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Ishapore 2A1 questions

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Shamu View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Shamu Quote  Post ReplyReply Direct Link To This Post Posted: December 20 2019 at 7:34am
I believe it was standard 7.62 NATO, but the rumor, once started kept running for ever.
Don't shoot till you see the whites of their thighs. (Unofficial motto of the Royal Air Force)
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Arminius View Drop Down

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Arminius Quote  Post ReplyReply Direct Link To This Post Posted: December 30 2019 at 12:20pm
I do not want to answer the specific question asked, as I don´t know the answer, and I am reading all about that theme myself.
But as a shooter and reloader for long:
a larger chamber will slightly reduce pressure.
cases from a larger chamber will not last long on reloading. Except, when you are adjusting the dies for that condition.
What I am missing in all that discussions, is Case PUSH BACK.
Read your P. O. Ackley!
Chamber pressure is contained by the steel of the barrel, the chamber section to be specific.
What the ACTION has to deal with is back thrust.
Ackley shot .30-30 AI ( Ackley Improved ) loads in Win 94´s WITHOUT LOCKING BOLTS!
And it worked. I wouldn´t do that EVER in my LIfe!
Back to topic: the .303 is an EXTREMELY tapered cartridge. May be because of filling with Cordite, maybe for cartridge feeding and extraction. ( the 7.62 x 39 is extremely tapered because of feeding and extraction reliability ).
The .308 is rather straight.
I BET ( no, I do not KNOW ), that the .303 for a given power pushes MORE back, than the same load in a .308 case!
So, if the action is mechanically sound, I think 7.62 ammo with higher pressure than .303 will do no harm. MAYBE the even higher pressure .308 is over the top. ( one could ream the chamber to the AI form ;-) )
I doubt it, given case form.
But I do not KNOW it.
Just for discussion, have fun, stay safe, Hermann
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Chris D View Drop Down

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Chris D Quote  Post ReplyReply Direct Link To This Post Posted: January 07 2020 at 2:46pm
I am not a reloader but in addition to my 2A, I have a civilian M14.  Plenty of info on 7.62x51/.308 on the M14 forum.   The main point is that comparing the pressures given for 7.62 and .308 is confusing because the Military measured 7.62 pressures differently than the civilian .308.  Here is an excerpt from a long discussion string
"The psi figures "quoted" are apples and oranges. The 62,000 psi is the current figure for SAAMI based on piezo transducers pressures. The 50,000 psi quoted for the 7.62 NATO is based on the C.U.P. (Copper Unit Pressure) measuring systems. Those are total different and a comparison is totally meaningless.

The actual psi maximum average working pressures (MAPs or PMAPs) for both the 7.62 NATO and the .308W are pretty close to the same; 50,000 C.U.P. or 62,000 psi. The military is now also using a case mouth transducer to measure the psi of 7.62 NATO and 5.56 NATO BTW. I have been measuring the pressure of various lots of US and foreign 7.62 NATO and commercial .308W ammunition in 3 test rifles using an Oehler M43 for several years now. I can show you 7.62 NATO ammunition that is higher that .308W ammunition and visa versa all tested in the same test rifle on the same day under the same conditions. Point is both cartridges are close to the same and it just depends on the specific type and lot of ammunition as to which will have a higher psi. Also don't assume that because the MAP is listed at 62,000 psi that all such ammunition is loaded to that....it isn't. I've yet to find a commercial .308W or a 7.62 NATO MAP that hits that. A couple of each have hit 60,000 psi but most quality .308W/7.62 NATO that meets spec is in the 56 -58,000 psi range. The loading criteria is based on a velocity specification (+/- a lot more fps than you would believe) that stays within SAAMI psi specs (more than just the MAP BTW) and gives adequate "accuracy".

The measured "difference" between the two cartridges is most often in the velocity. The .308W many times will have a higher velocity for a given bullet weight (say 150 vs 147) than the 7.62 NATO. Many assume the higher velocity means a higher psi. That is not the case most often. It means a slower powder was used in the .308W as the time/pressure curves demonstrate. The cartridge case external specifications are the same also. Ever try to buy a set of "7.62 NATO" loading dies? No, probably because they are all .308W dies because the two cartridges are the same externally. Yes milsurp chambers are more generous (need to be to ensure reliable functioning) than commercial .308W but comparing chambers and concluding the cartridges are there for different is an erroneous conclusion.

With bullets of 150 - 165 gr in the .308W there isn't enough of a slower time pressure to harm the M1A/M14. There is just an increase of 75 -100 fps or so velocity. The gas port psi is about the same with those .308W loads as with M118SB or M118LR ammunition.

Newer .308W ammunition of "super performance" or "extended range" have specially blended and much slower powders and should not be used in gas guns. This because of a much slower time/pressure curve that puts too much psi at the gas port. It is not because of increased MAPs in the .308W. Those types of ammunition have MAPs that are like any other .308W commercial ammunition that are within SAAMI specs.

Bottom line is; before going to different sites and cross referencing the "data" one should be aware of how the "data" was obtained. Most often, in the case of .308W vs 7.62 NATO, the "data" was obtained by totally different methods and is meaningless in comparison."
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