I have to echo a sentiment expressed earlier; I can't stand people who don't wear face pro and then go ballistic when they get hit in the face. If it's a full auto dump from 10 feet, then by all means, but don't be that person (on either end of that scenario).
Everything else has already been said - don't ever call another players hits period, nothing good will be achieved.
Better to err on the side of caution when calling your own hits as well. If it's marginal, just call hit and you'll always be doing the right thing. I recall one game where I was crouched behind a tree and my dump pouch was sticking out and took an audible semi-auto hit from long range. While I was internally debating whether or not I was hit, a nearby team-mate actually told me "that's a hit, you need to call that" and since then I've been pretty cautious with it.
That's not to say it won't come up because it does. Last game out I made an aggressive pistol rush on a group of players, hit the deck behind a plywood barricade, and did some peek shooting. After my squad cleaned up and the dead were leaving the area, one of the dead players asked me if I was sure I hadn't been hit as he could have sworn he hit me in the helmet. I didn't feel anything, and I only heard BBs smacking off wood. It's just the nature of the game.