A question about low-powered .308 rounds...

Discussion in 'The Ammo & Reloading Forum' started by CCubed, Jun 4, 2005.

  1. CCubed

    CCubed New Member

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    I'm not going to start any kind of .308 Win. vs. 7.62 NATO dispute: the two are different and the same - that's it. I would, however, like advice on shooting low-powered .308 Win. ammo in military 7.62 x 51 caliber rifles. The specs for the 7.62 NATO round, according to a credible Internet source are 2750 fps and 2520 ft-lbs. Now, I recently found a vendor who is selling current production commercial .308 Win. ammo with the following specs: 2643 fps and 2326 ft-lbs. Naturally, you don't have to be a genius to realize that these numbers are lower than a NATO round's spec. Therefore, is it safe, and indeed possibly even better, to shoot these commercial rounds in my 7.62 rifle? I know the metal thickness and headspace and all those other specs are slightly different between the 7.62 NATO and .308 Win. round. The most important difference is their CUP and PSI. Personally, I am not technically orientated - especially in the realm of gunsmithing and reloading - so I need advice: Is it okay, or even better, to buy this low-powered, current production .308 Win. ammo than old made-who-knows-where 7.62 NATO ammo?

    I'd appreciate answers and opinions. Thanks! :D

    - I should add, about safety, that I've already fired a couple 150 grain .308 Win. rounds in my rifle with no ill effects - and the specs for these were a little over NATO round specs, but not by much at all.
    Last edited: Jun 4, 2005
  2. mtnboomer

    mtnboomer New Member

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    If it is factory-loaded ammo and they say it will exit the barrel without any problems, the only concern I would have is whether it would cycle a semi-auto military rifle.
  3. CCubed

    CCubed New Member

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    mtnboomer, thanks for the response. Since my rifle is not semiauto - although I would like to own an M-14 or FAL :D - I don't have to worry about how well it cycles. (Although, it does have to be able to cycle through my bolt action.) So, thanks for the response, but that really wasn't the question. The question is: if a factory loaded round has lower fps and ft-lbs specs than a NATO round, does it also follow that this round's CUP or PSI - whichever you prefer to use - is lower than NATO spec, and consequently, safe or safer to fire in a 7.62 NATO caliber rifle.
  4. inplanotx

    inplanotx New Member

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    I do not understand why you will not realize that the two rounds are the same. Pressures stated in CUP (copper units of pressure) are NOT the same measurement as PSI (pounds per square inch). They are two different measurement systems and can be equal to each other although the numbers are different. CUP measurements WILL ALWAYS be a lower number than PSI but could equal each other!!!!

    In your numbers above between velocities, the only major different are the components used to assemble the two rounds by two different manufacturers. The 107 feet per second difference in velocity is MEANINGLESS as you can have that much variation from shot to shot in ANY gun using the same lot of ammo.

    The difference between the two energy figures you show is caused due to the fact that one round has a slightly lower muzzle velocity than the other. Muzzle velocity and retained velocity are part of the equation to figure out muzzle energy and retained energy!!!! If one has a lower muzzle velocity and hence lower retained velocity, it's muzzle energy and it's retained energy HAS to be lower mathematically!! The only difference in the two rounds is that the brass thickness of military rounds is usually thicker thus raising pressures if you reload. However, they are BOTH loaded to SAAMI specs which keeps them the same!!!

    Do yourself a favor and take one piece of each brass (military and commercial), fill them with water to the top of the neck and then weigh them. The commercial brass will hold more water and thus weigh more than the military brass filled with water because the military brass is THICKER!

    Again, the military brass and the commercial brass are both loaded to the same pressures, but are measured differently. CUP does not equal PSI nor can they be equated. The measurements systems are different, but the resulting pressures are equal!!!!

    I hope this finally explains what you are looking for.

    IPT
  5. CCubed

    CCubed New Member

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    So, what you're saying is that it is just as dangerous to shoot .308 Win rounds in a rifle action rated for 4600 CUP (7x57) as it is to shoot 7.62 NATO ammo? I understand that I'm not getting it. Look, here's my problem: First, I don't know much about this stuff...at all. This is my first experience trying to decode all this CUP, PSI, etc., gibberish. My second problem is that when you go to purchase a box of ammo, you don't find its CUP on the box. That's all I care about. I don't care about energy and velocity, necessarily. I am concerned with the CUP. My 93 action was designed to shoot 7mm Mauser bullets. Apparently, these have a CUP level of 4600. The 7.62 NATO round has a CUP of something like 5000, and the .308 Win has an even greater number. All I want to know is can this change? Can I go to the store and find commercial ammo with a military or lower CUP level in 7.62x51 caliber? I ask this because I've noticed that some commercial .308 loads have energy and velocity levels that are less than the 7.62 NATO levels. Therefore, my question was, if these levels are lower than the 7.62 NATO round's levels, does it follow that their CUP measurement is also lower? Do you get what I am saying? Is a round's CUP directly related to its velocity and energy? If not, then correct me. It simply seemed reasonable to me, and to the other shooters who I asked, that it would be. I understand I don't get it. I'm trying to.
  6. inplanotx

    inplanotx New Member

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    You are making a mountain out of a mole hill because you do not understand these pressure readings. So let me put it to you this way. YOU DO NOT NEED TO! PERIOD!!!

    Ammunition is loaded to SAAMI specs. PERIOD! Unless a box of ammo EXPLICITLY states, on the box, that the rounds are loaded to "+P" pressures, then you have nothing to worry about. Take the damn thing out, buy some Winchester 'white box' .308 147gr FMJ ammo at Wally World and go shoot and enjoy your rifle. When they did the arsenal rebuild on your rifle, the rifle was PROOF tested to pressures higher that the SAAMI spec as a precaution. All arsenal rebuilds are handled this way.

    Forget about PSI, CUP, LUP and anything else pertaining to the ammo spec as all ammo is loaded to SAAMI specs and is safe in your rifle. If the barrel is marked 7.62x51 then it was pressure tested to greater than NATO SAAMI specs. If it is marked .308 Winchester, it was pressure tested to greater than .308 Winchester SAAMI specs. ALL ammo manufacturers load ammo to SAAMI specs!!!!!!!!!!!! Some niche ammo manufacturers load ammo to '+P' (higher than SAAMI specs). It will be stated as such on the box. Do not use +P ammo in your Mauser!

    I hope you will take what I have told you and forget about all the crap that you are trying to ingest about things you do not understand and are obviously causing you to not enjoy that 1893 Mauser as you should be. I have two 1893's and enjoy shooting them. Of course, I reload, but I make sure that the rounds are loaded to MINIMUM SAAMI SPECS!

    Now, go to Wally World, buy the ammo I stated above and have fun with it. You have a perfectly good weaqpon and should be enjoying it rather that delving into all this crap you do not understand. It will NOT blow up on you unless there is a barrel obstruction or if you give it a steady diet of '+P' ammo.

    Nuff said.
  7. CCubed

    CCubed New Member

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    Okay. I get the picture.
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