New Army "Caliber Configuration Study"

[Tony Williams]

Much of the earlier debate in this general area centered on having the bullet have a specified kinetic energy at distance.

Have I missed something in the apparent shift to hit probability as being of very high performance? (I happen to agree with this.)

What are we asking the bullet to do when it gets there for CLAWS and LDAM?

Good questions.

There has been a lot of debate in various forums (especially mine) concerning the performance requirements of a GPC. This has been complicated in recent years by the insistence of the US Army on lead-free ammunition. This is a particular problem for a GPC in which the bullets must already be long for their calibre in order to provide good performance at long range. Lead-free implies even longer bullets (which brings stability problems) or lighter ones (which reduces the ballistic coefficient).

My take on the GPC as it currently stands is that, if it is to replace the 7.62mm in light weapons, it is an absolute requirement that it at least matches the hit probability of the M80 out to long range (however that is defined, but I generally take this to be 1000m): which means in its trajectory, flight time and wind-bucking capabilities. I also still believe that the muzzle energy must be held down to c.2,500 J/1,850 ft/lbs in the interests of ammo weight and recoil.

The traditional approach to the GPC relies on a relatively heavy bullet (e.g. 6.5mm/120+ grains) with a significantly better BC than the M80, so that although it starts with less velocity it gradually catches up with the M80; first in terms of velocity, and subsequently in energy. This is clearly the optimum solution for the best long-range performance.

The probable need to use lighter lead-free bullets indicates that a slight shift in the specification may be appropriate. At the extreme, this could mean a bullet which has the same BC as the M80 and is fired at the same MV: this will, by definition, match the velocity and hit probability of the M80, but it will never match the energy. The bullet will probably weigh no more than 7g/108 grains (assuming 6.5mm calibre - there are of course other options), which will bring some benefit in ammo weight and recoil. This may be satisfactory if LDAM results in a lightweight .338 MMG which could be assigned to infantry patrols for long-range engagements.

My feeling, from following the various technical debates about bullet design, is that it is possible to achieve something of a compromise here: a light, lead-free bullet which has a better BC than the M80 and therefore still closes the energy gap as the range increases.

It is of course possible to match the hit probability of the M80 with other calibres than 6.5mm: but smaller calibres will inevitably widen the energy gap with M80, making it less likely that the Army would accept them as a 7.62mm substitute. After all, while the primary reason for the retention of 7.62mm is its longer effective range than 5.56mm, the fact that it hits considerably harder at all ranges is also valued.

So, to sum up: matching the long-range hit probability of M80 is essential; matching or at least approaching its long-range impact energy is desirable (but not essential, especially if a .338 MMG is adopted). The optimum specification will of course depend very much on what can be achieved with the bullet design; the higher the BC, the easier the task becomes and the more options are available.