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I was in the military when we changed from M16 A1 weapon/ammo and started using the A2. Shooting A2 ammo in an A1 was not allowed....
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That's for a different reason. The M16A2 has a tighter twist to the rifling in order to keep the heavier (62 grain) bullet used with the A2 stable. It will also shoot the lighter (55 grain) A1 bullets. However, the M16A1 rifle with it's looser twist doesn't shoot the heavier A2 bullets that well.
Either way, the military zeroes the weapons at 25 m. You can't have one zero procedure for the M16A2 firing a 62 grain bullet and another proceedure for firing a 55 grain bullet. Considering you are trying to hit a rather large circle, I am not sure it would make much difference, but it's the military after all.