So I'm starting up a Deathwatch campaign and one of the players wanted to play an Adepta Sororitas rather then a Marine. She also like big guns and was considering a multi-melta.
But when we looked through the rules the only multi-melta we could find was the Astartes version which look way to heavy for a sister to carry.

So the question is, is there any other multi-melta in the rules or should we just modify the astartes version?

Fluff-wise there's not supposed to be a difference, and rules-wise it's easier to use the Astartes armoury for everyone. Soo.. That's what I'd do.

So the question is, is there any other multi-melta in the rules or should we just modify the astartes version?

Inquisitor's Handbook.

So the question is, is there any other multi-melta in the rules or should we just modify the astartes version?

Inquisitor's Handbook.

Thank you, I'll have a look there.

I'd still recommend letting the SoB use the DW armoury. As mentioned, they're supposed to be the same in the fluff, and in practical terms the game lines use different weapon profile. Multi-Melta being a nice example.

		DH		DW
Class:		Heavy		Heavy
Range(m):	60		60
RoF:		S/-/-    	S/-/-
Damage(E):	4d10+12		4d10+6	
Pen:		12		13
Clip:		10		12
Reload:		2Full		2Full
Special:	Blast 1		Blast 1
Weight(kg):	55		72

You'll already have to deal with the fact that Dark Heresy characters, while starting out far weaker, are generally superior to DW Marines in pretty much every way, when they hit the same RAW experience pool. Giving the SoB special access to a generally better armoury as well, will not make your life easier.

After Errata DW Multi-Melta has 2d10+16E dmg, Pen 12. Now it looks very strange in comparison with the DH MM.

DH was written before the invention of the Melta Quality,

DW too don't use Melta Quality.

I'm pretty sure it's in the DW errata. (?)

Nope

I say thee nay!

"Melta Weapons (page 148): The sentence “Astartes melta
weapons deal an extra 1d10 Damage to targets at Short Range or
closer” should change to “Astartes melta weapons double their
Penetration when firing at Short Range or closer.”

**** . I was only looking for "melta quality" and looked through the tables.

But formally, I was right: the " melta" special quality is not there

Anyway I assume that is why the damage was dropped. It relies on Pen 26 to get through tank armour, not on high damage.

Otherwise it would have too much overlap with the plasma cannon, I think.

Anyway I assume that is why the damage was dropped. It relies on Pen 26 to get through tank armour, not on high damage.

4d10+6 and 2d10+16? Just build the probability curves. They are not identical, but very close.

range 10-46 vs. 18-36. Average damage 28 vs. 27, slighty higher average damage and greater maximum damage in older version

BUT this does not include than Melta was formerly +1d10 damage at short range, making it actually 11-56 vs. 18-36, average damage 33.5 vs. 27.

As well as chance of Righteous Fury dropping from 40% (50% at short range) to 20%.

Whereas plasma cannon does, assuming the BC stats are the same, 14-32 (average 23), on Maximal 15-43 (average 28.5). (20% or 30% chance of RF.)

EDIT: look at it this way.

Against the front armour of a Chimera at short range (assuming the Only War stats, i.e., AP30) the pre-errata Deathwatch version of the multimelta does 17.5 average damage; the new version does 23. (Whereas the Dark Heresy version does 16 at all ranges.)

As opposed to a Deathwatch, BC, or OW lascannon, which will do 19.

While tha plasma cannon on maximal will do 10. BUT will have Blast (5); it's a heavy-infantry killer than can double against light vehicles.

I'm exhausted but I think my math is right.

To get back sort-of to the original post -- I think your player wilol be disappointed. The multi-melta, while a large gun, is a specialized weapon that is lackluster against anything but its prefered targets -- tanks and vehicles at short range.