In answer to your questions regarding the SA80: Yes, I would know and did know less than the manufacturer. As the end user all I needed to know was the effective range and the basics; but mainly that it worked when my soldiers and I had to pull the trigger.
As for knowing the muzzle velocity and trajectory of the rounds... 
I didn't need to know it and neither does the average infantry soldier. The sort of bloke who knows all of the useless facts is usually trying to mask inadequacies in other areas in my experience.
An interesting and amusing reply. You seem to be at least narrowing down your possible former unit with your reply, if nothing else.
I'll stick with being inadequate, and knowing after how many rounds my weapon may fail, or at which temperatures or conditions. I will be doubly bad, in knowing the limitations of the weapon, including how the trajectory would be affected by other countries ammunition. It might be boring to you, but then it can be and is a life saver. That said, and in your defence, a complete ignorance of velocities and trajectories could make folks feel a lot happier. But I guess knowing the reality of where to hide, and where not to was probably far more useful if the rounds are coming at you
It is up to you if you want to call yourself a Rupert (rather than say ex army or green for example), Maybe your reasoning was similar to mentioning your degree
