Early usage of the M16 version of the AR-15 in Viet Nam resulted in intense criticism of poor reliability. On-scene studies revealed that the barrel and chamber tended to accumulate rust pits in the humid conditions in Viet Nam that caused difficult extraction of fired cartridge cases. Chrome plating of the barrels and chambers solved the problem and the M16 has gone on to great approval. While the early AR-15s also were built with un-chromed barrels the moderate climate in the US and gentler use by owners resulted in few problems. 