CREATE TABLE table_17839 ("Colt model no." TEXT, "Name" TEXT, "Stock" TEXT, "Fire control" TEXT, "Rear sight" TEXT, "Forward assist" TEXT, "Case deflector" TEXT, "Barrel length" TEXT, "Barrel profile" TEXT, "Barrel twist" TEXT, "Hand guards" TEXT, "Bayonet Lug" TEXT, "Muzzle device" TEXT)

Name the barrel length for rear sight a2 for factory compensator
SELECT "Barrel length" FROM table_17839 WHERE "Rear sight" = 'A2' AND "Muzzle device" = 'Factory compensator'
SELECT "Barrel length" FROM "table_17839" WHERE "Muzzle device" = 'Factory compensator' AND "Rear sight" = 'A2'