38 receiver weapon mods are available for the pipe bolt action gun at Gun Nut 2, pipe revolver at GN 3, hunting rifle at GN 3, and combat rifle at GN 4.  I'm a big fan of the combat rifle but .45 ammo availability is low early on, and by the time the .38 receiver becomes available there is plenty of .45 ammo.  The .38 receiver is so weak compared to a fully upgraded .45 receiver (or .308 receiver that also becomes available at GN 4) that it's not a viable alternative so late in the game.
 
This mod is an attempt to make the .38 receiver a viable alternative in the early game by making the .38 receiver conversion available at GN 2 for all
three rifles (hunting, pipe revolver and combat).  I also increased the damage for the .38 modded rifles to be the same as the unmodded base gun (i.e. no 25% penalty).  I added one further small advantage by making the .38 receiver weight match that of the standard receiver.  For the combat rifle, this saves about 3 weight units.  So advantages of the .38 receiver are ammo availability and slightly lower weight which are useful early on.  Disadvantages are lower damage compared to the more advanced receiver mods available for those rifles even at GN 2 -- and the damage deficit grows with each level of Gun Nut.  By the time you achieve GN 4 the .38 combat rifle has damage that is less than half that of a fully modded .45 or .308 combat rifle.
 
This mod was made using the Creation Kit beta and cleaned with FO4Edit.