Fallout : New Vegas weapon mods Overview . < section begin = " overview " / > One of the features added to " New Vegas " is the ability to custom modify certain base weapons . While in " Fallout 3 " this was limited to the creation of custom weapons , " New Vegas " takes this a step further by allowing the creation of weapon variants based on basic weapons by adding weapon mods . Unique weapons cannot be equipped with weapon mods , with the exception of the weathered 10mm pistol , the Mercenary ' s grenade rifle and the holorifle . While unique guns usually have better base stats , Obsidian Entertainment introduced this inherent drawback of unique weapons to balance out unique and normal weapons and make the choice less of an obvious decision and more of a preference . However , some unique variants have both mods and better base stats . Such is the case of the varmint rifle ' s unique variant , the Ratslayer , which has all of the mods the varmint rifle can be equipped with . By themselves , the weapon mods are weightless and serve no function but , once attached to their specified weapon , will change the characteristics , look and / or function to create a distinct variant . The weapon mods range from scopes and silencers to extended mags and several in - between . In the case of the varmint rifle and other such weapons there exists more than one different type of modification . While you are not limited in the amount of total mods you can apply to a single weapon , you are limited to one of each type of mod per single weapon and therefore are not able to stack attributes like increased rate of fire . Once a weapon mod is attached to a weapon it cannot be removed and remains a permanent feature of that weapon , along with its added attribute . However , this could not always be a good thing . For example , extended magazines is almost always beneficial unless the magazine must be reloaded on a shell by shell basis ( ex : cowboy repeater ) , forcing a longer reload ( not necessarily forces the reload to be longer , since pressing the fire button stops the reload ) , or a scope on the other hand has pros and cons , as it may increase maximum range , but makes it harder to use in close combat situations . The addition of a mod to a weapon will permanently increase the overall monetary value of that weapon by the value of the mod , even if the weapon is completely broken at the time it is applied . < section end = " overview " / > Usage . Weapon mods are attached via the Pip - Boy . To attach a weapon mod you must have both the weapon it is intended for and the mod in your inventory . Highlight the weapon you want to mod within the items section of your Pip - Boy , then select the modding menu via the button as indicated in the top right of the info panel . After you have selected a mod , the weapon name will have a + after it to show it has been improved , e . g . varmint rifle + . Once a weapon mod has been installed it cannot be removed . Vendors . There are currently 10 vendors who offer weapon mods in the Mojave Desert , with additional vendors included within add - on areas . Inventory of mods changes with stock rotation every 72 hours game time . Achievements / Trophies . Attaching 20 mods to weapons grants the Mod Machine achievement / trophy .