I have a similar 18 cubic foot "26 Gun" Tractor Supply Winchester safe as well. I have
14 rifles,
4 shotguns, and
5 handguns. Rifles vary from from AR's, magnum bolt actions, lever actions and 10/22's - 31" to 45" OAL.
They all fit - BARELY (I could get another 4 to 5 handguns on the top shelf and I am working on that
). I'd be happier with another safe and loading half of what I currently have in it so it is easier to access my guns. So, 8 long guns and 5 handguns would be a very comfortable fit. I would say up to 12 long guns would be easy, with easy access, without starting to get creative.
I haven't removed the shelves with grooved barrel rests, but I've used some eye loops to anchor bungy chord to so that the rifles can sit at a different angle and not require gravity to hold them in place, or they can sit barrel down. I remove some rifle bolts so bolt handles don't conflict. One AR lower and upper get separated. Just ordered some gun socks to keep them a little better protected, but I was always very, very careful when putting them in and taking them out. I actually had to take a picture so I could remember just how they would fit back in the safe.
I haven't tried rifle rods to increase the capacity. It seems if I stick more guns into this safe, no matter what method I use, it will just make access more difficult. If I get any more long guns, I'm definitely looking at another safe, or a bigger safe.
Fire rating and theft resistance didn't come much into play. Mine is mainly for safe storage with kids at home and offering resistance to smash and grab burglary. I am just assuming if my house burns down, they will be a total loss and insurance will cover it. Regarding theft resistance, it will definitely stop a smash and grab. I have it bolted to my floor and it is tucked in a corner with a full freezer. That and locked doors will slow them down a few minutes and hopefully cameras, motion detectors, etc. will help notify me, so when they hear my voice telling them the police are on the way, they'll decide they don't have those few minutes.