Port pressure in a M1a

[rammac]

What is being described is adjusting the base volume in the gas cylinder. This will require more time for the gas to get to pressure to move the operating rod thus delaying openin a fraction of a millisecond. The adjustment in volume available here isn’t enough to matter. The gas plug is about a half inch in diameter and the thread pitch is 32 TPI if I recall (.032/rev) correctly. So using round numbers, the area of the piston is 3/16 sq inch and the length removed from the plug is .050. But the plug has a hole in the center and this would reduce the volume of material by about half. So, this means the volume increase is about .005 cubic inches.

If you want to increase the volume, ream the bore of the gas plug a little. Reaming to .325 for an inch will do the same thing as you described.

I disagree, just changing the volume is not the goal. The goal is to take a longer time to open the bolt (giving the case brass time to cool and contract) and reducing the harshness of the extraction process. So while you could bore out the gas plug to delay the dwell time it wont have any effect on the wear and tear on the bolt or the brass.

Moving the op rod's static position forward (by shortening the piston or the plug) means that it will have a longer travel distance before it moves the bolt. The op rod will gain a certain amount of momentum from the strike of the gas piston and that will result in a specific amount of inertia. That inertia will create a specific amount of energy that is used to move the bolt to the rear. If you increase the distance that the op rod moves then some of that inertia will be bled off and less energy will impart to the bolt. The end result is that it will take slightly longer for the bolt to open and it will experience less of a jerk when it's opened which results in less wear on the bolt roller and the brass.

A side benefit is that the rifle recoils smoother, barrel harmonics are less extreme, and accuracy improves a little.

 

[rammac]

Very interesting. Thanks guys and gals.
Does a slotted piston also affect dwell time, or only weaken the op rod action?

A slotted piston simply allows gases to move freely along the piston's outside. If I recall correctly the main idea was that a slotted piston would create less wear on the piston and the cylinder because it would float in a bubble of gas as it moved as opposed to resting directly on the bottom of the gas cylinder. I've never seen any real data on how it effects the rifle and not having competed with it at long range I'd defer to the experience of people like Bamban in regards to it's effectiveness.

 

[PDS]

The National Guard armament shop slotted the M14 gas piston as a shooter's aid. With the slotted piston and the bolt open the shooter would tip the muzzle up and down. If the piston was clean you could hear it slide back and forth in the cylinder.
The slot just released the air trapped in the piston and cylinder so it would slide faster.