This is a good point to address. If I borrowed a rifle on a hunt, I would insist the professional allowed me to play with it and run rounds thru it AND shoot it some before hunting with it. I wouldn't let the tyrrany of the urgent prevent me from getting familiar with the gun.

I get to shoot other fellows' guns here on my range when testing bullets, etc, and am pretty comfortable now picking up an unfamiliar rifle and shooting it BUT, I find I am much more confident when I know the quirks of the gun, especially trigger pull. And boy howdy do some guns have quirks...

If a fellow is forced to borrow a rifle, I would strongly suggest he take a deep breath and go thru the same motions he would if he bought a new gun back home; handle it, test it for functioning, dry fire it and definitely shoot it at a reasonable range of something like 100 meters to check zero. And know enough about general ballistic performance to understand shot placement at other ranges.

About the strangest "borrowing" experience I've had was when a friend brought his AR50 .50 BMG single shot over to the house and we shot at 200 and 400 meters. Stock design was limited to shooting off the right shoulder...being a lefty I was glad I practice from the off shoulder from time-to-time! How interesting it would be for the poor lefty to show up at a camp deep in the bundu and be forced to shoot a Winslow with a Tsunami Wave rollover comb and all the controls on the "wrong" side...