OAL is a nominal value. It can be varied and often is. In repeating firearms that are box fed or magazine fed, these structures can determine OAL. In high pressure handgun cartridges like 9mm Luger, 38 Super, 357 Sig, and 40 S&W a minimum OAL is a significant measurement that should be observed. In the OP's example he can experiment without concern but must be aware that making the cartridge too long would cause the bullet to stick in the rifling and remain there if the the chambered and unfurled round is extracted.