I am considering (thinking it out is as far as it has gotten) building a 'short barrel rifle' and was looking for some help/advice.

First, who is the registration done with? I am assuming it is the ATF, but their website blows and I haven't located (not saying it isn't there, I just haven't found it yet) any information.

Second, what part of the rifle is registered? Let me clarify. Let's use an AR15 for a hypothetical example. It is my understanding that I can swap uppers on a legal SBR. I can take off a 7" or 11.5" upper and slap on a 16" or 20". Correct? So, is it the lower that is the portion that is registered since the uppers are changeable (and the fact the lower has the serial number)?

Any drawbacks to registering a SBR? Can I sell it later if I need to/decide to or does the SBR become non-transferrable?

Any help is appreciated.

Thanks.

You register it with the NFA.  Only the lower is registered.  You can sell it as an SBR or unregister it.

go here:  http://www.recguns.com/Sources/IIF1.html

and here:
http://www.titleii.com/

Thank you very much. I will read the links this evening. You definately answered the one question I was looking for that I didn't spell out; that it is the lower that is registered and that is can be unregistered.

Make sure to check state and local laws and regs as well, just because the federal government allows it, doesn't mean that it's legal in your locale.

Yeah, I had already taken care of that part. I am in GA (thank God). Gun laws, although they have tightened somewhat over the years, but comparitively, are still fairly conservative (thank God again!!).

For those wondering, here is state by state breakdown:

state MG  SI  SR  SG  AOW  LBDD  EXPDD  Comments
AK    Y   Y   Y   Y   Y    Y     Y  
AL    Y   Y   N   N   Y    Y     ?
AR    Y   Y   Y   Y   Y    Y     ? (state registration of pistol
cal. MG's over .30)
AZ    Y   Y   Y   Y   Y    Y     Y  
CA    Y   N   Y   Y   Y    Y     Y (requires discretionary and
rarely issued permit for mg, lbdd or expdd from state Dept. of
Justice; no AOW pen guns; C&R sg, sr only)
CO    Y   Y   Y   Y   Y    Y     Y (requires state permit for
expdd)
CT    Y   Y   Y   Y   Y    Y     ? (no select fire mg's-full
auto's only, after 1993 assault weapon ban, state registration of
mg's)  
DE    N   N   Y   N   Y    Y     N (no smooth bore pistol AOW's)
FL    Y   Y   Y   Y   Y    Y     Y  
GA    Y   Y   Y   Y   Y    Y     Y (no incendiary expdd's)
HI    N   N   N   N   N    N     N (A clean sweep!, the only
state like this)  
IA    N   Y   Y   Y   Y    Y     Y (only si, sr, sg, lbdd and
expdd designated  as collector's items by the Comm'r of Public
Safety, basically the C&R list)
ID    Y   Y   Y   Y   Y    Y     Y
IL    N   N   N   N   Y    ?     N  
IN    Y   Y   Y   N   Y    Y     N  
KS    N   N   Y   N   Y    Y     ?
KY    Y   Y   Y   Y   Y    Y     ?
LA    Y   Y   Y   Y   Y    Y     Y (mg's require a permit to
purchase - war relics only; mg's, sr, si, sg and some expdd's
require a permit to purchase)  
MA    Y   N   Y   Y   Y    Y     N (license for mg's required)  
MD    Y   Y   Y   Y   Y    Y     N (mg's must be registered)  
ME    Y   Y   Y   Y   Y    Y     Y
MI    Y   Y   Y   Y   Y    Y     Y (apparently approved form 4
suffices for "license" for mg, si or some expdd (bomb) despite AG
opinion reprinted in ATF Green Book; no incendiary expdd; C&R sr,
sg only)
MN    Y   N   Y   Y   Y    Y     ?  (C&R mg, sg only,
registration required)
MO    Y   N   Y   Y   Y    N     N  (C&R mg, sr, sg only to non
FFL holders, C&R FFL holders any mg, sr, sg)  
MS    Y   N   Y   Y   Y    Y     Y
MT    Y   N   Y   Y   Y    N     N  (pistol cal. mg's over .30
must be registered with state)
NE    Y   Y   Y   Y   Y    Y     N
NC    Y   Y   Y   Y   Y    Y     Y  (sheriff's permit required
for mg's; must be FFL holder (including C&R) or must be for
"scientific or experimental  purposes" for a mg, si, sr, sg lbdd
and expdd)  
ND    Y   Y   Y   Y   Y    Y     Y (fed. "licensees" required to
register mg's, si, expdd with state when possessed for
"protection or sale")
NH    Y   Y   Y   Y   Y    Y     Y
NJ    Y   N   Y   N   Y    N     N (mg requires discretionary and
rarely issued permit from state court)
NM    Y   Y   Y   Y   Y    Y     Y  
NV    Y   Y   N   N   Y    Y     Y
NY    N   N   N   N   ?    Y     N (some pen guns may be allowed)
OH    Y   Y   Y   Y   Y    Y     Y
OK    Y   Y   Y   Y   Y    Y     Y              
OR    Y   Y   Y   Y   Y    Y     Y (no incendiary expdd's)
PA    Y   Y   Y   Y   Y    Y     N  
RI    N   N   N   N   Y    Y     ?
SC    N   Y   N   N   Y    Y     ?
SD    Y   Y   Y   Y   Y    Y     Y
TN    Y   Y   Y   Y   Y    Y     ?                        
TX    Y   Y   Y   Y   Y    Y     Y
UT    Y   Y   Y   Y   Y    Y     Y  
VA    Y   Y   Y   Y   Y    Y     Y (state registration of all
mg's)
VT    Y   N   Y   Y   Y    Y     Y
WA    N   Y   N   N   Y    Y     N (silencer may not be used on a
gun)
WI    Y   Y   Y   Y   Y    Y     Y (permit required for expdd, no
incendiary expdd's; no pistol cal mg's w/o permit)
WV    Y   Y   Y   Y   Y    Y     ?
WY    Y   Y   Y   Y   Y    Y     ?  

KEY
MG - machine gun
SI - sound suppressor (silencer)
SR - short barreled rifle
SG - short shotgun
AOW - any other weapon
LBDD - large bore destructive device
EXPDD - explosive, incendiary or poison gas destructive device