What regulation or policy states that females are allowed to have their bun down in a ponytail while firing at the range?

If FM 3-22.9 stated you could have your hair down in a pony tail when on the range but TC 3-22.9 now supercedes that FM....and the new TC does not clearly state you can have your hair down.....then guess what. No more hair down.

Okay, I think I found the "why" to your question. Here is the excerpt from FM 3-22.9:

"When in the prone position, the IBA's back plate tends to shove the personnel armor system for
ground troops (PASGT) helmet over Soldiers’ eyes. To minimize the PASGT helmet positioning
problem, make sure that the helmet is properly sized and fitted. Female shooters with long hair
will find that wearing their hair in a bun adds material between the IBA and helmet, further
forcing the helmet down over their eyes. Encourage female Soldiers to wear a short (chin length)
haircut or cornrow hairstyle. If the female Soldier chooses not to wear short hair, allow her to
wear her hair down when firing. Tightening the suspension harness and sweat band (raising the
helmet higher on the head) can lessen interference with the IBA, hair, and helmet."

We no longer use the PASGT (haven't since around 2005) and no longer use the IBA (since around 2010). So the equipment limitations that necessitated this are no longer an issue.

It's not mentioned in TC 3-22.9 anywhere and AR 670-1 para 3-2 states that although long hair can be worn in a ponytail for physical training, even if the physical training is in utility uniforms, if the helmet is worn, hair must be secured in a bun. Train as you fight.