i only just had the 2nd shematics (so first update) from the Dart Gun,and when you use it correct,its verry handy wapon
as it say`s in you pipboy -1000 R.leg and - 1000 L.leg

when i first tryd the gun and i just fired at raider,it had verry little effect and because its a slow reload gun,your [censored] before you can shot a 2nd time

but i just tested it on some Ghouls and this time shooting at the Legs,-after the ghoul gets hit it takes a vew steps and the DIES,(i hope it also works like this on other enemy`s,i asume it dos)

fun part whit the Ghoul mask,you just take a litttle cover and they have no idead where its coming from,when normally when you shoot a ghoul with a normal gun all the other ghouls comming at yeh,not this time
i could clear out the whole basemant/room under the Tennypennytower with ghouls whitout them comming after me

ps,iam not even shore if the wapon is upgraded,because it has the same nrs.as the standart DartGun,i just had the second schematics,but not shore if i have to update on workbench or if its done automaticly

To get the upgrades on the crafted weapons, you have to build one with the new schematics. Upgraded Schematics mostly increases the initial condition which is good as Repair for these is not something you can do yourself.

And, yes, used properly, the Dart Gun can be used to slow down Deathclaws enough for you to kill them with just about any other weapon. But, you have to use it on the legs. I think it also cripples arms, but that is not it's best tactical use.

From what I understand, it only cripples legs, but the effect will kick in regardless of where the dart hits.