Testing, testing...

[stanc]

I'm sure that Gene's 50% is more rhetorical than not. The facts are, however, that we will gain exactly zero traction if we are able to demonstrate only a 5% improvement absent a political decision to go to a new cartridge.

Also, the move from each of the cartridges to the next in the list had more than 5% improvements in one or more metrics. For example, the .45-70 had a slight improvement in velocity for a significantly smaller case. The .30-40 Krag had, IIRC a 220 grain bullet replacing the 400 grain+ 45-70 bullet and a lot more cartridges, and so on.

So yes, the improvement in some metrics in each of these may have been flat or a slight improvement, there were almost always significant improvements in one or more metrics. The exception may have been the 7.62X51 -- the improvement was in weight of brass and a mild reduction in recoil at the expense of a slight reduction in velocity. The political issue was imposing a common cartridge on NATO and getting everyone to go with our beloved 30-06 would not have worked.

The US never wanted NATO to adopt .30-06. We wanted .30-06 performance and bullet size. The desired solution was 7.62x51, partly for the savings in brass (as you noted), and partly because it enabled a lighter rifle than was possible with .30-06.

SecDef imposed the 5.56 DoD-wide because his metrics told him there was much more than 50% — doubling (100% improvement) the numbers of rounds per pound.

Doubling the number of rounds was a definite advantage for 5.56x45, but I don't remember ever reading anything about that being the reason for the SecDef's decision. What I recall is that it was due to ongoing difficulties in M14 manufacture; the directive to buy M16 was meant to supplement the shortage of M14s as an interim measure until the SPIW -- expected to enter service in the late 1960s -- could replace both M14 and M16.

But, the reason for the SecDef's directive is immaterial to what I wrote. The 5.56x45 was forced upon the Army, therefore it cannot be taken into consideration as part of the historical trend in Army cartridge development. Left to itself, the Army would have stayed with 7.62x51 for infantry rifles, until the leadership decided to make another incremental change. My guess is that after prolonged combat experience (against the 7.62x39) in Vietnam, the next incremental step might've been something similar to the .30 HRT.