Gentlemen,
The Harrier close-support a/c does not have a 25mm gun. The only weapons available to it are the air-dropped bombs or rockets.
Source:
http://www.raf.mod.uk/equipment/harriergr7.cfm
http://www.raf.mod.uk/equipment/harriergr9.cfm
Bit of a procurement shambles: although the early a/c were fitted with gunpods, they were empty. The GR5/7/9 were never able to use the gun operationally, although the pods remain: they have a useful function in managing airflow whilst in the hover, or some such.
NB1 - these comments do not relate to the AV-8B family
NB2 - the Harrier GR1/3 had a gun that worked!
Again, not earth-shaking or vitally important to gameplay - but for the sake of accuracy you might wish to note this.
And I did try to list the Unit No.s, but there were so, so many for the GR5/7/9........