It will require using the GECK and a few simply boxes checked! In the GECK, open FalloutNV.esm and wait for it to load; hit the button that looks like a disk on the top left and it will prompt you to create a new plugin file (or open another plugin file of your choice to make things tidier by going to 'open' double-clicking FalloutNV.esm as well as the plugin you wish to modify, then hit 'set as active' on the plugin so that it is being edited). Once everything has loaded, go to Weapons on the left which will bring up all the weapons in the game. Scroll down to WeapNVServiceRifle, right click and Edit. Click the box that says 'Automatic' on the right side, above the tabs. In the first tab Game Data, adjust your Fire Rate to your desired rate of fire (an example being that the Assault Rifle is 12.0). May have to adjust a few other things, such in the next tab Art and Sound, but you should essentially just go to another automatic weapons like WeapNVAssaultRifle and compare your options. Other things that may need adjustment: Attack Anim, Projectile (most automatics use AutoTracerProjectile), and Attack Multiplier for balance.