Many replacer mods exist on the Nexus. 

However most do this by actually replacing the weapon.

While this sounds like a good idea it can cause many problems.

A simpler (and safer) means to do this is to modify the levelled lists, though it comes at the downside of requiring a new game.

This also works for placed items due to how items are placed in fallout 4.

This is a very simple thing to do and allows for the replacing of a single weapon with many (something I have done in many of my replacer mods) or instead expanding levelled lists to include more weapons in addition to a vanilla (or mod) levelled list; if done "improperly" it can also result in multiple unique weapons which will vary from play through to playthrough (e.g. my SMG replacer has this "issue", and a patch to "fix" this, where the silver submachine gun can be of any of the types of SMGs I added).

This can be done by opening up the CC, and finding the levelled lists for the item you wish to replace.

Most weapons have several, some have one.

In the case of the several simply replace each item (generally just the weapon) with the replacer weapon (you may need to also modify the level lists rules to ensure it actually spawns something in).

If its a single levelled list the easiest thing to do is to create a new levelled list (with multiple weapons) and place this new levelled list as the item in side the vanilla levelled list.

This should make sense but a cursory look at my mods in FO4edit should clarify the points.

I may make a video showing what to do in the CC but it is quite simple to do, the pain is the "light and lore" patches I make, where I rechamber weapons, reweight mods and delete mods that either don't fit with the weapon or with fallout (or my idea of fallout, cannon means nothing these days sadly).