This mod tries to replace the vanilla Submachine Gun with the Defense Gun (made by YonaTaku) but particularly leaves the Silver Submachine Gun as is, making it a true unique weapon. Quite specific, no?  
  
Now, a warning: My motherboard is bickering with my video card, so I couldn't play the game to test the mod properly. I've done other personal replacers without problems and I believe that this one should work well too, but I can't be 100% sure. If you wanna do a small test, just open the console in game, type "player.placeatme 00083AD6" without the quotes and press Enter. This will spawn a Triggerman and he will be carrying either a Defense Gun or a SMG (Or an assault rifle depending on your level). If a Defense Gun appears, everything is okay. You can spawn him other times just to be sure, if you want.  
  
If you want a more detailed description of what this mod changes, here it is:  

• Changes the LL_SubmachineGun entry, so that anything that uses this (be it vendors, enemies, legendary lists, weapon spawns) will appear with a Defense Gun instead of the SMG.
• Changes the LL_SubmachineGun_SilverShroud entry, so that it calls for a new entry called LL_SubmachineGun_Silver that should use the Silver SMG instead of a regular Defense Gun.
• Changes another SMG spawn somewhere in Andrew Station, so that it spawns a Defense Gun instead. In the DLC version, it also changes other 2 SMG spawns around Far Harbor.
• Changes the QAWeaponChest container in the QASmoke test room, so there is a Defense Gun there instead. This is for testing purposes, really. 
• Changes the AspirationalItems special quest, so that the Spray n' Pray unique weapon correctly uses the Defense Gun modifications instead of the SMG ones (possibly resulting in a frankenstein weapon otherwise). I tried to make it so that the mods that appear on this Spray n' Pray are equivalent to the ones that would appear on the regular SMG version, so it includes a Powerful Automatic Receiver, Snail Drum Magazine, Fixed Stock and a Silencer. Legendary effect wasn't changed and other mods will be random.
  

  

Possible Issues and Incompatibilities:

It's possible that the Defense Gun will be a more common weapon around the Commonwealth, this is because the original mod already places the gun in several leveled lists, and this mod will replace another weapon with it. Don't know how much more often you will see it, though.  

Having another mod that manually edits the AspirationalItems quest (like the Unique Uniques mod) or the LL_SubmachineGun and LL_SubmachineGun_SilverShroud leveled lists (like weapons that don't use LL scripts) will cause problems. Place the esp below the other mod that edit those entries to make this one work.  

If you are already in the middle of a playthrough, SMGs that you already saw will probably continue being there. Only "new" ones will become the Defense Gun, be it in vendors, enemies, etc.  
  

Installation:
  
Just drop one of the .esps in the Fallout 4 Data folder, then activate the mod either manually or with a Mod Manager. Please place the .esp below the original Defense Gun in the load order. 
  

Credits to YonaTaku for the amazing original mod!
  
Have fun! ^-^