What is the rear sight type on the model with the acr muzzle brake device? 
CREATE TABLE table_19901_1 (rear_sight_type VARCHAR, muzzle_device VARCHAR)
SELECT rear_sight_type FROM table_19901_1 WHERE muzzle_device = "ACR muzzle brake"
<s>[INST] Write mssql query to answer the following question given the database schema. Schema: CREATE TABLE table_19901_1 (rear_sight_type VARCHAR, muzzle_device VARCHAR) Question: What is the rear sight type on the model with the acr muzzle brake device?  [/INST] SELECT rear_sight_type FROM table_19901_1 WHERE muzzle_device = "ACR muzzle brake" </s>