Actually, a weight of 180 grains is never mentioned in the regs. Here's what it says in the pertinent sub paragraphs:
C. To hunt deer pronghorn or bear they must be min. of .40 caliber.
d. To hunt elk or moose they must be min of .50 caliber.
e. From .40 caliber to .50 caliber, bullets must weigh a min. 170 grains.
f. If greater than .50 caliber, bullets must weigh min 210 grains.
e. above may be the source of some confusion wherein some hunters think the reg refers to ball size, when in fact it refers to the caliber of the rifle!
Note that when correctly interepreted, a .45 caliber rifle loaded with a PRB
is not legal for deer hunting since even a .450 round ball weighs only 137.1 grains.
Well, that was probably more detail than anybody wanted on the subject!!