OAL, or Overall Length, can be a very important measurement for firearms, or ammunition. Most states have parameters around how long or short certain weapons can be. For example, the standard AR-15 rifle has a barrel length of 16” which is the minimum length for a rifle barrel. If you attach a shorter barrel to your AR-15 without the proper paperwork and tax stamps in place, you are technically breaking the law! To use a short barrel on a rifle platform you need to officially make it a Short Barreled Rifle, or SBR. This involves a $200 tax stamp.
Ammunition uses OAL measurements as well. The overall length of an ammunition cartridge is a measurement from the base of the brass or steel shell casing to the tip of the bullet, seated into the casing. Cartridge overall length, sometimes shortened to "COL", is extremely important to the safe functioning of reloads in firearms.