When you fill out the form and send your $200 out on a dream, do you have to provide the S/N for the future purchase and transfer, or does the S/N get recorded once approved at the FFL transfer?
Which form are we discussing? For a silencer or short bbl rifle (SBR)? In either case yes they will need a serial# when you apply for the stamp. If you are manufacturing the device you can provide your own number. If it’s a SBR the receiver serial number is what will be used unless you manufactured it. Same with the silencer, you manufacture it you provide a serial number.
I am thinking more along the lines of having a preqalified stamp in case an SBR, automatic firearm, or suppressor becomes available and not have to have the seller wait around for the stamp approval to sell it.
I obviously don’t know much about this, are there different unique forms for each item I just listed?
The Form 4 is an “Application” to transfer and register a Class3 item. You are applying to the Gov’t to get permission to own your toy once you buy it. So yes you have to buy it first because the serial number is what you are registering and paying the tax on. Most shops that I am familiar with usually have a small fee in case you are not approved. Which makes sense because while you both wait that item is not available for sale or transfer. Shops with ranges on the premises might even let you shoot your toy before approval. The wait times are just the cost of doing business, they are used to it.
Sadly prequalifying or approval is not available. But as Craig6 noted the fine folks that deal with these items are good about the wait times. Personally I hate the wait time.