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Chamber Discussion: Picking the right chamber for your rifle build

This is a discussion on Chamber Discussion: Picking the right chamber for your rifle build within the Accuracy forums, part of the M14 M1A Forum category; So as I sit here over-analyzing variables that contribute to accuracy or lack thereof (something I un-nececessarily do a lot of), I spent a lot ...


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Old February 17th, 2017, 07:22 AM   #1
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Lightbulb Chamber Discussion: Picking the right chamber for your rifle build

So as I sit here over-analyzing variables that contribute to accuracy or lack thereof (something I un-nececessarily do a lot of), I spent a lot of time thinking about the different types of chambers that are available to the M14/M1A owner/builder.

The intended purpose of this thread is the help people understand what ammo they should buy or load to get satisfactory results out of their rifles and which ammo or loads to avoid considering their chamber.

I thought it might be helpful to list the common chamber types available for our rifles and give a summary of what application the chamber was designed for. Also list what would be the purpose of the chamber dimension and what bullet weights work well with specific chambers. I don't think there's a need to get into the mind-boggling discussions of throat dimensions and freebore, but the data could still be useful to some people.

It's common for new M14/M1A owners to know nothing about chamber options. Most people think that the rifle is simply chambered in 308, and know nothing of the other chamber options available.

So I guess I'll list the common types of chambers, but what I'd need help with are filling in the blanks below that would help someone pick the right chamber for them. Please provide links, if you can, to commentaries or articles that highlight information for the specific chambers listed. I'm excluding the Palma chambers, but if you think they should be listed, I can add them.

.308 Winchester
Intended use: General purpose/hunting

Bullet weights that work well with this chamber: 125gr through 168gr bullets

Bullet weights that don't work well with this chamber:

Benefits/Advantages:

Draw-Backs/Disadvantages:

Additional info/commentary:

7.62 NATO
Intended use: General purpose/hunting

Bullet weights that work well with this chamber:

Bullet weights that don't work well with this chamber:

Benefits/Advantages:Looser tolerances and headspace which reduce chamber pressure and allow the rifle to safely shoot a wide range of military and commercial ammunition.

Draw-Backs/Disadvantages:Looser chamber dimensions allow for more case stretch which reduces brass life for hand-loaders.

Additional info/commentary:

308 Match
Intended use:

Bullet weights that work well with this chamber:

Bullet weights that don't work well with this chamber:

Additional info/commentary:

M852 Chamber
Intended use: General purpose/Hunting/Match Shooting

Bullet weights that work well with this chamber: 168gr SMK

Bullet weights that don't work well with this chamber:

Benefits/Advantages:

Draw-Backs/Disadvantages:

Additional info/commentary:

Quote:
Originally Posted by rickgman View Post
Tony, the M852 cartridge was based on the "Mexican Match" ammo. The "Mexican Match" ammo was configured by pulling the projectile from an M118 Match cartridge and inserting a 168 grain Sierra Matchking pojectile. The overall length of the M118 Match ammo and the M852 Match ammo is identical. Both are 0.030" longer than M59 and M80 cartridges. That reduces the "effective" bullet jump without changing the freebore dimension. While a custom barrel (like an Obermeyer barrel) might have a "custom" freebore dimension, I would be very surprised if USGI barrels have a "custom" freebore dimension. Certainly that is the case with the M14 barrel and M14's have been used to shot M852, M118 and M118LR ammo. Rick
M118LR Chamber
Intended use: Long Rage Military/Match Shooting/Hunting

Bullet weights that work well with this chamber: 175gr SMK

Bullet weights that don't work well with this chamber:

Benefits/Advantages:

Draw-Backs/Disadvantages:

Additional info/commentary:

Obermeyer Chamber
Intended use: Long Range/Match Shooting/Hunting

Bullet weights that work well with this chamber:

Bullet weights that don't work well with this chamber:

Benefits/Advantages:

Draw-Backs/Disadvantages:

Additional info/commentary:

Jon Wolfe Chamber
Intended use: Long Range/Match Shooting/Hunting

Bullet weights that work well with this chamber: 168gr SMK/175gr SMK

Bullet weights that don't work well with this chamber:

Benefits/Advantages:

Draw-Backs/Disadvantages:

Additional info/commentary:


Please add your knowledge! What's your favorite chamber reamer, and why?

Tony.


Last edited by tonyben; February 17th, 2017 at 01:02 PM.
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Old February 17th, 2017, 09:42 AM   #2
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Quote:
Originally Posted by tonyben View Post
Bullet weights that work well with this chamber:

Bullet weights that don't work well with this chamber:
My large scale guess is:: a skilled reloader can make any match grade bullet shoot well in any chamber.

Another concern is that 155 gr military pull downs will never shoot as well and match grade Lapua, Berger, Sierra,... bullets. So it is not bullet weight that ends up determining whether the cartridge shoots; it is the quality of the bullet, the meticulous nature of the reloader, and the gun it is being fired in.

More than likely, it the the bullet nose profile {secant, tangent, hybrid being matched to the throat and leade of the chamber so that the proper jump coincides with proper magazine loading length.

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Old February 17th, 2017, 10:07 AM   #3
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Not all bullets are made alike, especially military bullets. We found long ago that M118 bullets didn't (usually) shoot well in Barnett barrels while 168 grain Sierra bullets (M852) shot exceptionally well. It had everything to do with the rifling and nothing to do with the chambers. M118 preferred four groove barrels. M852 would shoot well in either four or six groove barrels. Note the difference in quality of GI Match vs commercial match bullets too.

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Old February 17th, 2017, 10:19 AM   #4
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I thought the barrel's twist rate was what dictated the bullet grain weight to use?

Is there one of these that is like a .223/5.56mm Wylde - designed to work equally with both .308 & 7.62mm?

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Old February 17th, 2017, 10:21 AM   #5
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Tony, some years ago I tried to understand what the dimensional differences were between .308 Win ammo and 7.62 NATO ammo and what the differences were between .308 Win chambers and 7.62 NATO chambers. The ammo data was pretty straight forward. However, the chamber dimensional data wasn't as straight forward as I would have liked. When it came to .308 commercial rifle chamber data, SAAMI data told the story. However, military rifles do not adhere to a standard such as SAAMI standards. I did get the chamber data for the M14 rifle which was the one I was most interested in. See the data below:

.308 Win / 7.62 NATO Headspace

Cartridge .308 Win (SAAMI Standard)
1.62 Min; 1.634 Max

Cartridge 7.62 NATO
1.628 Min; 1.634 Max

Chamber Min Max Allowable

.308 Win Chamber (SAAMI Standard)
1.630 Min; 1.640 Max Allowable


M14 Chamber
1.6355 Min; 1.6455 Max Allowable



.308 Win Headspace Gages
Go = 1.63; No Go = 1.634; Rebuild Max = N.A.; Field = 1.638

-
7.62 NATO M14 Headspace Gages
Go = 1.6355; No Go = 1.6375; Rebuild Max = 1.6415; Field =1.6455


I concluded that when it came to ammo, .308 Win ammo was essentially the same as 7.62 NATO ammo in terms of the critical dimensions. I also concluded that M14 rifles might have a looser headspace than a commercial rifle and the difference could be as much as 0.0055". It is possible to have up to a 0.004" interference fit in a SAAMI compliant .308 Win rifle. However, in an M14 rifle, there is no interference fit even with the longest cartridge and the tightest chamber - in fact, there will be 0.0015" clearance. In a way, that makes perfect sense since a hunting or match rifle will likely never be subjected to to kind of environment and usage as a military battle rifle and someone's life can rest on whether that battle rifle is totally reliable even under adverse conditions. Rick

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Old February 17th, 2017, 10:26 AM   #6
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I understand that twist rate should be matched with bullet weight, but some chambers are made specifically for longer bullets. An Obermeyer or a M118LR, AFAIK, have longer free bores to facilitate heavy bullets like 175 grain and 180 grain SMK's. That means 125 grain bullets have to jump like grasshoppers before they hit the lands. This can sometimes be a bad thing.

As far as I understand (but I'm not sure about), the M852 was designed around the 168 grain SMK, or vice-versa.

I'm giving people a chance to comment on what they're sure of as far as matching chambers to bullets and specific applications.

Bullet weight and twist rate is a whole other topic which warrants it's own thread.

Tony.

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Old February 17th, 2017, 12:15 PM   #7
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Tony, the M852 cartridge was based on the "Mexican Match" ammo. The "Mexican Match" ammo was configured by pulling the projectile from an M118 Match cartridge and inserting a 168 grain Sierra Matchking pojectile. The overall length of the M118 Match ammo and the M852 Match ammo is identical. Both are 0.030" longer than M59 and M80 cartridges. That reduces the "effective" bullet jump without changing the freebore dimension. While a custom barrel (like an Obermeyer barrel) might have a "custom" freebore dimension, I would be very surprised if USGI barrels have a "custom" freebore dimension. Certainly that is the case with the M14 barrel and M14's have been used to shot M852, M118 and M118LR ammo. Rick

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Old February 17th, 2017, 12:31 PM   #8
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Oh boy, you opened up a big can-O-worms.......Without getting into the resizing/reloading debate, there are really just little subtle differences between the chambers.

My .02 cents worth anyway, and list my opinion in reamer order.

.308 Winchester

Generous freebore and leade angle. 125-180grn bullets @ 2.80 COAL and although some 180's many fit deep into the case hampering the M/V the original barrels 1/12 rate of twist are the real reason for most inaccuracy.

7.62 Nato 147-155 bullets

This is really a loose compromise chamber generous everywhere, one never knows when a rifle is going too be cleaned next or fired under what conditions, one never knows where there going too get ammo so its a comfortable feeling knowing your allies have ammo that will fit in your rifle and hopefully it will still work? With that in mind the freebore length is kept long too stay out of pressure issues just incase some allied ammo is loaded hotter.

.308 Match 150-180 bullets.

This is just a min spec .308 Winchester chambering with a snugger neck, the freebore and leade angle are better shaped too fit the 168SMK nose.

Obermeyer Match.

This is a older reamer that works really well with almost everything. Way back in the day when M118NM ammo was issued, it was fired is std. NATO chambered rifles. Not really the best for optimum accuracy. Long story short the Obermeyer is a NATO body and the freebore and leade is adjusted too better fit the USGI 173 bullet profile.


M852

The Military likes acronyms and M852 sounds cool, its really just a 7.62 NATO body and the freebore and leade adjusted too fit the 168SMK for less Evil Kinevel jump into the rifling.

M118LR

Almost the same as the Obermeyer but its cut too fit the profile of the 175SMK bullet better.

Wolfe Chamber.

Its my understanding this was a much needed/updated Obermeyer, a page was taken from Bill Wylde's book. The neck diameter was snugged up and still give the bullet reliable release while maintaining good chamber/case aberration then the freebore and leade are encapsulated in there own diameter so the bullet is held center/true into the throat, the leade angle was also laid back a smidge so the bullets jacket and core distortion are minimized as much as possible when the bullet swages into the rifling(throat).

Me, being a 2 dimensional thinker. Nez or Jon can give you the big picture thinking and the why's behind the changes done.

Thanks from tonyben and sac troop
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Old February 17th, 2017, 01:01 PM   #9
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Thanks Rick & Phil! That's exactly the kind of information I was looking for! Absolute gold!

Some people think I have all the answers, and I can generally answer most questions, but I lack the historical development information that's helpful in explaining why there are so many different chamber offerings.

That's the kind of information we can use to help someone who's building a new rifle pick what they want for their specific needs.

Further commentaries are welcome.

HUGE THANKS!

Tony.

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Old February 17th, 2017, 01:13 PM   #10
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Try not to over science it Tony.
Takes the fun out of the hobby.
Won't make all that much difference in the M14 anyway.

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Old February 17th, 2017, 01:16 PM   #11
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Quote:
Originally Posted by nf1e View Post
Try not to over science it Tony.
Takes the fun out of the hobby.
Won't make all that much difference in the M14 anyway.
So what reamer do you use, and why?

Tony.

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Old February 17th, 2017, 01:52 PM   #12
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I have three for the M14 in the playroom.
PTG 662634 and a couple of Clymer's. Plain old .308 win because I am only finishing up the headspace by moving the shoulder slightly on short chambered barrels.
Same thing for the M1 barrels, just setting headspace.
Lately I have been using M14 chrome lined barrels so they are finished when I get them.

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Old February 17th, 2017, 02:11 PM   #13
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Talk to PTG and see what they say. Probably tell you to use a palma reamer with a 344 or 342 neck.

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Old February 17th, 2017, 02:23 PM   #14
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Quote:
Originally Posted by noob14s View Post
Talk to PTG and see what they say. Probably tell you to use a palma reamer with a 344 or 342 neck.
Why would they say that? You have a reason for saying that which means there is logic behind your opinion. That logic may be helpful to someone who is on the fence about what chamber to use. Is your logic to support a general purpose, best of both worlds, or optimum accuracy function?

Tony.

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Old February 17th, 2017, 03:29 PM   #15
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The MAIN CONCERN is to size the chamber so that it will accept the ammo that will be used.

When I had a 308Win barrel installed on my M1, I asked that it be chambered for M852 and Fed GMM 168's. I don't know if that made any difference, but it works fine with that ammo.

If a rifle will be used with some of the various non-US military 7.62Nato ammo, then it might be best to avoid 'tight chamber headspace'.

While I'm certain that some chamberings can give a little more accuracy when the proper ammo is used, I think the amount of improvement is small.

My guess is that there is so much 'interest' in cartridge and chamber dimensions is because REAL NUMBERS can be specified about them. And we like to believe that we're smart enough to understand what the numbers mean and how to use them.

Jay Kosta
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