Lets start over: how do you design a GPC?

[mongoosesnipe]

Designing a military cartridge, like any other piece of kit, is a matter of choosing the right compromises.

I believe that to be of interest to any military, a new cartridge will have to do two things: at least match the long-range ballistics of the 7.62mm M80 while saving at least 25% of the ammo weight; and generate recoil about half-way between the 5.56mm and 7.62mm so that burst fire is controllable at short range. It will then be able to replace both calibres in the infantry squad, simplifying procurement, supply and training as well as providing an overall capability boost to the squad.

I agree that the 7mm calibre is too close to the weight of 7.62mm to achieve this. Using lead-cored bullets, 6.5mm seems to offer the optimum balance between weight saving and effectiveness, although 6.8mm might be better with lead-free bullets - that remains to be determined. Around 2,500 J (1850 ft lbs) muzzle energy seems to be about right to deliver the performance required while keeping weight and recoil down.

The 6.5mm Grendel is probably a bit too small to deliver what is wanted, especially when the use of lead-free and tracer bullets, plus the probability of polymer-cased ammo coming along, reduces the space for propellant. I think that there are basically two options: a Grendel case stretched to around 44-45mm, or a Rem SPC case stretched to 47-48mm (these are rough estimates). Whichever case is chosen, the overall length must be enough to allow the use of long, finely pointed bullets (which the 5.56mm, 7.62mm and SPC cannot do). Obviously, neither of the options could fit into an AR-15 action.

The illustration below, from my web article on The Next Generation of small arms and ammunition (http://www.quarry.nildram.co.uk/The%20Ne...ration.htm ), shows several things. I took the Russian 5.45mm 7N6 bullet as an example of a well-shaped military bullet and electronically scaled it up to 5.56mm, 6.5mm and 7.62mm calibres. As you can see, a bullet of this form fitted into the 5.56mm and 7.62mm cases would result in overall lengths too great to be usable in existing military weapons. The two rounds on the right show what I have been talking about - 6.5mms based on the Grendel and Rem SPC cases. They are not intended to be definitive, just to give a rough idea of what the end result could look like.

so what your wanting is to take a cartridge like the 308 and trim it down to say 47mm so that we could use long high bc bullets in a 7.62 NATO mag well, neck it down to 6.5mm such as to get a little more velocity or less recoil than 7mm or 7.62 bullets of similar high bc and put a very efficient shoulder on it for accuracy and then build an entirely new platform to shoot it out of...

if the goal is weight savings and performance and you want to design a new metallic or polymer cartridge to do that imho your already doing it wrong you may be able to save a few grams with a conventional case but if your starting from scratch i would go caseless