Testing, testing...

[stanc]

I got the value from Gary's ammunition: http://www.inetres.com/gp/military/infan..._ammo.html

Note that the value is for 78 ft from the muzzle

Yes, that info is the same as in TM43-0001-27.

I couldn't find a call-out for barrel length.

Nor does the TM give it. However, giving MV from 24" barrel is pretty much industry standard for high-power rounds, so I think it's logical to assume (despite the proverbial warning about assuming anything) that the data is for 24" barrel. And IIRC, for the last 100 years, the standard barrel length of .30-caliber infantry rifles and machine guns has been 24 inches (except for the M14, at 22 inches).

The difference between 2700 and 2750 ft/sec is within the normal spread of chronograph testing.

Chronographed velocities can vary widely, depending on the operator skill, ambient temperature, ammo manufacturer, and other factors. IMO, it's better to use the nominal velocity, as listed in the manual.

When this came up earlier, I ran ballistics tables to determine what muzzle velocity would give 2750 fps velocity @ 78 feet. The result was just a hair over 2800 fps (more specifically, 2805 fps).

As with Tony's comment, we want to err a tiny bit on the high side in those performance parameters that we want to equal or best the M80.

That's prudent, but it will make a cartridge bigger and heavier than absolutely necessary. Remember, the only advantage of this concept is the reduction of weight and bulk relative to 7.62x51. The bigger and heavier you make it, the less that advantage.

Can't do it with everything, else we get some grotesque requirements. We also need to keep the parameters where we demand "significant" improvement to an absolute minimum for the same reason.

IMO, the only areas that need to show "significant" improvement over 7.62 M80 are cartridge weight and bulk. Indeed, they are the only areas that can possibly be significantly better.

Muzzle velocity should equal M80, for reasons previously stated.

Terminal effects must be very similar, if not equal, to those of M80, again for reasons previously stated.

Downrange velocities, trajectory, and wind drift should be comparable, if not equal, to M80. The closer they match, the better, partly for psychological appeal, and partly to permit use of optical sights with 7.62 ballistic reticles.