.308 vs 7.62×51 Ammunition My “Serious Purposes” Warning


.308 vs 7.62x51 Ammunition
.308 vs 7.62×51 Ammunition

Ft Collins, CO –-(Ammoland.com)- As most of us know, 5.56×45 is the “militarized” version of the civilian 223 Rem varmint cartridge, developed by Remington in 1957.

Military-rifle chamber dimensions for the 5.56×45 cartridge are more generous than the civilian 223 (SAAMI-spec), although both rounds will chamber and fire in NATO-spec military rifles.

SAAMI stands for “Sporting Arms and Ammunition Manufacturing Institute.” SAAMI was instituted in 1913 in order to establish and exchange consistent small-arms technical information between American factories producing military arms and ammunition.

Today, SAAMI continues to dictate chamber dimensions and pressure standards for all commercial calibers, and all gun and ammunition manufacturers dutifully comply.

Commercial rifles in 223 Rem caliber, intended for non-serious purposes, mostly come with SAAMI-spec chambers, which are tighter than NATO-spec chambers, as noted above.

As a general rule, when a barrel is stamped “223 Rem,” it has a SAAMI-spec chamber, and as noted above, it is intended only for non-serious purposes. Tight chambers boost accuracy but have no place on high-capacity, military rifles.

Conversely, when the barrel is stamped “5.56×45,” it has a looser NATO chamber, with adequate leade, or freebore (this is a short section of bore just ahead of the chamber that is not rifled, allowing the bullet to jump off the case before it engages the rifling). The combination of generous chamber dimensions and adequate leade is intended to prevent stuck cases, broken extractors, and pressure-spikes in autoloading rifles, particularly when they get hot.

Of course, loose chambers yield slightly less inherent accuracy than tight chambers. However, for serious purposes, this accuracy compromise is insignificant

Accordingly, I always recommend a genuine “NATO-spec” barrel for all 5.56×45 ARs and other autoloading rifles that are intended for serious purposes, and I specify it on all “Farnam Signature” Rifles.

308 and 7.62×51

308 caliber was introduced into the American commercial market by Winchester in 1952.

Two years later, DOD militarized it, designating their version “7.62×51 NATO,” and as a result of heavy persuasion on the part of DOD, it went on to be adopted my most NATO nations and remained the “standard” through the 1960s and 1970s.

With additional heavy persuasion on the part of DOD, the 7.62×51 was superseded within NATO in the 1970s by the 5.56×45 and remained there to this day.

We are told the decision has now been made at DOD to supersede the 5.56×45 with Remington’s 6.80×43 (6.8 SPC, introduced by Remington in 2003). How long this conversion will take, and whether or not our NATO allies will go along, once more, is anyone’s guess?

One important note:

While the military 5.56×45 chamber is looser than the commercial (SAAMI) 223 Remington chamber, as noted above, the exact opposite is the case with the 7.62×51/308 Winchester caliber!

Military chamber dimensions for 7.62×51 NATO are actually tighter than the those for the original “308″ chamber dimensions, and I’m not sure I understand why this is so.

In any event, this anomaly has become an issue for manufacturers of military-style rifles in this caliber, as you might imagine.

When you buy a FAL, RA/XCR/M, SCAR, POF Revolution, et al and intend it for “serious purposes,” you want a “308″ chamber (with adequate leade), not the tighter 7.62×51 NATO chamber, particularly when you plan on running a wide variety of ammunition through it.

For one, I want my serious rifle to chamber and shoot anything! When a manufacturer cautions, “use only this brand of ammunition,” I have no interest in owning that rifle, as I have no idea of what ammunition I may be one day forced to use.

With my FS 7.62×51 rifle, I’ve been thus compelled to specify “308″ barrels and chambers.

Once again. “serious” autoloading rifles have to be able to chamber and shoot any kind/brand of ammunition you can find, without stammering, without breaking extractors, without the extractor pulling-through the case-rim, and without the extractor ripping heads off of cases.

It’s a serious issue, and your life may one day depend on your rifle running reliably, despite “field conditions,” continuous lack of maintenance, and funky ammunition.


Defense Training International, Inc

About John Farnam & Defense Training International, Inc
As a defensive weapons and tactics instructor John Farnam will urge you, based on your own beliefs, to make up your mind in advance as to what you would do when faced with an imminent lethal threat. You should, of course, also decide what preparations you should make in advance if any. Defense Training International wants to make sure that their students fully understand the physical, legal, psychological, and societal consequences of their actions or in-actions.

It is our duty to make you aware of certain unpleasant physical realities intrinsic to the Planet Earth. Mr. Farnam is happy to be your counselor and advisor. Visit: www.defense-training.com

  • 34 thoughts on “.308 vs 7.62×51 Ammunition My “Serious Purposes” Warning

    1. Shot both 308 an 7.62×51 in several different 308 I have an even reloaded the same for years,an never had no problems.

    2. This is what happens when you let someone who doesn’t know what he’s talking about write an article. 7.62 chambers have greater head space than .308. It can be dangerous to fire thin walled .308 in a maximum but within an in spec. 7.62 rifle. 7.62 cartridges have thicker walls to allow them to stretch without having a case head separation that may occur with .308 cartridges.

      .308: go – 1.630, nogo – 1.634, field – 1.638
      7.62: go – 1.6355, nogo – 1.6405, field – 1.6455
      SAMMI cartridge length: .308 – 1.634/1.627, 7.62 – 1.634 -.006

      1. Well I’m making the jump from 7.62×39 so this for me is helping me choose. 308 or 7.62×51 for shtf? Any suggestions? And can I get those calibers in a AK platform. Sorry love my AK it may not hit distance but it’s rugged and ammo is plentiful and cheap.

    3. The 556 round was not developed by FN until the late 70’s which means there were many AR’s marked 223 & in combat prior to that date. So were these non serious rounds for prairie dogs? I think not as many a soldier bet their life on them!

      1. Actually that is the SS109 62 grain steel tipped penetrator bullet that was developed by FN in in the early 70’s and adopted as the NATO Standard M855 cartridge in 1980. Prior to that the standard US 5.56 round was the Cartridge, 5.56mm Ball, M193, which featured a 55 grain FMJ bullet. M193 was never accepted as a NATO standard round. I can assure you that US military issue ammo and gun barrels were never marked .223, the Army used 5.56mm or 5.56x45mm. Some prototypes may have been, but that is all.

    4. Why s it that the 308 no go headspace gauge will close on 7.62 chambered rifles if the chambers are tighter than 308. I was always under the impression that the 7.62 was slightly looser, like the 5.56. And because of this you had to be careful firing 308 with thin brass. The 7.62 x 51 uses thick brass and because of this has no issues with the larger chamber.

    5. What is the obsession with labeling AR’s “high capacity military rifles”. It comes off as very uneducated for such a good article.

    6. I have been shopping for a Springfield socom chambered in ,308 caliper. Does this rifle come with a .308 chamber or would I have to order it that way specifically?

    7. Best article and responces I have observed on Ammoland. I have looked at manuals regarding both and could see there were differences but always wondered why everyone talks about them as if they are the same. Your real life experences arent duplicated by manufactures. Thanks for all the great responces and references.

    8. The 5.56 Nato rounds should only be fired in a firearm marked “5.56”, but 223 Remington rounds can be fired in both guns marked 5.56 and/or 223. The problem is that some manufacturers of AR-type firearms aren’t consistent with their marking of their barrels. The .308, both military (7.62) and commercial (.308) are completely interchangeable for ANY firearm marked either .308 or 7.62 and one may perform better than the other in a specific rifle, but it is NOT dangerous to the shooter to interchange the ammo in either marked rifle.

      1. As a general rule and statement 556 should not be fired in a 223 marked firearm. Particularly a bolt gun! However, many of the early Colt AR’s were marked 223. After a discussion with Colt regarding this topic I was told the AR’s stamped or marked 223 would have no problems firing 556 since cambers in semi auto & full auto firearms are typically cut larger.

        1. Realist, are you stating that Colt chambers, though marked 223, are spec’d 5.56? And therefore, their rifles can handle both cartridges?

    9. Should desire to shoot 556 in a barrel chambered for 223 just run a Wylde finishing reamer into the 223 chamber then you can safely shoot both 223 & 556.

    10. I have “unloaded” several firearms over the years after encountering a refusal to eat or properly digest a particular ammunition brand or loading. All things being equal, I just wont own a firearm that has this issue.
      My .308 rifles are civilian 308 chambered for the reasons set out in the article. My bolt gun for hunting is more than accurate for me at or sub MoA. It has the advantage of accepting aics magazines. My 308 auto loader will eat anything and while it isn’t MoA accurate, it is MoM accurate, and that’s all I really need at longer range.

    11. None serious purposes????? Let me assure you a varmint heavy barreled bolt action rifle in 223 is for very serious purposes and will typically allow for more precise bullet placement than a 556 semi auto rifle even though the 556 operates at slightly higher pressures. This is the reverse with the 308 and 7.62×51 as the 308 operates at higher pressures than the 7.62×51. So according to your analogy this would make the 7.62×52 for non serious purposes? In both cases there are slight chamber size differences and problems may arise in shooting 556 ammo in a barrel chambered for a 223. The 308 & 7.62×51 typically do not share this concern. Poor article?

      1. Sir no offense, but popping prairie punchers is not the serious purpose being contemplated here nor sniper shots.The reliable operation of auto loaders with multi mag dumps in a life treating situation is the concern being addressed. You are correct that even most $300 bolt guns are more accurate than self-loaders, especially those setup for quantity over quality.

        1. No offense taken here, but it appears you need not look at prairie dogs as the targets and rather look at the performance ballistics of both cartridges with the same weight of bullet! Also take into consideration most AR’s have shorter barrels than full length rifles which lowers velocity. Personally, I would prefer a 308 for combat! Additionally, your early AR’s were marked 223!
          No offense!

    12. Please take note that milspec no longer is a guarantee of right thinking. This article clearly shows on head scratcher situation created by milspec. The lessons are clear for those of you who are preparing for dark times (again). When selecting ammunition families for you critical service firearms the rule used to be milspec because that is what was available in large numbers. In include reloadable Warsaw Pact families of ammo in that category. Back during the Obama Ammo Drought those who had exotic calibers had lots of wall hangers. Those who had common ammunition families and stockpiled components and bulk purchases were able to continue training. Marshall Zhukov, leader of the Eastern Front in WWII liked to tell his people that “the best is the enemy of good enough.” Going to the latest high speed, low drag innovation can put you in a corner if you cannot feed your supply chain during dark times. Always remember, while amateurs are talking tactics, professionals are talking logistics. Semper Fidelis

      1. You might as well shoot steel case ammo through your hk. The roller lock system chews the hell out of cases and makes them damn near impossible to reload

      2. How doe the PTR guns compare to guns like the HK MR762A1, SCAR 17S, and SR-25 in terms of recoil, reliability, accuracy?

    13. Can you please help me find a source for the planned adoption for 6.8SPC beyond special forces?

      Or is it just a rumour at this point?

        1. All press releases, and scuttlebutt in intel community (no longer have connections to ordnance circles), is that only 6.8mm caliber has been settled on. In other words, no one has specified that the 6.8 SPC cartridge has been settled on, leaving open whether it will be 6.8 SPC or a yet to be developed 6.8 mm cartridge.

    14. I have a 7.62×51 chamber/leade in a POF Edge Gen4 18″ SPR. Gas piston operation and E2 extraction (fluted chamber a-la HK MP5/P7) makes feed issues non-existent, with all the accuracy benefits of the NATO geometry chamber/leade (.6 MOA @ 200 yrs w/168gr Hornady Black A-Max)
      POF actually run thousands of Wolff steel case rounds in testing their ARs.
      No need to give up accuracy by opening up chamber/leade. Purchase an AR engineered to overcome the feed issues.

    15. I used to have an HK91 and Springfield M1A1 (M14 clone). The reloaded brass fired in the HK often wouldn’t fully chamber in the M1A1. I had to run that brass through a standard resizing die and again through a small base die so it would chamber reliably in the Springfield.

    16. Sort of like how firearms makers are using tighter throat and leade dimensions in their 9mm Parabellum chambers these days. I bought a new G19 in 2006 and it had, and has, no problem chambering any of my loads including those using .357″ jacketed bullets and .358″ cast bullets. Not so for the G19 Gen 3&1/2 I bought in July 2018. I ended up buying a LoneWolf barrel and had them open up the throat so my loads would fit. I also recently bought a Combat Armory barrel and polished out the throat myself. If all you were going to use is factory ammo I’m sure the barrels would work well, just not when using larger diameter bullets. Then again, if you are not using 9mm Para spec bullets is it still a 9mm Para? Anyway that has been my experience and how I managed it.

    17. I have a 7.62×51 sig sauer Patrol G2 can I shoot 308s safely through this rifle. I hear yes and on witch is it ?

    Leave a Comment 34 Comments

    Your email address will not be published. Required fields are marked *