IIRC the SBR classification doesn't limit you to a rifle stock, you should be able to put a pistol stock on it if you want. Many people make "pistols" out of Savages and just keep the barrels over 16 1/4" and the OAL over 26" (not sure on the 26", may be 28" can't remember) and as long as the OAL of the gun from muzzle to the rear most part is over 26" and the barrel is over 16" its still legally a rifle.
As far as Striker barrels, many people that have True Strikers or the "Pistol Styled Rifle" (as described above) just use standard rifle sporter barrels and cut them down to 16 1/2 of 17 inches.
If it were me I'd just trim down your rifle barrel to 16 1/2" and then you don't have to mess with SBR legallities and still have a very short carbine length rifle and be totally legal.
JMHO and check all laws yourself before precceeding.
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