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DCRA #4/MK1* 7.62

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britrifles View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote britrifles Quote  Post ReplyReply Direct Link To This Post Posted: May 14 2020 at 10:44am
7.62 and .308 maximum pressures are not the same.  And they are both higher than .303.  Higher pressure = higher bolt thrust = more wear and tear on the action.  Think fatigue cracks in the bolt lugs.  Think bolt bearing surface repeated loading.  It’s nothing about what the rifle can take as a single proof load, it’s the cumulative effect of higher pressure loadings.  I spent my entire career working in metallic fatigue, I know a bit about it after 35 years.  You can choose to load to .308 max pressures if you want, use any .308 commercial ammunition if you want. But I won’t in my rifles, I want to pass them on, I want to shoot thousands of rounds and minimize the wear and fatigue of the action.  In fact, I prefer to shoot mild loads in my .303 as well, I want to save wear and fatigue life of the action.  And I don’t want to beat up the forend bedding either.  

Watch that video again from bloke on the range shooting 20 or 30 rounds of .308 Win Mag, that resulted in a very fast propagating fatigue crack on the bolt lug.  That wasn’t a “manufacturing flaw” that suddenly appeared.  Shooting full pressure .308 rounds will do the same, will just take longer.  Shooting full pressure .303 rounds will take even longer.  It’s not linear to the pressure level, it’s an exponential relationship between fatigue crack initiation and load level.  

Alright, I’ve said my piece, I won’t address this subject again.  





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Post Options Post Options   Thanks (0) Thanks(0)   Quote Goosic Quote  Post ReplyReply Direct Link To This Post Posted: May 14 2020 at 11:29am
Bloke on the range purposely fatigued that rifle. DCRA and Enfield purposely proofed their 7.62 rifles to maximum pressures. You continuously reference maximum charge weights and pressures and I continuously refer to either a minimum or mid range charge weight. Commercial 308 ammunition sold in north America cannot be loaded to any maximum proofhouse test specific load and then sold to the public. Commercial ammunition is made to a very minimum specific charge weight so as not to invoke lawsuits and as such pressures will never be at a maximum. I have never loaded any of my cartridges to maximum pressures as well. This is an assumption you have made on your own accord.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Zed Quote  Post ReplyReply Direct Link To This Post Posted: May 14 2020 at 1:48pm
I have a question regarding the factory ammo labelled 7.62/.308
Is this .308 ammo specifically loaded to NATO 7.62 velocity? (Or possibly NATO spec that's been repackaged to sell to .308 owners!)
In France I have seen some suppliers listing Surplus 7.62 as .308; I think it's just to catch a bigger client group.

With regard to the loading discussion; I think we are all on the same page regarding safety and not wanting to damage ourselves or our rifles. 

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Post Options Post Options   Thanks (0) Thanks(0)   Quote britrifles Quote  Post ReplyReply Direct Link To This Post Posted: May 14 2020 at 3:40pm
Agree Zed.  

My comments are not anything to do with safety.  The No. 4 action is strong, it’s been proven to hold even .308 Win Mag pressures.  I’m attempting to address cumulative effects of fatigue and wear, not safety in the sense of what loads a new (or low time) action is capable of withstanding.    Metal will fatigue and fail at loads far less than proof static loads, I see this every day of my career.  It’s not my opinion, it is a scientific fact.  We all get to have our own opinions, but we don’t get to have our own facts.   I’ve been to many accident sites and labs around the world investigating fatigue failures.  They occur from repeated loads that are much much lower than static proof loads.  They are even lower than what we would consider as “safe” operating loads.  

 The LE rifle was designed before fatigue concepts were even known or understood.  Shoot 1,000 rounds of any commercial .308 is likely no issue, maybe even 10,000 rounds.  But the higher pressures of typical .308 commercial loads compared to typical .303 loads will take its toll.  Most of the guys on this forum will never shoot that many rounds in their lifetime, but I shoot a lot, 5,000 rounds a year on my No. 4 rifles.   I want to pass these rifles down to the next generation.  And besides, reduced loads are easier on me, more comfortable to shoot, and just as accurate at short range, at least to 200 yards. Would I pump the load up a bit for 1000 yard match?  Sure I would, but I would not use this as my every day match load for 200 yard work.  This is the beauty of reloading, full control of the important parameters. 

I’m not insinuating that anyone on this forum is pushing the safe limits for the No. 4 rifle, in .303 or 7.62.  Please understand this.  If they were, believe me, I would be much more direct and state that.  

 Now, this is my humble opinion, not fact, that No. 4 rifle that bloke on the range tested with .308 Win Mag had a small fatigue crack in the lug from shooting a LOT of 7.62 Service ammunition over its lifetime.   It would have continued to grow slowly with 7.62 service ammo until eventual failure.  The action was basically worn out.  I estimate it would take 10x the amount of .303 Mk 7 ammunition to produce that same fatigue crack than it did with 7.62 NATO ball ammo based on the pressure difference.  The .308 Win Mag loads then greatly accelerated the crack propagation until it failed after 30 or so rounds (can’t recall how many).  The encouraging thing was that the test did show the failure was “fail safe”, the rib lug held the load and the bolt did not fly back. But, I don’t want to start cracking bolt lugs on my rifles thank you very much.  So, I make a choice to shoot relatively light loads (light for .303 pressures) and I will not shoot commercial .308 ammo in my No. 4 7.62 conversion, regardless of what anyone says on this forum.   That’s just my educated opinion.  I’m fine with others who want to ignore my rambling, I’m just stating what I have chosen to do.  

It’s amusing actually.  Some years ago I had this same argument about the M1 Garand, except I took the opposite side.  I was relatively new into vintage rifle shooting at the time.   I firmly believed that the best load was one that duplicated the .30-06 M1 or M2 ball ammo service velocity and pressure.  Heck, this is what it was designed to shoot!  How could that be wrong! Well, several much more Knowledgeable shooters jumped all over me giving the reasons I state above.  I argued, and eventually they just told me to knock myself out, knock the rifle out while your at it.  But, several years later I eventually came to their way of thinking.   Re-read what Brian D said, then read it again.  He knows what he is talking about. 

I think this post should clearly state my position.  I do not wish to insight discourse.  

So now, I really need to shut up!  


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Post Options Post Options   Thanks (0) Thanks(0)   Quote The Armourer Quote  Post ReplyReply Direct Link To This Post Posted: May 15 2020 at 6:02am
From the Ammoland (USA) website.

Says it much better than I can :




7.62 NATO vs .308 Win – Case

Measuring the thickness of cartridge cases is kind of a pain, especially since I tend to mash them all up when trying to cut them in half with my Dremel tool. So, I took the shortcut to illustrate the difference. From my big bucket of .308 / 7.62 brass, I selected some representative samples of both commercial .308 Winchester and 7.62x51mm NATO brass and weighed them. I picked several of each and averaged the weights. I didn’t measure the cases because they’ve been fired, so that won’t tell me much other than the general size of the chamber from which they went bang.
Hornady .308: 169.6 grains
Winchester .308: 163.3 grains
Lake City 7.62x51mm NATO: 183.5 grains
That’s a significant difference! Thicker walls combined with similar exterior dimensions means less powder capacity and a lower “top end” and all else the same, lower pressure and velocity.
The thicker brass of 7.62 cases is a significant factor for the reason we'll discuss next.

7.62 NATO vs .308 Winchester – Headspace

Last but not least we get to the real difference. Military rifles for 7.62x51mm NATO can, and usually do, have longer chambers. In things like machine guns powered by ammo made all over the world, there’s got to be some slack for reliable feeding and operation with all that violence going on during the feeding and ejection process. The solution is to make the chamber headspace a bit longer. If you’re not familiar with headspace, think of it as the distance from the bolt face to the point in the chamber that stops forward motion of the cartridge case. If chamber headspace is too long for a cartridge, it will float back and forth in the chamber. If headspace is too small, the bolt will not close properly or will require excess force to close.
How much different is the headspace? The .308 Winchester chamber headspace is between 1.630 and 1.6340 inches (SAAMI Info). The 7.62x51mm NATO is between 1.6355 and 1.6405 inches. While the published numbers show about six-thousandths of an inch difference, it’s not unusual for the headspace in a surplus 7.62 rifle to be 10 or even 15 thousandths longer than that of a commercial .308. That doesn’t sound like a big deal, right up to the point where you fire thinner commercial brass in a long-chambered rifle. The brass will stretch, possibly enough to contribute to a dangerous case rupture. Doing the same thing with thicker military brass is no big deal and the way the system was designed. Thicker brass can handle some extra stretching into a longer chamber throat, so it's no big deal.
The solution to the question is to know your rifle and what its headspace really is. Only then will you know if its safe to shoot commercial .308 Winchester ammo in a 7.62 chamber.
Figuring out if your headspace is safe is a fairly straightforward deal. You can order a set of .308 Winchester Go / No-Go headspace gauges. After removing extractors and/or ejectors as appropriate in order to remove all sources of tension, use the gauges to check the chamber size. The bolt should close easily on a Go gauge and not close on a No-Go gauge. A third type, a Field Gauge checks the maximum published chamber size. With some 7.62 rifles, you might find that the bolt closes on the No-Go gauge. As long as the bolt won't close on the Field gauge, you're still within maximum published limits.

The net-net-net

Technically speaking, in terms of specifications, there are differences, but mainly in the chambers of rifles designed to fire each cartridge. 7.62 brass is a bit thicker, and commercial .308 is sometimes loaded to slightly higher pressure, but other than that, the cartridges themselves are pretty much the same.

If you want to be ultra safe and conservative, fire only 7.62x51m NATO in 7.62 chambered rifles and .308 Winchester in .308 rifles.

Next on the “risk” spectrum is the scenarios of using 7.62x51mm NATO ammo in a .308 chamber. In theory, you might run across ammo that’s particularly long. Ammo might not chamber at all or might require undue pressure to the chamber. That could result in dangerously high pressure. In reality, that would be really unusual. While 7.62 ammo could be significantly longer, that's a pretty rare thing, at least to a significant level, so most people don’t consider it a big deal to use 7.62 ammo in a .308 chambered rifle.

Where you need to be careful is using .308 Winchester commercial ammo in a 7.62x51mm NATO chambered rifle.

While most modern 7.62 chambers are probably fine as they tend to be cut closer to .308 dimensions, it’s always safest to know exactly what you have in terms of headspace. If your rifle has long headspace, stick to 7.62 NATO ammo – don’t use commercial .308.







Author :

Tom McHale is the author of the Insanely Practical Guides book series that guides new and experienced shooters alike in a fun, approachable, and practical way. His books are available in print and eBook format on Amazon. You can also find him on Google+, Facebook, Twitter and Pinterest.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Canuck Quote  Post ReplyReply Direct Link To This Post Posted: May 15 2020 at 9:33am
Thank you for this information, I greatly appreciate all of the recommendations from everyone concerning this thread.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote britrifles Quote  Post ReplyReply Direct Link To This Post Posted: May 15 2020 at 3:57pm
Canuck, can you post some pics of the rifle?  Are you sure it’s a 6 groove barrel, the Long Branch made 7.62 barrels are 4 groove RH twist, lands are fairly narrow, compared to Enfield 5 groove.  It will be stamped 7.62 and CA near the muzzle.  

I suspect these chambers were reamed to 7.62X51 NATO dimensions, based on the throat length.  My rifle had never been fired, except for the proof firing at Long Branch.  My Dad had sent it out for conversion when he was a member of DCRA (1960’s), it was a very reasonable price when done thru DCRA.  I did check the headspace with .308 commercial headspace gages and the bolt closed in the GO gage and a long way from closing on NOGO.  

I believe there are some 2000+ such conversions done by Long Branch.  



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Post Options Post Options   Thanks (0) Thanks(0)   Quote Canuck Quote  Post ReplyReply Direct Link To This Post Posted: May 15 2020 at 4:08pm
Typographical error on my part. It is indeed a four groove rifling. I'll post pictures soon, I am not at home at the moment. The rifle came to me with just the fore end so I am trying to find suitably colored top hand guards and butt stock in my boxes of NOS wood that I have that I may need to lightly stain to match the fore end. I have all the other LB parts to complete the build. The muzzle does have the 7.62 and CA stamps. I have already ordered commercially available head space gauges. I am leaving nothing to chance, I don't want any surprises the first time I pull the trigger. I have also put out queries looking for 7.62 NATO ammunition and have a few leads to explore. I want to be safe for sure as well as enjoy my rifle.   
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Post Options Post Options   Thanks (0) Thanks(0)   Quote britrifles Quote  Post ReplyReply Direct Link To This Post Posted: May 15 2020 at 4:45pm
I can probably get you some Canadian DA 59, 60 or 62.  Cases are thick, so use minimum charge weights if using .308 load tables.  This barrel likes the 168 SMK.  
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Post Options Post Options   Thanks (0) Thanks(0)   Quote britrifles Quote  Post ReplyReply Direct Link To This Post Posted: May 15 2020 at 4:48pm
Well, crap, on second thought, I think I’ve brought all the 7.62 Dad had back with me to the States.  But, I still can probably get you some, but might be a while.  Should be plenty of 7.62 available on the market tho.  The DA cases are very good, I’ve had no issues at all in many reloads.  Neck size until the bolt won’t close easily.  
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Canuck Quote  Post ReplyReply Direct Link To This Post Posted: May 15 2020 at 6:22pm
Thank you for the kind offer! I don't know the rules regarding Canada/USA ammo shipments, maybe spent brass would be okay? Maybe....we will see but gee thanks again!
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