It can also be down to how the rifle has been held.

Recently I was coaching a mate and was offered (force fed) hot advice by 2 experts who should have been minding it. Had he taken any notice and not shot L1A1s before he may have had a nast bruise.

Sadly, the lee really shoots best off the elbows, so if you rest it and get too low behind it the plate does not nestle into the pocket under the collar bone and belts it instead.

Likewise shooting with a sling will help pull it into the right spot. We had very young cadets who used to shoot without any bruises....nearly 40 years later I get my first bite by being sloppy and not snugging in there.

I am not for one minute saying this is the cause, but it might help.