As one of the earlier posters pointed out, having both a rifle and pistol model at the same time, whether at the range or at home is not illegal, but having a rifle and an unattached pistol upper is cause for suspicion. The logical question anyone would ask is "Why do you have that?".
Definately do not put yourself into a situation that can, at the very least, cost you thousand of dollars to get out of.
Realistically tho, if the authorities are rooting around your house or person you've probably already got big time problems, that's just icing on the cake for them.
The process of obtaining an SBR, provided they are legal in your area, is easy. It just requires some leg work on your part, some spare cash and a bit of patience. If you want one go ahead with the process.
SBR's and Suppressed weapons are the hottest areas in firearms right now, people seem to have realized that full auto weapons have been pushed out of the price range for the average Joe/Jane and with the cost of feeding the same are just not feasable.
But anyone who is capable and has the initiative and funds can go thru the process and have something just as cool, but with a much smaller appetite.
I have a 10.5" AR (5.56) that i really like, does everything I want except make a loud, sustained blast. Really gets the attention at the range and is surprisingly accurate. I also have paperwork in for a YHM suppressor for my SIG 226 TAC, very much looking forward to gettiig that.
Enjoy the hobby but do be aware of the laws and consequences.
RD