I still remember the VD films from basic training. She's such a nice girl! She looks so clean! She says she has never been with anyone else!
50 years later and I've never had VD (honest, baby, I swear!), I've never had to worry, and I've never fired anyone else's reloads in my guns.
YMMV
Many years ago as a young and impressionable youth I caught one dose of something I shouldn't have. This was back in the late 70's before AIDS/HIV. I was a callow 18 yo and the lady was the other side of 35 (and you think Cougers are a new innovation?).
I was once given some .308 reloads to shoot in my L1A1 SLR. I had no knowledge of the loader at all so I pulled them down, salvaged the bullets and reloaded the primed cases.
Early last year my gun broke down first day of a two day comp and couldn't be repaired. Another shooter who I have shot with several times but don't know away from the competition circuit let me shoot his spare 1911 .45 with my reloads.
A few months ago I was running a club shoot and someone turned up to observe. I offered to loan them my Kimber 1911 9mm as I had been sighting it in before the event started and I had enough ammo for it onhand. The other shooter replied he had ammo. My club president immediately said "We don't let others fire their reloads in our guns". Turns out the guy had factory ammo but had never shot a 1911 (only a CZ clone) so I was happy to let him borrow my pistol.
The first weekend of this month I was shooting my Kimber in a match when, on day 2, my front sight drifted loose and I couldn't keep it tightened (I had been relighting it after some sight mods and forgot to put some loctite in the dovetail after removing and replacing the sight) . My club president offered to let me gun share with him, but only on the condition I shoot his ammo. Not because he didn't trust my reloading (he has watched me shoot my reloads for years) but because
his ammo was what
his gun was sighted in for. Suited me, I didn't have to withdraw form the event and took all my ammo home.
This weekend I shot a competition. A club mate and off range friend was having issues with his 1911 A1. His ammo wasn't firing. He thought his firing pin was broken so I offered to let him shoot my spare 1911. That too suffered some misfires. I thought it was his ammo (his primers were quite high) but didn't have enough ammo with me for him to shoot too. Overnight he pulled out the firing pin and found the spring gunked up and semi compressed. Today he still had the odd misfire. But I have shot with him for years and trusted his reloading.
Bottom line, if it's someone I know and have shot alongside for some time then maybe I'd let him shoot my ammo. But if I don't know them then its factory or
my ammo. If I'm borrowing a un then it is the owner who makes the decision.