CREATE TABLE table_19901_1 (rear_sight_type VARCHAR, muzzle_device VARCHAR)

What is the rear sight type on the model with the acr muzzle brake device?
SELECT rear_sight_type FROM table_19901_1 WHERE muzzle_device = "ACR muzzle brake"
SELECT "rear_sight_type" FROM "table_19901_1" WHERE "ACR muzzle brake" = "muzzle_device"