Workbook

Copyright 1997-2001
by T. Mark Graham. All Rights Reserved.
LESSON 205: TUNING THE GAS SYSTEM
(Revised 09-22-00)
There are several factors that contribute to proper cycling in a FAL. Most important is the gas
pressure created from the burning propellant that forces the projectile down the barrel. As the
projectile passes the gas port, the pressure works in three directions. First, it continues to propel the
projectile down the barrel; second, it is exerted outwardly from the inside of the cartridge case
against the chamber walls. Third, it travels through the gas port, through the gas block/sight
assembly, and puts pressure on the face of the gas piston, driving it back against the bolt carrier and
initiating unlocking and cycling. The timing of this step coincides with a drop in the pressure exerted
inside the cartridge case against the chamber walls, which allows the cartridge case to extract. When
the projectile exits the barrel, pressure drops to zero because the gas vents through the end of the
barrel. There is a pressure-time curve; critical to cycling the action is not just the total volume of gas
and gas pressure, but how long that pressure is maintained. The longer the barrel, the longer the
projectile blocks gas from escaping out the end of the barrel and directs it through the gas port. A
shorter barrels mean a shorter time that the projectile is between the gas port and the end of the
barrel, and must have a larger diameter hole to bleed the same volume of gas in a shorter time frame.
If the hole is too large, the outward pressure exerted by cartridge case has not dropped sufficiently
for the case to be extracted, but the piston is forcing the bolt carrier to the rear anyway resulting in
extractor failure or the extractor rips the rim off the cartridge.
Another factor is muzzle devices. L1A1 style flash hiders are designed to dissipate and cool gas in as
large an area as possible, thereby reducing the flash signature. Muzzle brakes that have an exit hole
of a smaller diameter than the inside of the device, trap gas in this expansion chamber. As the larger
volume of lower pressure gas strikes the portion of the muzzle device where the exit hole is reduced,
it creates back pressure, which adds to the pressure on the gas piston. This is why removing a
muzzle brake from a gun that was tuned with one in place may prevent it from cycling.
A gas plug fits into the front sight assembly and both retains the piston and directs gas flow. There
are many styles and not all are compatible with all front sight variations. Israeli are shorter than
metric and use a longer gas piston. Inch pattern
are also shorter than metric, but use a gas piston
that is slightly different, but interchangeable with
metric. There is a hole in this piston that directs
gas to the piston. Rotating this piston 180
degrees blocks the gas from hitting the piston.
This position was used for imparting maximum
pressure on rifle grenades, but has caused many a
novice panic at one time or another over their
"single-shot" rifle. A line, a scallop cut, an "A"
or a Hebrew aleph (א) usually indicates the firing
position. The single shot grenade position is either unmarked or marked with a "G" or a "Gr." As the
gas passes through the plug and hits the piston, the piston begins moving rearward. At this point, the
gas pressure may vent through a hole at the rear of the front sight assembly. A rotating ring called a
gas regulator covers this hole. The gas regulator is numbered, depending on variation, from 1 up to 7
on metric, from 1-11 on inch (sometimes not numbered at all, such as Brazilian guns). At 1, the hole
is completely covered, imparting maximum pressure on the piston. At the high end, the hole is
uncovered, allowing gas to bleed off and reducing pressure on the piston. The pressure may reduced
so much as to prevents cycling.
Different ammunition has different pressure curves, so you must experiment with each type of ammo
you use. The higher the number (the more gas bleeding off), the less violent the recoil and the less