in 5.56×45mm NATO as the NATO specified rifles can handle much higher chamber pressures than the .223 Remington is capable of producing. Using commercial .223 Remington cartridges in a 5.56mm NATO chambered rifle should work reliably, but until recently, it was believed this was less accurate than when fired from a .223 Remington chambered gun due to the longer leade. Although that may have been true in the early 1960s when the two rounds were developed, recent testing has shown that with today's ammunition, rifles chambered in 5.56×45mm NATO can also fire .223 ammunition every bit as accurately as rifles chambered in .223 Remington, and the 5.56×45mm NATO chamber has the additional advantage of being able to safely fire both calibers. Using 5.56×45mm NATO mil-spec cartridges (such as the M855) in a .223 Remington chambered rifle can lead to excessive wear and stress on the rifle and even be unsafe, and SAAMI recommends against the practice. Some commercial rifles marked as ".223 Remington" are in fact suited for 5.56×45mm NATO, such as many commercial AR-15 variants and the Ruger Mini-14 (marked ".223 cal", except the Mini-14 "Target" model, which only fires .223), but the manufacturer should always be consulted to verify that this is acceptable before attempting it, and signs of excessive pressure (such as flattening or gas staining of the primers) should be looked for in the initial testing with 5.56×45mm NATO ammunition. The upper receiver (to which the barrel with its chamber are attached) and the lower receiver are entirely separate parts in AR-15 style rifles. If the lower receiver has either .223 or 5.56 stamped on it, it does not guarantee the upper assembly is rated for the same caliber, because the upper and the lower receiver in the same rifle can, and frequently do, come from different manufacturers – particularly with rifles sold to civilians or second-hand rifles that have been repaired with spare parts. Since all parts are interchangeable, a shooter must take great caution to check for markings of 5.56×45mm on the barrel before attempting to fire 5.56×45mm NATO ammunition out of it. In more practical terms, most AR-15 parts suppliers engineer their complete upper assemblies (not to be confused with stripped uppers where the barrel is not included) to support both calibers in order to satisfy market demand and prevent any potential problems. Some manufacturers have begun offering a hybrid .223 Wylde chamber