Logistics

I am a staunch advocate of 6.8x43mm SPC II.  It outperforms 5.56 in every meaningful way and most 7.62x39 loads as well.

It will never be adopted by the military.

Why?

Logistics.

Let's look at my AR in 6.8 compared to one in 5.56.

Which one is 5.56?  This is problem one.  When the transition begins, there will be weapons of both calibers in the system.

The 6.8 (the green one by the way) takes a different magazine than the 5.56.  But you can insert the wrong magazine into either gun!  I've taken to marking my 6.8 mags with electrical tape around the base for a visual and tactile indicator.  Knowing what gun takes those mags is up to my memory.

The logistics problem is getting the right ammo and magazines to the troops with the new rifle.  This is not insurmountable; but the Army no doubt remembers the early 1980's.  That's when the M16A2 began to be issued along with its new M855 ammo.  Lots of commands had units with the old gun and units with the new gun.  When I arrived in Germany in 1988, my unit had the M16A2.  During the orientation zeroing lots of the people there had M16A1's.  The range had to have both M193 and M855 on hand so we could zero our weapons.

At least the magazines were the same.

More than once we went to the range and the crated ammo was M193 and we couldn't shoot.  I can imagine that the reverse happened as well.  In a pinch, though, we could have fired the old ammo.  That will not be true of a transition to 6.8.

This was peace time.  It gets a lot more complicated in war.

This is just where the fun begins!  What about optics?  An ACOG with a bullet drop compensator is marked wrong for 6.8.  Now you have to replace the scope along with the rifle.  Although this could be a blessing to a civilian shooter as all of those fine optics are dumped as surplus!

What we have is a rifle that looks just like the old one.  A magazine that can be inserted, but not fired from the wrong model and ammunition with the same issue.  It's a logistics nightmare.