Widest Point has to be 6.5"

[BGun]

I have been confused about this for a while now but I just thought of asking it on the forums.
When it says the widest point has to be at least 6.5", does that mean from the inside to the inside of the head, or does it mean from outside to outside? That probably doesn't sound really clear, but thats the best way I could think of describing it.
Thanks alot!

[DanHS]

It is the inside dimension - from the inside of the left wall to the inside of the right wall

[MaKiMaKi13]

inside to inside i think almost all heads would be legal if it was outside to outside