Workbook

powerful the ejection of the spent case. On weapons with a last-round bolt hold-open, open the gas

port all the way, then fire with one round in the magazine. Close the port one number at a time until

the bolt locks open consistently 5 times. This is your maximum gas setting. For reliability, drop one

or two numbers (2 to four clicks) more. If you have an L1A1 style gun, or one that lacks the last-

round bolt hold-open device, you will have to experiment to determine the ideal setting. If it only

works on 1 or two, there is not much room for error, such as when your gas port begins to foul. The

remedy is to increase the diameter of the gas port.

Access the gas port by disassembling your front sight assembly, removing the gas regulator, plug and

piston. On metric guns, remove the front sight. On inch pattern guns, the gas port is drilled

diagonally from behind the front sight into the barrel. On metric guns, the hole is vertical through a

hole behind the hole through which the front sight is threaded. Both guns with standard length

barrels have a nominal gas port diameter of .096 - .098”.. A conservative gunplumber will increase

the diameter a bit at a time, until the desired pressure is achieved. With an 18" barrel, I am usually

successful with a .116" to .118" . 16" barrels usually work with a .118, occasionally with a .120. At

one time, inch pattern rifles with 21” barrels were “product improved” to a 2.75mm (.108”) I

recommend, however, that you make the hole no larger than necessary for the desired results. Too

large a gas port can "shave" bits of copper off the projectile, causing fouling and malfunctions, or rip

cartridge rims due to the chamber pressure being too high at the moment of attempted extraction.

The following table identifies relevant drill sizes.

#41 .0960"

#40 .0980"

#39 .0995"

#38 .1015"

#37 .1040"

#36 .1065"

#35 .1100"

#34 .1110

#33 .1130

2.9mm .1142"

#32 .1160"

3mm .1181"

#31 .1200"

3.1 mm .1220"

1/8" .1250"

If a full length barrel with a .096"+ gas port is not cycling, it is probable that the gas piston is too small in

diameter or the tube is too large. There should be no lateral movement of the piston in the tube. This allows

gas to blow by the piston, instead of actuating it. The easiest remedy is to buy a new piston. Failing that,

you can build up the tube or piston with nickel plating or hard chrome until they are a tighter fit. Insure also

that the gas tube is indexed properly and the retaining roll pin installed correctly. There are two holes

pointing downward oblique to the gas tube. As the piston reaches its maximum travel, it clears these two

holes, allowing any excess pressure to bleed off and reduce wear on the gun.