Quoted: If the military is using 55grain then why the 1/7 twist?
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Because the military doesn't use 55grain primarily

Even if they did, 55gr ammo shoots just fine in a 1/7ROT barrel.
There are some units with older 1/12ROT barreled rifles who are still issued 55gr M193, but M855 is the standard now.
Edited to add: Here are some pics of my own 6400C to give you a better idea of what it's about:
The Colt 6400c has an M4 marked upper and lower:


Heavy duty "H" buffer and heavy duty extractor spring for reliability:


Barrel has proper 1/7ROT, chamber and barrel are chrome lined:

Upper is RAS numbered:

Side sling mount:

Standard pushpin lower and no protruding searblock:

Proper M4 handguards with dual heatshields:

Proper M4 entended feedramps:
Lifetime service agreement: