other than technical discussions of ammunition design and performance will be deleted.

Not vacation, working.

Aortic repair conference, organising, chairing and co. Computer links not working, and crap.

Cardiac surgeons, cardiologists, sonographers, radiology working together to fix aortas from valve to iliacs.

Had a ripper CT of split mid aorta, blunt trauma, car vs tree.

Just an idea. I'll call it the 6.5mm ANMG, which stands for "Assault rifle 'N' Machine Gun", but really "Anthony-Nathaniel Middle Ground". ;) The idea is to explore a slightly different concept than what we've been discussing so far, which shouldn't be taken too seriously but hopefully will be thought provoking.

I based the case on the 6.5 Grendel, with the shoulder angle changed to 20 degrees and the case length increased to 40.3mm, neither of which matter too much but are based on preferences of mine. SolidWorks gives a case volume of 38.2 grains H2O, which sounds roughly correct if a bit optimistic. For all intents and purposes, this is like the Grendel, but I wanted to add a little more length so long bullets didn't intrude so much into the cartridge case. Below is the case and its volume-of-material:

The point of the round is to provide a lighter round than is possible given the GPC requirements, while still having the potential to meet at least some of the GPC requirements with heavier loads. To this end, the standard round would use a very light-for-caliber bullet of 85 grains; a weight easily achievable in lead-free form, such as an EPR. This round or a lead-free frangible training round could be used for training anywhere, and is useful for all squad-level small arms due to its light weight. In this ballistic example, the bullet used is flat-based, but it doesn't have to be; that's just the model I was using at the time:

Clearly, this does not meet Tony's requirements, but for those concerned about 5.56mm's terminal performance and the performance of .22 caliber rounds in general, it does have some interesting characteristics. For example, it gives almost 60% more energy at 500m than M855, while adding 40m onto the range at which M855 holds on to 2,000 or more ft/s velocity. It gives the same energy at 650m as M855 does at 500m, as well, plus an extra 40% energy at the muzzle. Sectional density is about the same, but given iron/copper alloy construction I don't think SD will be so important at short ranges anyway.

The round has a light bullet to make me happy - total round weight in brass comes to 15.23 grams - heavier than I'd like, but getting there. In steel, cartridge weight is 14.75 - still an increase of 23% over 5.56mm, but quite a bit better still than rounds like 7.62x39, 6.5 Grendel, or 6.8 SPC. So pretty light, all things considered.

Here's where I maybe make Tony happy - the light 85gr bullet satisfies the Army's requirement for a round that can be used in training ranges. The Army has shown that they will accept combat rounds with lead cores (e.g., Mk. 262 being used outside of SOCOM) as long as those rounds are not being used for training. Great, so here's where you cheat. If more range is desired, additional loads can be introduced or brought out from stores that have heavy, lead-cored bullets in either OTM (for DMRs) or steel-jacketed FMJ (for SAWs/MGs) bullets to meet those additional requirements, and these rounds can meet Tony's energy at a kilometer requirement. This does mean there would need to be additional suites of tracers, etc, but this problem becomes much easier if OWL succeeds, and anyway, there's a wide variety of tracer subtypes in service already. Again, in normal service everyone would just use the 85gr load and be pretty happy, but for special cases like Afghanistan the users best able to take advantage of longer-ranged ammunition have access to it. Further, the heavier projectiles are by definition more specialized ammunition, so their requirements can be adapted to whatever barrel length works. 

Loaded with the new Berger 130gr Hybrid just as an example, even the raw Powley values allow the round to handily meet the klick energy requirement from a 16.7" barrel:

And that even works out to a pretty good 17.47 grams cartridge weight, too.

This is just an exercise - I'm hoping it will spark some additional discussion beyond the normal GPC: good/bad? back and forth. I am not totally happy with this solution, and I suspect neither would Tony be, but it's maybe a meeting halfway between our two positions that we can be equally unhappy with. ;)

NathanielF said...

the biggest factor here is additional magazine weight

Would poly mags change the argument a bit?

I mean, where would be the sweet spot for case dimensions x loaded magazine weight, keeping projo weight the same? And how much case volume would this solution squeeze out?

It would take a bit more dedication from me to solve that problem; however my current preferences are for cartridges that have about a 1.75-1.85 caliber case base and maybe 6-7.5 calibers of case length. That should give the best all-around performance, based on what I know about interior ballistics. The cartridges 6mm PPC, 5.45x39, 6.5x47 Lapua, and .284 Winchester all fall within or very near this range.

In general, while the case head of a round is one of the biggest contributors to weight, in the end I think the squatter cartridges (see the above set) win out anyway, as they improve things in so many other ways (lighter magazines due to more favorable dimensions, better, more consistent propellant burn, etc) that they're worth their disadvantages (greater bolt thrust, greater cartridge weight).

But there's really too many 6.something rounds in this thread, so I'd better balance it out with some SCHV, hadn't I? ;)

Stays above 2,000 ft/s out to over 500m, and punches with 91% more energy than M855 at that range... Not bad for a .22 cal "mousegun" round, eh? ;)

It certainly looks a lot better than the 5.56mm, Nathaniel, but I have a reservation about the dual loading approach - it never seems to work. 

In the interwar period the French introduced heavy (MG) and light (IW) loadings of the 7.5x54. Postwar they dropped the heavy load and concentrated on the light for everything. Germany was in a similar situation with the 7.92x57 during WW2 but decided to drop the light loading and use only the heavy. More recently, the Chinese introduced the 5.8x42 in light and heavy loadings, but dropped both after a few years in favour of one universal loading. In practice, two different loadings just seem to be too much trouble to bother with.

So if your round were ever introduced in that form, I suspect that the light and heavy loadings would soon be replaced by one compromise loading - something rather close to a GPC!

On the subject of the SCHV - Badcow54 has been doing a very interesting analysis of suppression, taking into account previous experimentation and working out aural and physical impact factors to calculate an overall suppression index (which varies with range). This makes it very clear why 7.62mm MG fire is perceived by its users to be far more effective than 5.56mm fire. This will be included in the revised version of his article which should be up on my website in the next week or so.

autogun said...

 

On the subject of the SCHV - Badcow54 has been doing a very interesting analysis of suppression, taking into account previous experimentation and working out aural and physical impact factors to calculate an overall suppression index (which varies with range). This makes it very clear why 7.62mm MG fire is perceived by its users to be far more effective than 5.56mm fire. This will be included in the revised version of his article which should be up on my website in the next week or so.

 

 

That would be pretty interesting to see. I always enjoy reading Emeric's work, even if we don't always see eye-to-eye.