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Difference in barrel contours?

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Old July 29th, 2012, 10:20 AM   #1
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Thumbs up Difference in barrel contours?

I have been trying to find how much difference it is between the regular USGI barrel profile and my Springfield Inc. M1a NM.

I have been trying to understand which mounts that can be used for it, the Ultimak, which i really like, doesn't seem to fit NM barrels.

But then again, it seems that there are regular profiled NM barrels and some are medium weight and some heavyweight??

I'm confused... :)

Can anyone straighten this out for me? I've searched and googled but apparently my search-fu is weak...

Any help is greatly appreciated :)

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Old July 29th, 2012, 11:32 AM   #2
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2Sharp,

The easiest way to learn the differences is to just have a look at some good photos of the various barrel types. Smith Enterprise and LRB both have good photos on their websites.

http://www.lrbarms.com/m14parts.html

http://www.smithenterprise.com/products03.01.html

M14 barrels usually fall into three categories: Standard weight (the original "skinny" barrels on issued military M14 rifles) medium weight ( made as fat as they can be and still use the original operating rod guide) and heavy (so fat that the original op rod guide won't fit and a special larger guide must be used.)

A medium weight barrel is about one pound heavier than a standard, and a heavy barrel is about two pounds heavier than a standard (that's a little less than a kilo to you...)

Just be aware that since these commercial barrels are not required to be made to the exact specifications of a military drawing, the dimensions of any one manufacturer's barrels may be a little different than the next and so it's hard to guarantee that a barrel mounted accessory like an Ultimak or SAI scout mount will fit perfectly...

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Old July 29th, 2012, 11:36 AM   #3
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Welcome to the forum,

We have terms for barrel contours that sometimes confuse us, so I can see how it would be really confusing to someone from Sweden.

The standard barrel contour found on real US M14 rifles is the lightest of all the barrels and has come to be known as a "light" barrel.

The next size going larger is what is commonly called the "Medium Heavy" Barrel. The breech is a bit larger in diameter and then it tapers down to the same size as a "light" barrel under the operating rod guide so you can use a standard "GI" op rod guide. From there to the front band, the barrel is slightly larger than a GI or Light barrel. In the area for the Gas cylinder forward, ALL NM barrels are the same size and contour so you can use the standard gas cylinder and Flash Suppressor. Of course, you can buy special Medium heavy barrels that are set up for different types of Flash Suppressors or Compensators. With the Medium Heavy barrel, one has to do some modifying of a standard handguard so it will go down over the heavier breech.

The "Full Heavy" or "Heavy Barrel" is larger from the breech all the way to the front band. From the front band forward with a NM barrel, it is the same size as the Light or Medium barrels so you can use a standard gas system and Flash suppressor. You can NOT use the standard GI operating rod guide and the barrel makers normally provide a special operating rod guide that fits the barrel. You have to heavily modify the handguard so it fits down over the Heavy barrel.

After the standard GI contour barrels, the next NM barrel that came out (chronologically) was the full heavy barrel and used in NM competition here since the early 1970's. The advanatage of the full Heavy Barrel is the weight as it makes the rifle super heavy and you can recover faster from each shot. However, it is so heavy, that it is not a good choice for a rifle that is going to be used for something other than NM competition or shooting from the bench.

The Medium Heavy barrel contour came out after the Heavy contour as the Heavy contour was too heavy for some people. The accuracy of the NM Medium Heavy barrel is so close to a full Heavy barrel that mos people can not see the difference when they shoot unless they are very expert shooters.
Also, the Medium Heavy barrel is not too heavy to use with a rifle for hunting.

If you have more questions, please ask.

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Old July 29th, 2012, 12:40 PM   #4
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Ok, i think i got it now :)

Thanks a lot for the guidance, much appreciated. Seems it'd be hard to modify an Ultimak to fit...

I have my mind set on the Troy MCS but also thinking about the Vltor system, but it seems there aren't any rails that would fit.

Oh well, MCS is nice... :)

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Old July 29th, 2012, 02:03 PM   #5
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Quote:
Originally Posted by 2Sharp View Post
Ok, i think i got it now :)

Thanks a lot for the guidance, much appreciated. Seems it'd be hard to modify an Ultimak to fit...

I have my mind set on the Troy MCS but also thinking about the Vltor system, but it seems there aren't any rails that would fit.

Oh well, MCS is nice... :)
You may be interested in reading the post in this link:

Can you fit a picatinney rail to a medium heavy or full heavy barrel?

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Old July 29th, 2012, 02:25 PM   #6
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Good info there! So now the idea of modification has been tossed out of the window...
MCS it is :)

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Old July 29th, 2012, 02:31 PM   #7
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Originally Posted by 2Sharp View Post
Good info there! So now the idea of modification has been tossed out of the window...
MCS it is :)
That will save you a LOT of time and frustration.

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Old July 30th, 2012, 01:18 AM   #8
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That will save you a LOT of time and frustration.
Sounds even better!!

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Old February 28th, 2014, 08:47 PM   #9
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Old February 28th, 2014, 09:20 PM   #10
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So the barrel in the SAI Loaded is a medium heavy?

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Old February 28th, 2014, 10:52 PM   #11
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Old March 1st, 2014, 08:27 PM   #12
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OK, so one of my NM has a standard contour barrel and one has a large barrel for about 8" before it tapers down to a standard shape. The light one is marked NM, and the thick one is marked NM 308. Why the differences? The light one has a 4 digit ser no. 0061xx and the heavy one is a 151xxx but has a F0075 bolt.


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Old March 2nd, 2014, 09:43 AM   #13
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anyone?

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Old March 2nd, 2014, 10:16 AM   #14
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Quote:
Originally Posted by Jrr051468 View Post
OK, so one of my NM has a standard contour barrel and one has a large barrel for about 8" before it tapers down to a standard shape. The light one is marked NM, and the thick one is marked NM 308. Why the differences? The light one has a 4 digit ser no. 0061xx and the heavy one is a 151xxx but has a F0075 bolt.
I don't know what your asking but I will try too read you mind? Original NM barrels were made too the same conture as Standard barrels, your rifle having a low ser number was made/built before med/heavy barrels were allowed in NRA High Power shooting. Years later the rules changed, your .308NM barrel is a med weight conture barrel and it was more than likely made by Wilson, the extra weight helps add a bit of stiffness so it flex's less, takes longer too heat up so you can shoot longer, and adds a small amount of recoil reduction from its added mass.

Your Bolt if that's all the markings on it was made by SAI, its Forged not cast.

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