Home Forums Hunting & Shooting Ask The Gunwriters Pressure 308 Win vs 7.62 Nato

My apologies for posting something so liable to be misunderstood that you would take it personally when I had no such intention. Would you prefer that I edit the line to omit real and to read nationally recognized experts with broad experience and easy to find material that forms a useful background to beating this long dead and rendered horse?

I did intend to suggest that when authorities all say the same thing there is liable to be some truth in it. As Jerry Pournelle writes on his own blog - when he's not sure he'll assert something and be promptly corrected when a simple question might be ignored.

On the other hand Clint and Walt are quite properly described as industry professionals including successful professional gun writing - the world has beaten a path to their door. If you are accusing them of plagerism have at it but it's really only common knowledge.

I have no personal claim to expertise myself but I have had attorneys persuade a Court that I was qualified to give what was my opinion as an expert.

Further as I tried to indicate by proximity I believed and continue to believe the limited quote from the Fulton Armory site was within the limits of fair use and also was a particularly apt answer to a question posed and so a useful assist to a board member. And once more as was implied in the original post the whole issue has been done to death and become a matter of opinion with personal stories and beliefs - what some call fly talk - an unwarranted assertion, a flat contradiction and much acrimony.

Apology accepted. I would prefer that you amend the post to read,

"nationally recognized experts with broad experience and easy to find material that forms a useful background to beating this long dead and rendered horse?"

Walt and I have corresponded in the past. I've never met Clint.

Done to death? No. It rears its ugly head constantly. Unfortunately, there is no Internets FAQ.

I was only slightly interested in the thread, until I see your nice chart showing .013" different headspace, which left me going, "hmmm..." I had heard the warnings about powder burn rates in M1's and M1A's but not headspace issues in everything else.

Midway, for the record, has no 7.62 brass on hand at the moment, but I have since found some LC brass, so I guess I'll order some of that, along with some headspace gauges of one sort or another to see what I have in this Armalite. At least one respected maker (Noveske) states their chambers are designed for 7.62 ammo, and commercial .308 ammo may not work well, so there is conflicting info out there. Sometimes it takes a little work and experimentation to figure out who is the "...nationally recognized experts with broad experience and easy to find material..."

Yes, this subject will continue for a long time I think.

Many gunsmiths unknowingly use the wrong gauges when checking 7.62x51mm chambers. They tell owners of these rifles that their headspace is no good. They base this on 308 Win specs. Most will pay to have the headspace corrected when there is no problem to begin with.

The last rifle I dealt with this problem was an Ishapore made No 1 Mk III, chambered in 7.62x51mm. At the request of the owner, I dropped by the shop where the check was done and had a chat with the gunsmith. I asked him if they gauged according to 7.62x51mm chamber specs. I was told that there was no such thing.

After asking him to check one of his catalogs, he said that he did not have the proper gauges, apologized and refunded the fellow his money.

In any case, I do appreciate the information.

I don't understand how strain gauge data can be accurate. How do we know that x amount of strain equals x amount of pressure?

I have read that strain gauge systems are calibrated with ammo that produces known pressures, but how do we know that? How did we measure the pressure of the calibrated ammo?

Strain gauges have seen extensive testing for decades. When used to measure strain in a system for which the strain is known, there is good agreement.

In a barrel (ie. a steel tube), the equations to predict strain from pressure (and vice verse) were developed long ago. There are some errors introduced by the forces on the barrel near the receiver and by recoil. Both are considered to be small.

The trick for the home user is to apply the gauge well and to measure the dimensions of the metal on which it is applied. Errors in either mean the calculated pressure will be off.

Another complication is the brass. It acts to reduce the area over which the pressure acts, and it is also applies a resisting force to contain some of the pressure. You must, then, make careful measurements of the brass thickness under the applied gauge. Lastly, you use an average value for the strength of the brass to calculate its resisting effect.

There are several "ifs" here, but Ken Oehler has written that in practice it all works out pretty well. (If I recall, he said one gets decent results even if you use a set psi offset to compensate for the brass.)


No ammo produces a known pressure in every chamber. You'll see claims on the web that "SAAMI reference" ammo exists for this purpose, but it is not so. Reference ammo is extremely consistent ammo that has been tested in several pressure barrels known to be correctly cut. Such ammo is used to qualify a newly made pressure testing barrel. If the new barrel is correctly cut and if the sensors are right, then the reference ammo should produce the same psi and fps as in the existing test barrels. It's simply a check on the new system; it is not proof of accuracy.

At home, it's safest to use a variety of factory ammo to set an upper limit for the measured strain. With this, one doesn't care about the calculated pressure. If your gun is safe with factory ammo, handloads made to the same strain reading should be as well.

One must also use a chronograph as a check that the indicated pressures aren't too good to be true.

2525,

Excellent post! You answered my questions very well. Thank you.