My field-test for the settler ammo thing happened like this:
I was equipping a guard at one of my settlements with an assault rifle. I saw something moving in the distance just before I was about to open up the trade menu with him- VATS'd it - it was a bloatfly or something. I took a couple of shots at it, making sure to miss and get him and his buddies coming in closer, just to see how the ammo thing was going to work on a settler with a non-default weapon. I gave him the assault rifle and ONE round of 5.56 ammo. The fly got closer, my guard said "Whu...? HEY!" ~POPOPOPOPOPOPOPOPOP~ .... he fired like 20 shots at a dam bloatfly with only round of ammo, and it was still in his inventory afterwards.
On the other hand, actual companions eat ammo for real. I had one I had given the railway rifle to... she fired like 30 spikes at one guy to take him down. Seems like they will burn through ammo at the maximum fire rate of whatever weapon you give them.
It occurs to me you could test the rate of fire settlers give out with whatever weapon you give them by going to a settlement with very few settlers so you can really watch the results in detail, do a save, give them whatever weapon you're testing, and open fire and attack them or somebody else there and see how they respond with volume of fire when they turn hostile to you. Re-load and make nice afterwards.