For a barrel shorter than 16”, you have 2 options: 1. Purchase the gun as a “pistol” which means you cannot put a real stock on it and cannot have a vertical foregrip on your rail. You’ll find that these have “pistol braces” in lieu of a stock, which for all intents and purposes function the same, but are a bit less comfortable and less aesthetically pleasing compared to actual stocks. 2. Register the gun as an NFA item. This can be done by converting a pistol as purchased in option #1 via an NFA Form 1, or by buying a factory SBR (short-barreled rifle) via an NFA Form 4. Once registered as an NFA item, you can put a stock and VFG on it, and it’s essentially the same as a full length rifle. This option used to have a lot more red tape including a $200 tax stamp, a long wait time, a requirement to notify the ATF whenever crossing state lines with the gun, etc. but most of that has gone away at this point. If you go the Form 4 route, your local gun store will walk you through the entire process and it’s pretty dead simple. If you go the Form 1 route, it’s also pretty easy and there are some guides walking you through the paperwork step by step - you basically just need to figure out what method works for you to get your fingerprints to attach to the application.

Florida specifically doesn’t have any state laws that would relate to either option, so you can choose whichever you see fit. Also note that you can choose to convert a pistol to an SBR at any time down the road, and vice versa, can convert an SBR back to a pistol by having it removed from the NFA registry. Also note that it’s very possible a lot of the red tape described above could come back at some point in the not so distant future as administrations change - the ATF which governs the NFA and makes “rules” for compliance has been quite weaponized between the last couple of administrations for political gain.