Why are cartridge OAL’s with hollow point and full metal jacket bullets of same weight different?

Different nose profiles may require some adjustment to their over-all-length (OAL) for proper feeding and functioning. The correct OAL for a given bullet will depend more on its configuration and profile than on its weight, and this can be seen quite clearly in the Sierra reloading tables. This is why we include the lengths we used for specific types in developing our data, rather than just listing a SAAMI maximum OAL. Some experimentation may be necessary with your particular firearm to obtain best results.