Hi RecruitMonty
MGs 34 42 15 17 and 81 are all developments of the the origional MG30 of swiss origins and there seems to be very little diference between them. That said the most commonly listed effective range for 34 and 42s is, as you first stated, 24 hexes.
see
http://www.wwiiguns.com/display_articles.php?id=9 for example.
So it is probably very acceptable to give the 81 this same effective range or thereabouts. 20 hexes perhaps, due to the shorter barrel and more central bipod arrangement.
If you do look at HMGs later then you might consider this. The standard rifle gets a range of 10 hexes, Sniper rifles get a range of 15 hexes, why, telescopic sight.
If we apply the same reasoning to the HMGs we get,
The standard HMG gets a range of 30 hexes, so the telescopic sighted HMGs -should- get a range of 45 hexes.
What the spcamo thinking is on the topic remains an unknown.
Best Regards Chuck.