Artillery Assignment
 

I have an on-map artillery with a range of 202 but it's not able to be assigned to a hex on a map less than 50X50. Specifically, hex 18,23 can't be targeted by the 105mm Schneiders. Why?

Re: Artillery Assignment
 

From the Game Guide:

Artillery have a built-in minimum range, 20% of their max range and locked to a maximum of 20 hexes, minimum of 5. So only some very short range on-map infantry guns will tend to have a min range of 5-19 hexes in reality. (There has never been any distinction between guns, rockets, or howitzers as to min range in the game series).
It's not spelled out in any detail in any manuals since it is obvious from the fact that the "cannot fire" indicator is shown if you try to plot the gun on too close a target hex in the bombardment screen. Mousing over the red circle gives an explanatory message.
If you really need to fire indirect at close-in enemy targets - then issue mortars to your troops, not infantry guns.

Re: Artillery Assignment
 

My current approach is to limit on-map indirect fire support to mortars, with bigger guns purchased as off-map assets instead.

Mortars tend to be much more flexible on-map assets, since they do not need to worry much about minimum ranges, and they can change position on their own, without needing a transport vehicle. They pack plenty of punch against infantry targets, and often come in sections of 2-3 tubes for some concentrated firepower.