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I haven't done the math on bolt thrust between 5.56 and 7.62x39. But 5.56 operating pressure is around 62K psi, and 7.62x39 is around 45K psi. The bolt thrust might be closer than we think.
My thought on losing a lug might be all wrong, but I think rotating the bolt out of it's locked position is easier on all moving parts when pressure has subsided even a little bit. So the heavier buffer delays unlocking by a few milliseconds, and in that same short time, pressure is dropping in the chamber/barrel. I could be wrong.
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Did some math to compare bolt thrust between 5.56 and 7.62x39
5.56 NATO,
Case head diameter .378"
Case head area .112 sq-in
Max pressure 62K psi
bolt thrust: 6,944 lbs
7.62x39,
Case head diameter .447"
Case head area .157 sq-in
Max pressure 45K psi
bolt thrust: 7065 lbs
Measured the rear lug surface of both bolts. They are the same.
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I haven't done the math on bolt thrust between 5.56 and 7.62x39. But 5.56 operating pressure is around 62K psi, and 7.62x39 is around 45K psi. The bolt thrust might be closer than we think.
My thought on losing a lug might be all wrong, but I think rotating the bolt out of it's locked position is easier on all moving parts when pressure has subsided even a little bit. So the heavier buffer delays unlocking by a few milliseconds, and in that same short time, pressure is dropping in the chamber/barrel. I could be wrong. Based on surface area calculated by rim diameter of 5.56 case, at 62k psi, the force on the bolt will be 6,974 lbs. 7.62x39, using SAAMI spec pressure of 45k psi, will have a force of 7,058 lbs. on the bolt. 7.62x39 military loads have an operating pressure of 51,400 PSI and will place a force of 8,062 lbs. on the bolt The caveat, most American 7.62x39 AR barrels are cut with a .308 diameter bore. Pushing .311 diameter military loads down the .308 diameter barrel will produce a pressure in excess of 51,400. Ruger's mini 30 also uses a .308 diameter barrel.
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Did some math to compare bolt thrust between 5.56 and 7.62x39
5.56 NATO,
Case head diameter .378"
Case head area .112 sq-in
Max pressure 62K psi
bolt thrust: 6,944 lbs
7.62x39,
Case head diameter .447"
Case head area .157 sq-in
Max pressure 45K psi
bolt thrust: 7065 lbs
Measured the rear lug surface of both bolts. They are the same. This is all valid. Now consider the difference between force on the lugs in a barrel with all bolt lugs seated evenly, and one where the barrel is not square to the bolt and one or two lugs take all the pressure. That, combined with less support of the lug bases on the 7.62x39 bolts, is what really breaks bolt lugs. Squaring up the receiver (and therefore the barrel extension) goes a long way to making the bolt live. Getting the rifle properly gassed to prevent premature unlocking during high pressure (as you were saying) is a good thing as well. If you do those two things and use a good bolt, you can expect good bolt life.
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I feel the reason as to why 7.62x39 breaks lugs on bolts is that the lugs on the bolt have to be much smaller than the lugs on a .223 sized bolt. Also, The pressure on the 7.62x39 bolt is much higher than the pressure on a .223/5.56 bolt. (7.62 Case has a larger bolt surface area than a .223 bolt area hence more load is applied to the bolt). I understand 7.62x39 does not have an issue with breaking lugs in an AR10 or similar platform. I am not comfortable running 7.62x39 loads in an AR15 sized platform due to the smaller bolt lugs and increased load on the bolt.
Who chambers the 7.62x39 in the AR10?
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Did some math to compare bolt thrust between 5.56 and 7.62x39
5.56 NATO,
Case head diameter .378"
Case head area .112 sq-in
Max pressure 62K psi
bolt thrust: 6,944 lbs
7.62x39,
Case head diameter .447"
Case head area .157 sq-in
Max pressure 45K psi
bolt thrust: 7065 lbs
Measured the rear lug surface of both bolts. They are the same. This is all valid. Now consider the difference between force on the lugs in a barrel with all bolt lugs seated evenly, and one where the barrel is not square to the bolt and one or two lugs take all the pressure. That, combined with less support of the lug bases on the 7.62x39 bolts, is what really breaks bolt lugs. Squaring up the receiver (and therefore the barrel extension) goes a long way to making the bolt live. Getting the rifle properly gassed to prevent premature unlocking during high pressure (as you were saying) is a good thing as well. If you do those two things and use a good bolt, you can expect good bolt life. That makes sense.
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Who chambers the 7.62x39 in the AR10?
The CMMG “Mutant” is reputed to have a 308 sized bcg with a 7.62x39 sized boltface, which means a lot more metal around the case head, and much bigger locking lugs, and the increased mass probably helps to keep the bolt locked during the highest pressures.
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Did some math to compare bolt thrust between 5.56 and 7.62x39
Measured the rear lug surface of both bolts. They are the same. Measure overall lug size instead.
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Did some math to compare bolt thrust between 5.56 and 7.62x39
Measured the rear lug surface of both bolts. They are the same. Measure overall lug size instead. There is no dimensional difference in the lugs. The 7.62x39 is milled out more in the bolt face to accommodate the larger rim diameter. So the front of the lugs on the 7.62x39 do have less steel below the lug. If tested to failure it would appear likely to let go before the 5.55 bolt would. 5.56 left, 7.62x39 right, ![[Linked Image]](https://i.imgur.com/I8l4rfh.jpg)
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Well, I stand corrected.
Two of the lugs on the 7.62x39 are narrower than the rest. The lug beside the ejector, and the lug just below the extractor are only .090" wide.
The rest are .100" wide, as are the 5.56 lugs.
Last edited by MontanaMarine; 11/28/18.
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I wonder if sending the 7.62 bolt for cryogenic hardening would help extend the life and prevent lugs from shearing off?
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John Stark.
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I don't know, but I'm just going to run mine as is, with either an H3 buffer, or a rifle buffer.
Probably not a bad idea to keep a spare bolt around.
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Does PSA sell 7.62 bolts?
I searched and did not find them.
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They have them listed in in 'AR15', 'upper parts', 'bolt carrier group', but currently out of stock.
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Armorally bolt carrier groups are in stock, and have 26 reviews and all are 5 star. I'll get one for T&E.
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