I'll start with a link to my blog post on "Technical Details and Probability vs. Possibility". I'm playing that card on weapons as well.
While I grew up with guns and was a career Marine, weapons for me have always been simply tools of the trade. I don't read up on them, I don't know exact specs off the top of my head and I usually don't talk about them.
Heck I can't even tell you model of my own shotgun, I simply know that it's a .12 gauge, Remington pump-action shotgun. I took that gun off of one of my knucklehead Jarheads a couple of decades ago and have had it ever since. I clean it monthly but I don't really ever use it. So that's pretty much my deal with firearms.
Technically I know the weapons I've been trained to know and use, I also know far too much about air-launched weapons but those aren't of much use here.
I did do basic research when putting together his weapons collection, most of my research revolved around weapons I'd like to have to decorate my own home. So basically I went window shopping (via the internet) and drew up my own personal wish-list.
For this story I had him purchase custom replica's so they all shot the same round, which is what I would do if I had the money so that's what Mr. Narrater did. It also makes for easier story telling as I don't have to track how many of which type of round he has in supply.
Please feel free to PM me with corrections/recommendations or even ridicule me in the comments regarding this topic, it's never going to be my area of expertise so I won't be the least bit offended.
I always appreciate the input,
-Emmeran