The $5 Tax is for the transfer of an AOW.
An AOW is:
1) A Gun that is neither a rifle, shotgun, or a handgun under 18 USC 44 Sec. 921. This means, that it must have never had a stock. Otherwise it would be a Rifle. It must also have a Vertical Foregrip as without one it would be considered a Pistol.
2) A smoothbore Pistol.
3) A Gadget Gun (Cane Gun, Ring Gun, etc...)
4) A smoothbore gun (Shotgun) that never had a stock, or it would be a Shotgun.
Any of the above firearms will transfer for $5. But, making on a Form 1 still costs $200. It might be cheaper to find a C2 Manufacturer who can register it Tax-Free. He will charge a fee for the service. Then you can get it back for a $5 transfer fee. If you make one on a Form 1, then you don't have to hassle with a C2. But, on an AOW you loose all the other advantages of not using a C2. Since an AOW cannot be assembled until the Form 1 comes back, I would go the C2 Route. But, as previously stated opinions may vary. One advantage is if you can find a C2 (Unlikely but possible) that has his own range, he can build the AOW, register it in 24 Hours and let you shoot it on his range until the Form 4 goes through for you to buy it. A Form 1 is definitely preferable on SBRs, and any other converted gun. This allows you to continue using your AR with 16"+ Barrel until the Form 1 comes through. Then you can convert it to whatever barrel length tickles your fancy. If you go C2, then you cannot get your gun back until the Form 4 goes through and it will actually cost you more than doing it yourself.