First you have to delete all old files :
in Data\Meshes : 
	\Weapons\2handhandle\autocannon.nif
	\characters\_male\ACReload.kf
			 \ACAttack.kf
			 \ACreload1stperson.kf
	\Ammo\40mmsingle.nif
	\Ammo\40mmpack.nif
	\Projectiles\40mmproj.nif
		    \40mmcasing.nif

in Data\textures : 
	\Weapons\2handhandle\ <All that has "autocannon" in its name>
	\Ammo\40mmsingle.dds
	\Ammo\40mmsingle_n.dds
	\Projectiles\40mmshell.dds
	\Projectiles\40mmshell_n.dds
	\Projectiles\40mmproj.dds
	\Projectiles\40mmproj_n.dds
	\Architecture\Wasteland\MilitaryFacilityhatch.dds
	\interface\icons\pipboyimages\items\items_grenadepack.dds
	\interface\icons\pipboyimages\items\items_singlegrenade.dds
	\interface\icons\pipboyimages\weapons\Autocannon.dds
	\interface\icons\pipboyimages_small\items_small\items_grenadepack.dds
	\interface\icons\pipboyimages_small\items_small\items_singlegrenade.dds
	\interface\icons\pipboyimages_small\weapons_small\glow_autocannon.dds
	
then copy the new content to your Data folder