Passing Gas & Head Space Issues
Apparently some think the gas ports holes in the barrel and gas cylinder need to be perfectly aligned. This isn’t true. The gas port in the barrel is specified to be .076+.001”. The gas port in the gas cylinder is specified to be .11+.03”. This plainly allows a certain amount of offset in the alignment of the two parts. That amounts to .034”.
The manual says that the gas lock may be hand tight at any point from it’s 6 o’clock index to as much as one o’clock past index. It should then be turned back to the 6 o’clock index and the gas cylinder should be brought forward against the lock. This should result in the gas ports being in acceptable limits of alignment.
As it happens, I have two gas cylinder alignment gauges. One is a commercial gauge made by Badger Ordnance. It is made right to the actual gas port sizes and will center the two ports when fully installed. This is handy in that it will present a realistic position for measuring the gap between the barrel band and it’s corresponding shoulder on the barrel. That’s the gap where the shims are installed for those who feel they need such things. However, the gas lock may not cooperate and index at 6 o’clock when the ports are aligned this way. In a perfect world, they might, but it’s rarely perfect. The gas lock probably won’t index tight at 6 o’clock. It may be too tight to get there or not tight enough and go past it’s required index point.
My other gage is GI. Part number 11015316B. This gauge is made undersized compared to actual gas port dimensions. It’s .104” and .056” which allows even more offset since there is a .048” difference. The military was very forgiving.
The point is, that as long as the barrel gas port does not extend past the gas cylinder’s port, when brought forward against the lock, there should be sufficient gas pressure to allow the rifle to function.
Shims have nothing to do with how the rifle functions. They do prevent possible movement of the gas cylinder which isn’t likely as long as the gas plug is tight. They may also restrict movement of the gas cylinder caused by heating which can result in binding the piston if the shims are not installed properly.
My personal opinion is that shims are not necessary nor is perfect alignment of the gas ports. I tend to agree with Army doctrine in this area, but I respect those who don’t. I do install shims when requested.
The National Guard shooting team has strict standards for it’s coaches and published guidelines for their successful use. I thought it might be wise to pass some of these on for the benefit of those who are contemplating becoming a coach. Here is the first such guideline.
“When you don’t have the slightest idea what the wind is worth, tell your shooter to “hold up, we have a direction change”. Then hope like hell that you can overhear one of the coaches around you, and that he isn’t lost and faking it too.”
The dotted line represents the point on the shoulder where the cartridge would headspace
I’ve been asked to comment on head space by one of my customers who probably has a lot of it. He lives in paradise and shouldn’t have much going on between his ears while basking in the sun and ocean breezes. Aloha Lester…
Much has been said about head space on this and other forums, in books, and other publications. While I agree that head space can present a significant safety issue, I’m not as concerned about keeping it within the narrow parameters usually sighted.
Since we are discussing the M14 rifle, most of our concerns are related to the differences between .308 Winchester and 7.62x51 NATO ammunition. .308 is usually specified to require head space of 1.630” go to 1.634” no-go with 1.638” reject. 7.62 ammunition is specified to be 1.634” go to 1.638” no-go with 1.6455” being field reject. That sounds reasonable, but what does it really mean?
The range between go and no-go is only .004” (four thousands). What this represents is the ideal range for both functioning and accuracy as it relates to head space. I say as relates to accuracy because there are many factors outside of head space that effect accuracy much more. Many rifles shoot very well with no-go head space or longer. Again, safety is the primary factor.
Normally, the acceptable range of head space for a given rifle cartridge is within a range of .010” (ten thousands). That’s quite a lot given the concerns of most shooters. Look at the .30-06 for instance. It has a range of 1.940” to 1.950”. Military armorers were issued a three gauge set of head space gauges to check Garands and other .30-06 military weapons. The gauges were 1.940” go, 1.946” no-go, and 1.950” field reject. It’s understood that a bolt closing on a field reject gauge required changing the barrel. However, a bolt closing on the 1.946” no-go gauge would remain in service. The only exception was rifles being sent overseas which would have the barrels changed if they failed the no-go gage. This had more to do with limited armorers services available outside to CONUS rather than safety issues.
Back to the M14. The safe range for both .308 and 7.62 NATO is actually .010”. The SAAMI specs for .308 are purposely short of this to satisfy civilian liability concerns. A .308 Winchester chamber doesn’t exceed safety limits until it reaches 1.640”. This is just not acceptable by civilian standards, but proof of this is found in the standards for military ammunition which doesn’t fail field reject until it reaches 1.6455”, slightly over the .010” normal limit. Military standards allowed new M14 rifles with head space up to 1.640” to be accepted from the manufacturers.
Even the close tolerances of both Army and National Guard MTU specifications suggested that M14 National Match rifles with head space of 1.638” (no-go) should not be rejected until they were tested for accuracy. If accuracy was within acceptable limits, the rifle was issued. It was thought that these long head space rifles may need a barrel change earlier than those with short head space, but I don’t know that it had been proven.
While most armorers, including myself, build rifles with head space on the minimal side (I like 1.632” to 1.633”), It seems that concerns about slightly longer head space are overblown.
Here’s another gem from the NG coaches school:
“All shots above and below the figure X that don’t catch for wind are the fault of the shooter. Note: Never allow your shooters to carry their scopes to the line. They may question your ability to read the wind, or perform some other obnoxious gesture, such as buying you a seeing eye dog.”
Commentary By Ted Brown
Shooters Den
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