Before you read the below, it should be noted the standard C7/M16A2 butt is 5/8 of an inch longer than the standard M16A1 butt.
Rifle butt sizing.
Please note the AKM here is just being used for an example, as the rifle only has one size of butt.
The correct way to size a butt for the C7A1 rifle is as follows:
1. Pick up the rifle
2. After your safety precautions, place the butt of the weapon between your bicep and forearm
3. Grasp the pistol grip, if things seem to be okay and a 'comfie' fit, this is your size. If its hard to grasp the pistol grip, you need a shorter butt, and vice versa if you need a longer one.
4. This is the proper wqay to size a butt, and this method goes back to the ole FN C1 days. It works. Give it a go at your unit.
See my silly crude pics below....
Cheers,
Wes