I'm a serving soldier and I must admit, sapper's post doesn't quite ring true, unless he's a crow trying to big it up which isn't that unusual. Point I'd like to make is that I've used most weapons in the British army from SLR to HK's and I don't know what grain round any of them use.
What I do know is that I'm extremely proficient at using weapons and have fired 100's of thousands of rounds down the range and more recently against human beings. Just because someone doesn't know every single microscopic fact about a weapon or it's ammunition system doesn't necessarily mean they don't know