Uscha,
I'm not sure where the 25 mils came from; the 'safety angle' under 1000 yards is 8mils for flanking fire and 5 mils for overhead fire, but mostly we would put a rough guesstimate of 15 degrees on it, which is actually a little bit bigger.
This came up before in conversation and a guy figured that this was just a peace-time range precaution, that we didn't really do that in combat. I replied that we absolutely stuck to that 15 degrees in combat as it's actually much safer on a range than in actual combat conditions, and riflemen that are worried about getting shot from the front by the bad guys really don't appreciate having to worry about getting shot in the back by the good guys.
"Technically if you stray more than that you need to move the tripod and screw up the settings."
Negative. If that's the situation you should be repositioning the gun into its supplementary position.
When you emplace a machine gun you pick that spot because it can cover your principle direction of fire (or PDF, a sector of fire your gun is responsible for in the overall company fire-plan) and your final protective line (a line that places fire across the front, or a piece of the front, of the unit's defensive position).
Within that position, you then pick a supplementary position, either to the left or the right (usually corresponding with which flank the gun is on), that is a right angle from your PDF, in case the enemy shows up in a place he wasn't expected.
Then you fall back a couple/few hundred meters and dig an alternate position, which can cover the same PDF as the primary. It will also have a supplementary position, though not able to cover the same area as the primary. This is why you see machine guns dug into L-shaped fighting positions; let's say the gun is emplaced. We're on the left flank, so the PDF is going to be to 12 o'clock, the FPL is going to be to 3 o'clock, and the supplementary position is going to be to the left, putting the new PDF at 9 o'clock.
The alternate position will be behind the primary, but the PDF will still be at 12 o'clock and the supplementary still at 9 o'clock, but it's a different axis than the previous 9 o'clock.
So, you're not screwing up the settings (the Range Card for the gun), the crew is actually trained to do that, as well as displace back to the alternate in a hurry.
V/R,
Jack