Blowgun, does it use Proficiency or not

As far as I can tell my level 3 Rogue/Thief with 16 Dex should have a +5 to attack with a Blowgun (which he is proficient with).  He is also proficient with a Dagger which is +5 to attack.

The character builder currently claims my Dagger is +5 to attack (correct) but is only +3 to attack with a Blowgun.

Is the character builder bugged regarding this or am I missing something?

Hmm.  

Oh.  I see the problem.  I am confusing Proficiency with Mastery.  I took at as a Mastery and (apparently) I assumed that meant I was proficient with it.

I guess you answered my question as "you are missing something". ;)  Thanks for drawing my attention to the appropriate rules.

Quote from wagnarokkr >>

Technically, you are only allowed to choose Weapon Mastery with a weapon you're already proficient with. Unfortunately, D&D Beyond's character builder doesn't enforce this correctly. It's only really an issue for Rogues, since all the other classes that have a Weapon mastery feature are already proficient with all weapons.

It would also be an issue for anyone who takes the Weapon Master feat (I think many of us forget it exists, intentionally or not).

Quote from wagnarokkr >>

[...] It's only really an issue for Rogues, since all the other classes that have a Weapon mastery feature are already proficient with all weapons.

 Barbarians are a little different too: only Simple and Martial Melee weapons.

I will just add, it is very strange to me that Rogue/Assassins do not automatically gain proficiency in blowguns. I can see the argument for not giving them proficiency in all martial ranged weapons, but it feels like there should be a cutout that gives assassins (at the very least, if not other subclasses of Rogue) proficiency in blow gun.

That's just my feeling though, and should not be interpreted in any way as me saying or implying that they get it, or I think that they get it, under the current Rules as Written.