Montana Fish, Wildlife & Parks' hunting regulations specify under the heading "Archery & Crossbow Equipment," the equipment that is legal for archery hunting in Montana.
For example, for FWP enforcement purposes, a bow is considered legal if it is at least 28 inches axel to axel in length. Bows shorter than 28 inches are prohibited for use in Montana. In another example, an arrow is defined as a projectile at least 20 inches in overall length. An arrow must weigh no less than 300 grains with the broadhead attached. Crossbows are not a legal weapon during the archery equipment only season.
To review these and associated details on archery equipment regulations in Montana, go online to the 2013 big game regulations, or visit the FWP website at fwp.mt.gov and click on the Hunting page and then Regulations.