You're missing out. I get these as soon as their available. With gun nut 3 and 4 it unlocks all the best mods for all other guns besides the 10mm (pretty sure you can fully mod the 10mm with just level 2). Even the .44 you need level 3 and 4 to have Powerful and Advanced receiver, respectively. When you have Powerful, it's like 70-80 dmg and with Advanced it's over 100 dmg. So if you're only level 2 you're probably around 30-50 dmg (depending on what kind of Perks you have, like Gunslinger for example). Of course you might be able to find a gun here and there with these parts, then remove them and add them to one your guns, but that can be tedious work to find all that shit.