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.308 vs 30-06 accuracy expectations - chambers

2230 Views 28 Replies 12 Participants Last post by  Phil “Murphy” McGrath
I’ve seen this discussion numerous times on this site and others and the .308Win always seems to get the nod. Nearly every discussion revolves around the efficiency of the .308Win cartridge and reduced recoil. Those are not difficult to understand or believe. What surprises me is that nobody discusses the chamber, or more specifically the throat area.

Nearly ever modern cartridge chamber (including 308Win) follows the same configuration.
1. A short straight section of parallel freebore just slightly bigger than caliber groove size followed by...
2. A tapered lead section between 1 and 2 degrees to the caliber dimension

The 30-06 chamber follows the much older design pattern of having no parallel freebore and instead uses a tapered throat from the case neck down to the caliber dimension. This makes me think the 30-06 has maybe been held back by this throat design in service rifle type matches. Thoughts?
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Certainly no expert, but some serious long distance competitors use the ‘German Serengeti’ or ‘German Salazar’ chamber for match 30-06 rifles:
…For example:
Match Chamber with 1.5° Leade, Throated for 190s
Clark chambered the barrel blank with the Serengeti 30-06 reamer, print no. 9685, from Pacific Tool & Gauge. This reamer was used to cut my other current 30-06 chamber and it produces great accuracy. The Serengeti has a 1.5 degree leade angle like a .308 as opposed to the 30-06's traditional 3 degree leade; I believe this change is an important factor in getting the best accuracy from a 30-06. The body dimensions are well matched to Redding dies, so the brass is not overworked and lasts for many firings. The freebore on the Serengeti is just right for the 190 grain range of bullets, but if heavier bullets such as 210s are going to be a rifle's main diet, then a slightly longer freebore, perhaps 0.070" longer, should be specified. This reamer's neck diameter is 0.340" which is the SAAMI standard for 30-06 and works well with modern brass. Clark cut the barrel to 28" overall for good balance.”

…That’s all I know re this topic.
 

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Chambers may need to be manipulated for a specific purpose. With most 30-06 a simple operation with a pull through finish reamer can create the rifle of your dreams giving you the lead you desire.
I think the fact that many competitive shooters re-barreling their M1s to .308 speaks volumes.
 

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I’ve seen this discussion numerous times on this site and others and the .308Win always seems to get the nod. Nearly every discussion revolves around the efficiency of the .308Win cartridge and reduced recoil. Those are not difficult to understand or believe. What surprises me is that nobody discusses the chamber, or more specifically the throat area.

Nearly ever modern cartridge chamber (including 308Win) follows the same configuration.
1. A short straight section of parallel freebore just slightly bigger than caliber groove size followed by...
2. A tapered lead section between 1 and 2 degrees to the caliber dimension

The 30-06 chamber follows the much older design pattern of having no parallel freebore and instead uses a tapered throat from the case neck down to the caliber dimension. This makes me think the 30-06 has maybe been held back by this throat design in service rifle type matches. Thoughts?
Really depends on the weapon in question.

While the SAAMI specified chamber is indeed a no-leade design, the M1 Garand chamber, as specified by the Army, has a lead of approximately the same dimensions as the .308.

And, some 7.62mm designs also use a no-leade chamber, including the accuracy test barrel used by the Army.

M1 Garand post gas-trap barrel drawing dated March 1940:
Schematic Slope Rectangle Font Engineering


7.62mm Accuracy and Velocity test barrel:
Rectangle Line Slope Schematic Font
 

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Discussion Starter · #8 ·
I think the fact that many competitive shooters re-barreling their M1s to .308 speaks volumes
Art- I fully agree on this. Even if the “mechanical” potential accuracy between the two was identical, I think the reduced recoil and economics (less powder to buy per shot) would cause the 308 to be the logical choice.



What got me thinking about this topic was my 308 Garand that is all NM tricked out, including a heavy Krieger barrel. Given that heavy Garand barrels are nearly extinct, could I get a few more thousand rounds of peak performance by reaming out to a match 30-06 chamber and running a ported gas plug? The chamber that Random Guy posted was what I was looking at.
 

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Discussion Starter · #9 ·
Lysander,
Thanks for posting that information. Didn’t realize the M1 had that chamber. Did that design go back to the inception of the Garand, or did it come later?
 

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Also, the C.I.P. chamber drawing shows a leade similar to the Garand chamber.

As to rebarreling M1s, other reasons: Match quality ammunition availability and commonality.

Lysander,
Thanks for posting that information. Didn’t realize the M1 had that chamber. Did that design go back to the inception of the Garand, or did it come later?
The gas trap barrels and the 1939 M1 Ball barrels have the no-lead design:

Rectangle Schematic Slope Handwriting Font


The M1903 and M1918 both had a no-leade chamber.
 

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Throat design does help accuracy and or hurt it. It’s not just the freebore length but the freebore diameter as well as the angle lead in. Some chambers throats don’t even have a straight/parallel section of the freebore. The throat just starts out as a funnel back at the case mouth. This does nothing to help bullet alignment when making the jump from the case and into the rifling. The SAAMI 300win mag is a absolute horrible design is a good example. At the case mouth it starts out at .317” diameter but the just tapers in like a funnel.

The 308w case capacity is almost the perfect case volume for bore size in 30cal. It’s the most efficient. This makes the 308w also more forgiving in terms of ignition. Typically the fuller the case is with powder the more uniform the burn vs having a case with an air pocket in it per say.

The 30-06 is just as an accurate as a cartridge as the 308w is. The 308w if you will if you want to say it a different way Is more consistent. This goes back to the burn of the powder.

There are differences between the 308w SAAMI, 7.62 NATO and even the 7.62 NATO NM chamber. Of those three the 308w SAAMI is a better design.

Also the 7.62nato chambers if I remember correctly (I don’t have the drawings in front of me as I’m not at work) but the case body of the chamber is also looser/bigger than 308w SAAMI.

Besides the above chamber spec’s we have three different 308 chamber designs we use at work on building custom rifles. The only thing different is in the throat design. Which we use depending on the ammo/bullets the customer will be shooting. One reamer with a short throat which is the 308w Obermeyer reamer. This is for 175gr and lighter bullets. It will also chamber most box ammo. Then we have a med. throat reamer which is +.020” longer than the Obermeyer. Then we have a really long throated reamer just for shooting 200gr or heavier bullets for like a FTR gun.

30-06…we have the SAAMI min spec. Which unless it’s called for in ammunition test barrels where it has to be SAAMI spec.I don’t use it for custom builds on bolt guns. We have the Serengti reamer but I wouldn’t use this if the customer is going to shoot box ammo as I feel the throat is too short/tight. Strictly a bolt gun match chamber. Then I have another reamer which I call the 30-06HVT. I designed the throat of this reamer based on factory 30-06 M72 box ammo. I tried to find the longest loaded round and measured it from the base of the cartridge to the ogive of the bullet and then added another .020” to the freebore length and made a straight parallel section with a freebore diameter of .3085” I think it is. With a 1 degree 30 min lead angle.

The one M1G barrel I did in a service rifle pattern when I rebarreled one of my Garands has that one reamer.

So back to the main question. Is a 308w more accurate than a 30-06? With standard chamber and loads…I’ll give the tip to 308w but not necessarily more accurate but more consistent. The 30-06 with the right density load of powder and a decent designed throat…I don’t think the 308w has anything on it.

I have two bolt guns built (one set up as a K98 sniper type resto mod gun) and another gun I used for testing back when we started Krieger. So it’s a single shot rifle I used for accuracy testing. Both of these guns have a chamber that I call 7.5HVT. It’s a 8mm Mauser case necked down to 30cal. Back in the 1930’s the French actually where playing around with the caliber and might have used it just for a short bit. I’d have to look at my notes but I think it was called 7.5x58mm. Has a tad more case capacity than a 308w but less than a 30-06. I designed my own throat for it. It is very accurate and easy to load for. The only down side is it won’t run thru a short action bolt gun.

Later, Frank

PS if your wondering what the HVT stands for…I call it high velocity target. Even though the caliber might not be a really high velocity round. The first round I gave this name to was 12.7x114mm chamber. I rebarreled a dewat Lahti to 50cal but instead of doing 50bmg as the rim is smaller than the 20x138B Solothurn round I took 14.5x114 Russian cases and necked it down to 50cal. The rounds fed out of the Lahti mags and I didn’t have to rework the bolt face or modify the extractor as the the 14.5 and the 20mm used the same case rim diameter. My everyday working load for that gun slung a 750gr AMAX bullet at 3100fps and I wasn’t stepping on it/beating up the gun.
 

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Discussion Starter · #12 ·
Frank- thanks for the thoughtful response. I think your comments/experience align with my belief about a good chamber bringing excellent accuracy to 30-06. The chamber you describe sounds like a good one (30-06HVT). That would be fun to try out in a M1903 USMC sniper clone too. I have not put a new 30-06 barrel on a Garand or 1903 in a while, but those used the SAAMI reamer. They shoot well for what they are, but next time it will be something better like the HVT. I also think my rechambering plan might have merit when the 308 throat is smoked....unless of course you start making some M1 Garand heavies..
 

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Also the 7.62nato chambers if I remember correctly (I don’t have the drawings in front of me as I’m not at work) but the case body of the chamber is also looser/bigger than 308w SAAMI.
It is a common myth that the M14, M60, M240, etc chambers are larger, looser, bigger than SAAMI spec chambers.

Here is an overlay of several "NATO" chambers, as you can see the diametrical difference between the bodies is 0.0007 to 0.0013. The headspace difference is around 0.010, with the MG chambers noticeably shorter.

Rectangle Slope Parallel Font Drawing


Slope Rectangle Parallel Font Diagram


Rectangle Slope Parallel Font Electric blue
 

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Frank- thanks for the thoughtful response. I think your comments/experience align with my belief about a good chamber bringing excellent accuracy to 30-06. The chamber you describe sounds like a good one (30-06HVT). That would be fun to try out in a M1903 USMC sniper clone too. I have not put a new 30-06 barrel on a Garand or 1903 in a while, but those used the SAAMI reamer. They shoot well for what they are, but next time it will be something better like the HVT. I also think my rechambering plan might have merit when the 308 throat is smoked....unless of course you start making some M1 Garand heavies..
I used that reamer in the 03 that I rebuilt also.

M1G heavies. Might make a run of them. Only have 3 guys at this time showing interests at this time in a hvy pattern and 3 interested in a service rifle pattern. If you are interested in one send me your email address and name along with a phone number. When I have pricing figured out I’ll notify everyone on the list.

The hvy pattern at this time will be S.S. only. All of the 30cal barrels for M14’s also will have 5R rifling and 1-11 twist at this time.
 

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It is a common myth that the M14, M60, M240, etc chambers are larger, looser, bigger than SAAMI spec chambers.

Here is an overlay of several "NATO" chambers, as you can see the diametrical difference between the bodies is 0.0007 to 0.0013. The headspace difference is around 0.010, with the MG chambers noticeably shorter.

View attachment 498313

View attachment 498314

View attachment 498315
Like I said I was going off of memory and I need the drawings in front of me. If I recall correctly the throat diameter on the 7.62 NATO is .316” and the 308w SAAMI is .310”. I don’t recall freebore length on those.

I can post the drawings on Monday. I have them all saved on my computer at work.
 

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It is a common myth that the M14, M60, M240, etc chambers are larger, looser, bigger than SAAMI spec chambers.

Here is an overlay of several "NATO" chambers, as you can see the diametrical difference between the bodies is 0.0007 to 0.0013. The headspace difference is around 0.010, with the MG chambers noticeably shorter.

On the headspace difference of around .010” are you talking about go +.010“ if using a field gauge vs saami on 308w being go +.006” typically being max.?

I know on the drawing I have for 7.62 nato it shows a min. dimension of 1.628” which is .002” shorter than a 308w min of 1.630”.

I didn’t know the MG chambers could be even shorter? Any idea as to why?
 

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Like I said I was going off of memory and I need the drawings in front of me. If I recall correctly the throat diameter on the 7.62 NATO is .316” and the 308w SAAMI is .310”. I don’t recall freebore length on those.

I can post the drawings on Monday. I have them all saved on my computer at work.
Ain't no such thing as an 7.62mm NATO chamber.

Free bore diameters:

M14 = 0.3095"
FAL/T48 = no freebore, a 0.06 TPI forcing cone with a .305 ref diam at 1.242 from datum plane starts at termination of neck chamfer which is an 80° included with a ref dia of 0.3247" at 1.101" from plane
L7/M240/MAG-58 = no freebore, a 2° 51' 50" forcing cone with a 7.747 ref dia at 6.6mm from datum plane starts at termination of neck chamfer which is an 80° included with a ref dia of 8.3mm at 12.67mm from datum plane
C.I.P. (.308 Win) = 7.85mm
C.I.P. (.308 Win Match) = 7.84mm
SAAMI (.308 Win) = 0.310"
M60 = 0.312"
M110 = 0.309"

Believe it or not, the tightest throat is the MAG-58, and the loosest is the M60 with 0.0035" diametrical difference between the two that everything else falls into. Not really surprising, with an MG a tight throat means more wear before muzzle velocity is lost. And the M60 was designed around a Stellite lined throat, so wear would be delayed.
 

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Well that cracks me up and not surprising.

If for example the 7.62 nato spec reamer is suppose to be a standard you think that would be the standard used in guns?

I’ve seen the same thing if you will in production guns (commercial) there is the saami min spec standard but that doesn’t mean a manufacturer uses that standard.

This is a good reason why whenever someone orders test barrels from us and don’t supply a print for everything and just call out 300wm or any other caliber/chamber I reference the tool part number on the drawing and put that info on the sales order confirmation copy so there is a track record of what we made the barrels to. They get all that info sent to them in the confirmation for review. That way it cannot come back on us making the barrels wrong.

I’m dealing with this headache right now with an overseas customers for finished M40 sniper rifle barrels for they’re military. The order is for two batches of 350. They’re prints don’t show enough detail when it comes to the chamber. They call it 7.62x51mm. So I’ve sent them several different prints for them to tell me which reamer to use and no one at the arsenal (can’t talk to anyone directly as is has to go thru a contractor which no one there knows anything about guns) can tell me which exact chamber spec it is!

So I’m hesitant on taking the job as they have a accuracy requirement at 200 yards the barrel has to meet. They say they are using three different types of ammo which are 308w Federal GMM with a 175gr bullet, M80 ball, and a 3rd some foreign 308 match round which escapes me right now.

So they ordered two barrels for testing. One with the 7.62 nato reamer and one with the 308w SAAMI reamer.

I’ve also asked how are they inspecting the chambers and bores so we do apples and apples for inspection. No one gets back to me. That’s why I’m hesitant on taking the job. Poor communication and I don’t want anything coming back at us.
 

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Well that cracks me up and not surprising.

If for example the 7.62 nato spec reamer is suppose to be a standard you think that would be the standard used in guns?

I’ve seen the same thing if you will in production guns (commercial) there is the saami min spec standard but that doesn’t mean a manufacturer uses that standard.

This is a good reason why whenever someone orders test barrels from us and don’t supply a print for everything and just call out 300wm or any other caliber/chamber I reference the tool part number on the drawing and put that info on the sales order confirmation copy so there is a track record of what we made the barrels to. They get all that info sent to them in the confirmation for review. That way it cannot come back on us making the barrels wrong.

I’m dealing with this headache right now with an overseas customers for finished M40 sniper rifle barrels for they’re military. The order is for two batches of 350. They’re prints don’t show enough detail when it comes to the chamber. They call it 7.62x51mm. So I’ve sent them several different prints for them to tell me which reamer to use and no one at the arsenal (can’t talk to anyone directly as is has to go thru a contractor which no one there knows anything about guns) can tell me which exact chamber spec it is!

So I’m hesitant on taking the job as they have a accuracy requirement at 200 yards the barrel has to meet. They say they are using three different types of ammo which are 308w Federal GMM with a 175gr bullet, M80 ball, and a 3rd some foreign 308 match round which escapes me right now.

So they ordered two barrels for testing. One with the 7.62 nato reamer and one with the 308w SAAMI reamer.

I’ve also asked how are they inspecting the chambers and bores so we do apples and apples for inspection. No one gets back to me. That’s why I’m hesitant on taking the job. Poor communication and I don’t want anything coming back at us.
Quite possible the folks ordering your barrels don't even know what chamber they want. Not unusual for military " mechanics" to just grab a component off the shelf and expect it to perform it's function.
Not long ago, on this forum , we had an individual order and receive a short chambered .308w barrel and couldn't understand why, once installed, it wouldn't chamber a round. He was under the impression, from reading on the internet, that short meant 1.630 and had not clue it should be 1.620 or even shorter and would require finish reaming. Way to many enthusiasts read and that's it , no comprehension of the data being used. Whatta Hobby!
 
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