All I've done in a level like this is have an AI set a satchel then run off and hide. Then set a trigger by the satchel so when a unit walks past it, the AI detonates it. Works rather well for player convoy ambushes.
I find the SPR to be somewhat inaccurate for long range, so that might not be your shooting thats the problem. Â But the M24 and M107 are more than accurate enough for 1000 meters (and over? probably my own error for greater distances to an extent).