Hello, I have had my eye on purchasing the KAC SR-15 block 3 11.5 SBR as my go-to home defense firearm. But I would also like the idea of owning a barrel with a length of 16". Is it perhaps possible to purchase a factory-new lower-receiver to the SBR and then purchase both an SBR upper and the 16" upper? Or possibly purchase the complete 11.5 SBR and slap the 16" upper on it? Just wondering if there are any legal laws that would add into this. Thanks!