Some of the factors in determining port size in a barrel may be barrel length, gas system (carbine, mid-length, or rifle), and barrel twist. Chambering may have some affect as well. A .223 REM or .223 Wylde may have a different port size than a comparable 5.56 chambered barrel due to the type of ammo used. I have seen a rifle-length gas system barrel (14.7") not do well with .223 REM ammo and do great with 5.56 NATO M193. This may be in part to have a slightly larger port opening. .223 REM ammo can be a bit under-powered and fail to cycle in some barrels with larger port openings. An example to note is that of Colt's Model 605. Initially, they used a full-length 20" M16 barrel (rifle-length gas system) of which they cut down to 15". They never made any compensation for the shorter barrel length and retained the same gas port size. This resulted in numerous cycling issues and the weapon was out for only a year before they scrapped it. Since then, others have figured out the issue and started opening up the gas port just slightly and it drastically improved the cycling. Mine works great.
"A Bad Day At The Range Is Better Than A Great Day Working"
USMC Force Recon 1978-1984
US Air Force Res. 1995-2004 (Air Transportation)
M16/AR15 shooter since 1978, gun collector and AR builder since 2004
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