Trying to get aftermarket gas block

If unsure the best way is just to measure the barrel diameter with a set of calipers where the gas block sits. Also keep in mind the other major factor when ordering a gas black is to make sure it will fit under whatever hand guard you have if it goes under the hand guard.. They are not all the same.

Agree with square target2. Both measurements are important. If you don’t know, measure. If you don’t have calipers, buy some; there are numerous available at reasonable prices.