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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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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!
Yes, that is a big problem when non-technical people get stuck dealing with technical issues. I'll bet your foreign customers don't even have engineering support to answer your questions.

If it helps, the USMC M40A3 barrel drawing specifies the chamber dimensions shall conform to ANSI/SAAMI Z299.4, .308 Winchester.

There is also a mistaken belief that one chamber fits all. If you read various reports on weapons development tweaking the chamber dimensions has a great impact on weapons' reliability. In some cases the difference is between acceptable and unacceptable performance. For example, during WW2 the British got acceptable performance out of the Hispano-Suiza 404 20mm, but the US never did, and the Brits were convinced the reason was the US never adopted the British chamber. This was particularly vexing for the British as the US was supposed to be a major supplier of 20mm aircraft cannon, but poor reliability relagated US made 20mms to training use only. In fact, a majority of US produced 20mm cannons were placed in storage and never used.
 

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Yes, that is a big problem when non-technical people get stuck dealing with technical issues. I'll bet your foreign customers don't even have engineering support to answer your questions.

If it helps, the USMC M40A3 barrel drawing specifies the chamber dimensions shall conform to AMSI/SAAMI Z299.4, .308 Winchester.
You are correct…it’s a big problem. Every question I ask has to go thru the contracting people if you will and they have to relay it to the Arsenal. I’ve begged and pleaded to be able to talk to someone directly at the Arsenal to no avail! Everything had to get relayed!

I’ve got the whole M40A3 rifle build drawings. It includes everything even the scope mounts etc…I’ve even made references to what some of our military uses in they’re sniper rifles being 308w SAAMI. Falls on deaf ears.

I’ve been in similar situations with our gov’t and an ammo maker when it came to the 300wm sniper ammo. What was funny about that was the contracting officer in charge which was a woman at the time (about 5 years ago this happened) actually figured out they had a ammo spec problem. When she made reference in saying that on a conference call…one of the military guys blew a gasket on her and told her to be quiet that they didn’t have a ammo problem. You just can’t make it up.

2 years ago I got an order for some 9mm test barrels for a ammunition maker. So when I was looking at the prints closely I noticed the bore and groove spec’s where not SAAMI and they also where not Nato. It was a hybrid of both. Then I looked at the chamber…again it wasn’t SAAMI and it wasn’t Nato. So I asked the ammo maker…do you have the chamber reamer? Because we don’t! They said no. Could I order one and at it to the bill? I said it was proprietary and the reamer maker couldn’t make it for us. Luckily I looked in the small detail of the print title block and I figured out who the manufacturer of the gun was. I had contacts at that manufacturer because we make precision barrels for them as well as R&D barrels for them. So I got a hold of the guy I talk to all the time there even though he deals with rifles only and could he put me in contact with whom ever is in charge of the handgun stuff. Bingo…got the chamber prints released to me in detail and they gave the ok to the reamer maker to make us the chamber tools.

It works out sometimes and sometimes….it is just a cluster!
 

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When I first started cleaning up and re-barreling rack grade M1s from CMP using Criterion barrels, I ordered a pull-thru Manson reamer from I believe Brownells. Imagine my surprise when I went to chamber a M72 spec round and the 175gr Sierra was jammed in the rifling and the bolt wouldn't close! The rounds chambered and fired fine in my CMP field and service grade M1s. I did a lot of reading about chambers. The Manson reamer was a standard SAAMI with basically no leade. I called and talked to Manson himself. Surprisingly he claimed he was not aware of the difference between the SAAMI and GI chambers. The SAAMI works okay with M2 ball but not M72 match. I ended up buying a throater and lengthening the throats on the two barrels. I ended up selling the Manson reamer and the throater once the two barrels were done. I found that JGS makes a pull-thru reamer specifically designed for the M72 match. I have used that reamer for three barrels with success. You live and you learn. I have two .308 pull-thrus. Both are PTG, one is a standard .308 and the other is a 118LR.
Bill
 

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I'll give you three data points (one of which was already brought up) for the 308 vs 30-06 debate.
1) German Salazar did as good an apples to apples test in a real world setting actually firing at distance for score. Same actions, same barrels, same bullets, ammo loaded both loaded to his high standards. The results on target were statistically indistinguishable.
2) One of the longest standing records in High Power Rifle competition was fired by Eric England shooting an M70 30-06 bolt gun in 1968. England fired a phenomenal 998-45x on the 1000 pt aggregate course. The targets fired on still current today, but then brand new NRA 10-X (decimal) targets. That record stood for 41 years (through the 308 era) until it was bested by Sherri Gallagher of the AMU shooting a TUBB2000 in 260 Rem.

So how was the 30-06 able to perform in points 1 and 2 even through a time when every service branch was trying to switch over to the 7.62/308W? It's because the M1 Service Rifle was limited by powder. The gas system of the Garand prevented the use of powders of the day that filled the case (achieved 100% load density). That airspace in the 30-06 case was detrimental to accuracy, and part of the routine for service rifle shooters of the day was to tip the rifle upward before settling into position to position the powder at least consistently toward the rear of the case. Rechambering to the 308W was the better solution.

The Bolt gunners like Salazar and England didn't have the same limitations that the 30-06 gas gunners did and could load whatever powder suited their need. And their results showed on target.

Different aside. If you've never heard of Eric England, treat yourself and buy "The Phantom of Phu Bai". England was not only one of the best Highpower Rifle competitors ever, but he was also one of the most decorated Snipers of the Vietnam War with 98 confirmed kills. The book tells a bit of his story.
 
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