Ammo Maths

Almost as soon as I posted Tracking Ammo it dawned on me that an obvious series of questions arises:

• If I find a handgun with a 5 rating, what's the likelihood that I will get at least ten shots from it?
• If I have a shotgun with a rating of 3, how many shots am I likely to get from it?

More generally:

• If I have a gun with a rating of X, what is the probability, p,  that I get N shots from it?
• If I have a gun with a rating of Y, how many shots, Z, am I likely to get from it?

These questions are all inter-related, so a bit of thought on one or the other will start to get some answers. I'll have a think and get back to you over the coming days.