If "the guy" has it on a Form1 or Form4, he can transfer the entire firearm to you for a $5.00 tax stamp.
This does not include the cost of the gun, transfer fees, shipping, etc.
I wouldn't recommend buying just the foregrip, without the required
Keep in mind that the tax for manufacture of an AOW is $200.
The $5.00 tax is for subsequent transfers.
It makes more sense to go the SBR route, IMO.