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Discussion Starter #1
I’ve been following the threads recently posted about the M14 gas system and adjusting dwell timing. There needs to be some understanding of just how the White cut off gas system works in order to understand what is affected when altering any part of the M14 gas system.

First as I noted in my posts, it’s important to realize the bullet is well out of the barrel before anything starts to move in the gas cylinder. We are talking micro seconds here and I won’t detail just how many micro seconds it takes to do this stuff. Gas pressure is bled off the barrel through the gas port and into the gas cylinder which starts just as soon as the bullet passes the gas port. However, the amount entering the port while the bullet is still in the barrel is not enough to fully function the rifle. After the bullet passes out of the barrel gas pressure drops dramatically in the barrel, but continues to expand in the gas cylinder. This starts the piston moving along with the op rod with a relative soft push. As soon as the piston moves a short distance the piston gas port comes out of alignment with the barrel gas port. This contains the expanding gasses within the gas cylinder as the piston moves to the rearmost position. Along the way, the piston passes another port at the bottom of the gas cylinder which vents off the gas pressure in the system. At that point, the operating rod continues to move under its own inertia to cycle the action.

Alterations to the gas system affect both how the rifle functions and can affect accuracy.
The M14 was designed with a certain amount of overkill in the gas system. That is, it has more operating pressure than it actually needs to function. However, this was done purposely to insure reliability under combat conditions and using NATO spec ammunition that may have originated in any of several countries, some of which didn’t have the best quality controls. For that reason, we find that the M14 gas system is easy to work with, forgiving most of our loading errors, and it can be modified to alter various parameters.

I don’t believe the gas system has a great deal of influence on accuracy. For accuracy purposes I’ll consider that the rifle we are discussing is a fully modified National Match rifle. Battle rifles are not designed for pinpoint accuracy in the first place. So, we are working with a gas system that is properly fitted with minimum tolerances to start. If one alters the size of the gas port, changes will occur in functioning, but not necessarily in accuracy. Enlarging the gas port has been done in some rifles to improve feeding and extraction problems. This is a last resort kind of operation, because enlarging the gas port can also result in over pressuring the system and can cause damage. Most folks seem to want to reduce the size of the port which slows down bolt velocity and can be effective in reducing blow back in suppressed rifles. The same results can be had by venting off gas pressure with a ported gas plug, either drilled or made adjustable and, to some degree, using a slotted gas piston. There are also gas plugs made with larger inside diameters along with gas pistons enlarged the same way that increase the volume of area for gas to fill which reduces pressure. Some think this is effective in very short barrels like the SOCOM system, but it really doesn’t make any significant difference since the SOCOM gas port is in the same position as found on all M14 type rifles. If anything, the SOCOM probably needs a larger gas port since the pressure drop from the bullet exiting will come sooner.

What I have found is that the standard gas system, as designed, works fine in most all situations and can handle bullet weights between 125 and 185 grains without problems as long as chamber pressures are kept within a normal (specified) range. There are special situations that require specific alterations to the gas system, like using a suppressor, that justify modifications.

One adjustment that can be of benefit is adjusting the dwell. Dwell is the time it takes for the bolt to cycle. When the bolt tries to open too fast or too soon extraction problems result. Increased bolt velocity also imparts noticeable felt recoil. Slowing down the dwell time allows chamber pressures to drop for easier extraction and the longer duration spreads recoil impulses longer to make them feel softer. This is largely between the operator’s ears, but it does feel like lighter recoil. Dwell time is lengthened by reducing the distance between the operating rod and the bolt roller (moving the op rod forward). It takes few micro seconds longer for the op rod to travel back and engage the bolt roller and open the bolt. I don’t recommend altering the op rod on either end. The op rod is not going to appreciate the removal of any material since these are fairly high impact areas that may become damaged if altered. Most armorers opt to shorten the gas piston and/or gas plug. I am not a fan of altering the gas plug. It’s made of relatively soft stainless steel that will get battered if the surface case hardness is removed. Once this happens it may be difficult to remove or install it in the cylinder. The piston, on the other hand, is much harder. My preference is to remove material from the back of the piston allowing the op rod to move forward. The operating rod must remain in contact with the tail of the piston when it is fully forward and it should not rest against the bolt roller. A gap will result in vertical stringing of the shot group on the target. It will also result in battering between the piston and op rod which may damage the rifle.

The M14 is a tough old bird. It is designed to function reliably under a wide variety of circumstances, in awful conditions with the worst ammunition. National Match M14 rifles are among the most accurate military rifle in the world (they are the best in my opinion) and altering them is like trying to re-invent the wheel.
 

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Dwell time is lengthened by reducing the distance between the operating rod and the bolt roller (moving the op rod forward)
I assume your are referring to the back (toward the shooter) side of the op rod cam?
 

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Thanks for this post. I've been reading many posts on this and have seen so many pros and cons of this or that. This lays it out pretty well and clearly.
 

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Ted you may have to elaborate on this but the way the old USMC RTE shop did it was they had Barnett leave the gas cylinder shoulder .035 long with NO gas port drilled/reamed. They put a gas cylinder on and clocked the lock around to see where it fell , then turned enough off the long shoulder to get the 5:00 o"clock fall of the lock. If memory serves it was .013 for ever 15 degrees of rotation of the lock. The gas cylinder was then pulled and put on upside down and the a small drill bit was used to spot the location of the gas port, again the cylinder was removed the port drilled and then reamed with 07 taper pin reamer. The splines where then peened the cylinder was driven to the shoulder. and the lock drawn down. A piston was s then inserted and a plug was torqued to 65 inch pounds WHILE keeping a .017 feeler gauge held between the op rod and tail of the piston The short chambered barrel was then finish reamed with a pull through reamer to a head spcae on the minimum side of the range. The purpose of all this was to induce dwell. As you have outlined there can be advantages to this. This practice was standardized for the 12's as all the guys on the truck had this in their notes.
Enjoyed speking with you the other day
GM
 

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Thanks to both Ted and NCshooter regarding this topic. I have an 1995 dated but still NOS/short chambered Barnett USMC marked barrel that lacks the gas port, and this info is interesting in that context.
 

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Discussion Starter #7
That's one way to do it and the Marines always had their own methods.
The AMU and NG timed everything off the gas port (opposite the Marine method) as they preferred to insure the gas port was centered in a groove. Many armorers think this improves accuracy as opposed to having the port in a land. Of course there isn't much choice when working with GI issue barrels. I don't know if there is any real significant difference, but I do select my own barrels with the port in the groove. I think Krieger does this too. One thing for sure, the Marine M14's were very good.
 

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The port in the groove was discussed but if memory serves me correctly an experiment was done on "the sled" where they deliberately barreled several guns with ports in different locations around the bore axis. It was determined it didn't make enough difference to matter. The fact was they were only trying to get 1 MOA guns consistently.
Many would do better than that but all you needed was 1 MOA believe it or not the acceptance criteria for the M40A1's was 1MOA @ 300 yds as well.
 

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OP,
Thanks for the knowledge and wisdom...
Did or does the 'compressed air' ahead of the bullet have any impact or influence on the gas port and all it's mechanics?


Does the compressed air ahead of a fired bullet impact the gas system and it associated parts and systems?
 

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It does enter the piston and it does add to the working pressure in the system. However, I can't see it doing much as the pressure is relatively low, a few hundred psi, and followed by a much higher pressure for a much longer duration.
 

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Discussion Starter #12
I don't think compressed air ahead of the bullet has any significant effect on the gas system. Keep in mind there is a .30 caliber hole (much larger than the gas port) venting air out the front of the barrel.
 

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Mr Ted Brown, I think I was in one of those recent posts you speak of, haha.

I mentioned somebody once telling me some fancy gun smith shaving a bit off the piston. I didnt do it, and never would.

I agree that the M14 platform is tough, and I realized my lesson on over tightening my Schuster plug (increasing gas pressure). It was a mess. Have since learned to set it and forget it, when it comes to Schusters. Or just dont Schuster it at all and let it do its thing.

Do you happen to have any knowledge on shooting suppressed? I've heard that you HAVE to have a Schuster plug or a Smith plug (one he makes for SOCOM suppressed). I've also heard (I believe from the Bula guy, has a customer that runs suppressors) that you really don't need to have a special gas plug at all to run suppressed. Do you have the truth to chime in on this?
 

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Discussion Starter #14
The only problem I have run into shooting the M14 suppressed is back pressure blowing back into my face. Some of the guys who use suppressors are installing a spindle valve in the gas cylinder that has been cross drilled with a smaller port. I've made up several with ports about .052" that seem to work OK. I made up an adjustable spindle valve for my rifle that seems to work well. It eliminated the blow back and still allows the action to function normally. Please don't ask me to make one for you. It's still experimental.
Note that the original Sonics suppressor used on the M21 sniper rifle had a pressure relief valve to mitigate back pressure. I don't know of anyone making one like it now. The Schuster gas plug may work, but I have not used one.
 

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... Does the compressed air ahead of a fired bullet impact the gas system and it associated parts and systems?
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I believe (no specific facts or knowledge) that the 'compressed air' does have some effect.

It might not be enough pressure to cause 'measureable movement' of the piston or oprod, but I believe there is enough pressure to cause some vibration or 'loading' on the piston.

Most of us have heard about the military 'test and swap' gas pistons method for fine tuning of accuracy. I think that demonstrates that 'something' is happening while the bullet is still in the barrel. Even if detectable piston movement occurs only after the bullet has left the barrel.

Jay
 

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M14 gas system VOODOO?

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I believe (no specific facts or knowledge) that ...
'something' is happening while the bullet is still in the barrel. Even if detectable piston movement occurs only after the bullet has left the barrel.
Jay
YEP!
With the M14, a closed system with many many interrealated parts, I believe there are indeed several " somethings happenning" between ignition and bullet exit.

CONSISTENCY is the mother of accuracy. And getting all those many M14 parts, rampaging along at high speed, to act consistently ...
gets complicated.

Turning off the gas to minimise all these many interactions makes things simpler, and should, in theory, make the M14 system more accurate.

In practice?
Has anyone actually demonstrated significant accuracy improvement by turning off the gas?

ENQUIRING MINDS NEED TO KNOW!
 

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YEP!
With the M14, a closed system with many many interrealated parts, I believe there are indeed several " somethings happenning" between ignition and bullet exit.

CONSISTENCY is the mother of accuracy. And getting all those many M14 parts, rampaging along at high speed, to act consistently ...
gets complicated.

Turning off the gas to minimise all these many interactions makes things simpler, and should, in theory, make the M14 system more accurate.

In practice?
Has anyone actually demonstrated significant accuracy improvement by turning off the gas?

ENQUIRING MINDS NEED TO KNOW!
No but in his early days I used too bust on Tonyben for doing his accuracy work and load development with the spindle off, I alway's pointed out too him it doesn't matter how accurate the rifle and load combo is if the rifle doesn't run as a system?
 

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^^^^agree^^^^
and run thru the magazines
 

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Discussion Starter #19
If you follow the rules, having the gas system on and the rifle functioning in it's semi automatic mode is required. They don't allow the operator to turn it off when shooting competition. Therefore it's essential that the rifle shoot it's best with the gas system turned on. Testing it with the system off doesn't accomplish anything.
 
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