It seems you're a bit confused.
OAL is total length stem to stern & so you don't need anything other than a caliper to measure that.
Measuring devices like comparitors are used for measuring things like Length to ogive, ot headspacing on rimless cases.
What are you actually trying to measure?
Allan Quatermain: “Automatic rifles. Who in God's name has automatic rifles”?
Elderly Hunter: “That's dashed unsporting. Probably Belgium.”