HI Guys:
I'm still looking form my firing tables (my library is a disaster right now) but based on the angle and a low charge (as Herb suggested- charge 1 is likely) the range is only a couple of klicks. My bet is there was a high value target on the reverse slope of the ridge in front of the gun. It may have been that the range slightly exceeded that of the 81mm mortars so the gun was dug into the very odd gun pit!
Unusual that the tractor is parked in front of the gun unless it is to screen the view of the gun from the ridge. The muzzle brake appears to be painted in red oxide primer which is also odd.
Clive: do any other photos from that series show any other guns? I suspect that this is a sniping gun task on a programmed shoot. I wonder where the observer was?
P.S.: Guns have "detachments", tanks and ships have "crews"!

Mike