Below is an context that describes a sql query, paired with an question that provides further information. Write an answer that appropriately completes the request.  # noqa: E501
### Context:
CREATE TABLE table_12834315_4 (barrel_length VARCHAR, rear_sight VARCHAR, muzzle_device VARCHAR)
### Question:
Name the barrel length for rear sight a2 for factory compensator
### Answer:
SELECT barrel_length FROM table_12834315_4 WHERE rear_sight = "A2" AND muzzle_device = "Factory compensator"