Don't know if this continues to work with Resistance 1.75 but under 1.46 or below if you alter the 'magazinetype' from the default '256' (ie clip takes up 1 slot) to a fraction such as...
magazinetype=0.1*256
then you have a single shot rifle.
However there are some bugs such as you can carry as much ammo as you want (100, 1000s of rounds), they don't show up in the GEAR section, and unless you fill up the soldiers slots with other clips or grenades he'll continually report he's out of ammo (which he isn't).
OFP as it stands now just isn't designed for single shot weapons like this unfortunately.