This is muzzle brake for relatively rare M14 left-hand thread. This type of thread was used for some Saiga carbines in .308 caliber, old AK 47 and few Vepr carbines. Main advantage of Saiga 308 muzzle brake – versatility. It can be used in day mode (without sleeve) and in night mode (with metal sleeve). Major part of recoil pulse if compensates by 6 properly shaped IPSC-style chambers. You will notice recoil damping even with powerful .308 Win caliber. At the night you usually shooting with night vision scope and metal sleeve is designed specifically to complete elimination of fire flash and therefore do not disturb night vision scope operation.