my only idea is to give them same availability as laslocks? any other advice?

The laslocks used in the 30k game alongside Mitralocks aren't too well represented by the laslock rules in Only War.

For Mitralocks I'd go:

Basic, Range 20m, Rof S/-/-, D10+4 E, Pen 2 Clip 8,Rld Half, Special: Scatter, Reliable

It should also have some rule that makes it easier to fire one-handed....reduce the penalty to -5 perhaps.