Hi, in dark heresy the ballistic dendrite came with the gun already mounted, but in OW it doesn't seem to come with a gun. Does that mean fresh tech priests with a ballistic dendrite just walk around with a stumpy tentacle until they can rustle up a compact pistol?

As written/implied, the mechadendrite is indeed offered weaponless. More lenient GMs might bundle a compact stub automatic or laspistol with a player's first ballistic mechadendrite. I am not familiar enough with OW's starting equipment system to know if Tech-Priests are privy to Compact Pistols as part of their standard kit.

As Asymptomatic said.

Personally, I'd definitely allow the player to start with a mounted, basic, compact pistol weapon.

If acquired during play, I'd expect them to sort that kind of thing out themselves, but it's a bit ridiculous to have someone deploy without arranging something so elementary beforehand.

Unless you're facing something terrifying, you'll pick something like that up from the first enemy that keels over, anyway.

I'd figure that the 'default' is a generic laspistol with an integrated power supply.

Utilizing a different weapon will require acquisition and probably modification of that weapon.

I'd warn player that wants to use it that it's the crappiest mechadendrite there is.

I'm now imagining a tech-priest in the field facing down traitor guard with his lascarbine and he's mumbling in binary:

"....would be more logical.....gave it the *redacted* laspistol"

As he proceeds to wave around his so-called "stumpy tentacle"

As for a real answer, I would say a laspistol can be powered by their potentia coil, and I would argue against it being the crappiest mechadendrite on the grounds that if the enginseer has asuch a weapon he is definitely in a bad place (granted it only shoots people rather than utilitarian jobs, but this is the IG son)