There is some debate as to the relative manufacturing quality of each nation's SKS production. The Chinese SKSs varied significantly even among new rifles with some having screwed in barrels, milled trigger groups and bolt carriers with lightening reliefs cut into them being at the top end and cheaper rifles having pinned barrels, stamped trigger groups and slab-sided bolt carriers – though overall quality and serviceability remained high. The main reason for the manufacturing variance comes from differences between rifles made for the Chinese army and those made for export. The Chinese types typically have chrome-lined barrels while the Yugoslav versions do not, resulting in some Yugoslavian carbines having bores in considerably worse condition than even the cheapest Chinese SKSs. The Yugo M59/66 rifles also are unique in having a gas shut off valve for grenade launching, which is a common source of malfunctions. While often encountered in well-used condition, Romanian carbines were as well-built as the Soviet versions. In general, carbines made in the USSR are considered the highest quality.