In a nutshell:
A regular 21" barrel has a certain pressure impulse for a time duration equal to the amount of time the bullet is forward of the gas port and before exiting the muzzle.
When you shorten a barrel the impulse at the gas piston remains the same but the time period the impulse exists is reduced due to the shorter length of barrel between the gas hole and the new muzzle crown.
By opening the gas port you are increasing the impulse to compensate for the decrease in duration.
On a metric FAL you should not need more than .116 to .120 for even a 16" carbine with new recoil springs.
I don't recommend going larger than .120 Some have gone .125-.130 with no ill effect. I just feel that practice is not the preferred remedy. Fix the problem preventing you from using a .115-.120 port size.
The causes could be many and are cumulative and could be problems other than with the gas system (carrier/rails/recoil plunger/recoil tube/etc)
drill # size chart:
40 .0980
39 .0995
38 .1015
37 .1040
36 .1065
35 .1100
34 .1110
33 .1130
32 .1160
31 .1200
A regular 21" barrel has a certain pressure impulse for a time duration equal to the amount of time the bullet is forward of the gas port and before exiting the muzzle.
When you shorten a barrel the impulse at the gas piston remains the same but the time period the impulse exists is reduced due to the shorter length of barrel between the gas hole and the new muzzle crown.
By opening the gas port you are increasing the impulse to compensate for the decrease in duration.
On a metric FAL you should not need more than .116 to .120 for even a 16" carbine with new recoil springs.
I don't recommend going larger than .120 Some have gone .125-.130 with no ill effect. I just feel that practice is not the preferred remedy. Fix the problem preventing you from using a .115-.120 port size.
The causes could be many and are cumulative and could be problems other than with the gas system (carrier/rails/recoil plunger/recoil tube/etc)
drill # size chart:
40 .0980
39 .0995
38 .1015
37 .1040
36 .1065
35 .1100
34 .1110
33 .1130
32 .1160
31 .1200
