GunTech, could you run me a 5.45x39mm round necked up to 5.75mm with a 80, 85 and 90 grain bullet? What kind of speed and energy do I get?
Unfortunately, I am stuck with bullets in the database, so unless I build a 5.75 bullet, such a task isn't doable.
It should be noted however that without changing the allowable pressure or case capacity, minor changes in bullets won't have much of an effect. You can trade things like short range energy for long range performance, but ultimately, you are just trading back and forth, not gaining.
A good example is the 6.5 Grendel. There are some good gains over the 5.56 because you are blowing out the case and sharpening the shoulder, and running the cartridge severa thousand psi over the 7.62x39mm. That will make a difference.
Now consider the 6x45 vs the 5.56x45 - the same case necked up to 243.
Muzzle velocity drops, meaning less muzzle energy. You trade short range energy for long range energy because the 243 bullets tend to have much better BC. You can also adopt a 223 bullet with better bc, and you get similar performance.
6.5 bullets, at moderate weights, tend to have very good BC. If yourt drive them at relatively decent velocities the have plenty of energy, and retain it well. Note that the 308 has much more energy, but the existing ball ammunition has relatively poor BC, so that the 6.5, starting slower and with less energy than the 308, eventually surpasses that cartridge (at around 500 yards)
There are high BC bullets for 308, but these tend to be heaver, meaning more recoil, every thing else being equal.
The real trick is to develop a cartridge that does a lot of things well, even if it doesn't do one thing really well. It a case of generalization versus specialization. If you have one round for machine guns, one for assault rifles, one for DMR and one for snipers, you can optimize each one for that one specific job. Of course, you've created a logistical nightmare, because now you have to supply 4 or more catridges.
Logistics is a huge factor in winning wars. So build you cartridge right, and possibly you can hace one round that performs pretty well in an assault rfile, not too bad in a machinegun, fairly decent in a dmr rifle and adequate as a sniper round.
And that is a very good thing.
The 6.5x45 decribed above is going to be a little too powerful for light and compact assault rifles - you are going to want to add a brake. It will be great for semi-auto fire, but much less contyrollable in autofire than 5.56.
In a full sized rifle with a 20 inch + barrel, it will do very well at intermediate range, and with good sectional density, shoud be good against walls and such.
Put it into a 24 inch DMR, or a prescision built 26 inch sniper rifle and it should be plenty accurate. If necessary, and where recoil isn't a factor, you can load the same round with a heavy, very high BC bullet and get long range performance that rewally shines - superior to M80, and very clode to M852 or M118LR special ball.
The rifle requires no change, and can be based on the same platform as the basic military rifle - highly refined and with match grade parts. in a pinch, the sniper can still shoot the general issue ammo, just losing some accuracy and long range performance. This is much more desireabe than issuing a completely separate catridge with a separate logistical train.
One Round to rule them all, One Round to find them,
One Round to bring them all and in the darkness bind them."