My No4Mk2 with the 2A1 7.62x51 barrel chambers and fires my reloads with no issues.  I use RCBS .308 reloading dies. I full length resize. I use the minimum 40.0grn charge of Norma 202 powder, primed with WLRM primers, and topped with either Sierra 168grn TMK or Hornady 168grn A-MAX projectiles seated to an OAL of 2.800".

The only difference between the .308 and NATO 7.62x51mm is avery small variation in tolerances owing to the military chamber being a tad more generous then that of a commercial sporting rifle,therefore, some commercial rifles will not handle military ammunition. I own two commercial rifles in .308 and my No4Mk2 in 7.62x51mm NATO.  I full length resize so I can use one cartridge for all three rifles. Again,no clambering or firing,and extraction issues at all with the Enfield. I used a .308 field gauge for my preliminary checks and will continue to do so.That rifle should handle reloads without worry.. A perfect duplication of a NATO round is with 37.0grns of IMR 3031,primed with Rem 9 1/2,using a 168grn jacketed BT. It gives you a velocity of 2336 with only 35,000 psi......

i shoot both in my M1A it works fine with either , some rifles do not - the no4 conversion i once owned hated commercial ammo , my 2A and 2A1 did not care much , but i never shot any heavy loads in any of them , 

The baseline answer to you would be that,you have the potential of owning a rifle that was proofed to 19 tons,if not a little more and will handle commercial ammunition or, reloads if one is to stay within reason of charge weights. 

The 7.62x51 NATO chamber has a more pronounced area similar to that of our trusted Enfield to allow for dirt and debris while in the field. The commercial rifles have a tighter tolerance. Now,ignore that last statement. 

If you have factory .308 ammunition,use it. It will fireform to that rifles chamber,you can then necksize if you so choose and those cases will be for that rifle only. If you full length resize with the .308 dies, you will never worry if that cartridge is going to fit into the NATO rifles chamber. 

This is how minuscule the differences are between a 7.62x51 NATO  and a commercial .308 round. The cartridge on the left was fired out of the 7.62mm Enfield, the cartridge on the right was fired out of my Remington M700 .308. Notice the shoulders on both? The NATO round has a more pronounced and elongated shoulder whereas the .308 round has a shallower shoulder. The fired NATO round is a smidge longer as well,because of the aforementioned chamber area.Both rounds where originally full length resized. Full length resize and use a minimum charge weight and drop those rounds into that 7.62x51 and go to work...

1.  A fired case drops into my .308 Wilson case gage and the case head protrudes just slightly past the end of the gage.

2.  The Canadian DA 7.62 ball ammo drops into the .308 case gage and the case head sits between the min and max “step” on the gage, indicating correct case headspace for a .308. 

3.  Using a case headspace comparator, the difference between an unfired service cartridge case and fired case is 0.004 inches.  The fired case measuring 1.628.

4.  Sample round of disassembled DA 7.62 1962 ball ammo has a 147 gr boat tail hard point bullet, 43.5 gr stick powder resembling IMR 4895.  Primed empty case weighed 188.6 gr.  This is what Canada was shooting in DCRA matches as “issued” ammunition.

5.  Nothing wrong with neck sizing .308/7.62 for a bolt rifle, it will prolong brass life, less time in brass prep.  

I’ve not checked the headspace on either rifle, I don’t have a .308 headspace gage.  But I do know the history of these rifles, who built them and who owned them.  Bolt SN matches the receiver and I’m not worried about the headspace.  Based on the case headspace increase after firing, I’m not concerned.  Would I check the headspace on a rifle I bought with unknown history?  Yes absolutely.  

I believe there are slight differences in .308 SAAMI and 7.62 NATO headspace dimensions, the SAAMI Field being a bit tighter than NATO. 

I do not plan to shoot any commercial .308 ammunition in these rifles.  Like Goosic does, I load to the low end of the .308 load data getting .303 velocities with equivalent weight bullets.  I get excellent accuracy.  I believe that all the complaints of poor accuracy from the No. 4 7.62 conversions had everything to do with the ammunition and nothing to do with the cartridge, mostly to do with bullet quality.  Did higher 7.62 pressures contribute to the problem?  Yes, possibly.  

If someone plans to shoot a 7.62/.308 barreled No. 4, I recommend loading your own ammunition to equivalent .303 velocities.  I happen to use 40.0 gr of Varget in both the .303 and 7.62 using a 174 gr SMK in the .303 and 168 gr SMK in the 7.62.  

 

The targets that I post on here with all those tight groups are not done with store bought ammo,it is done with my handloads and I pride myself in the knowledge that I am keeping all the chamber pressures to a minimum along with using the minimum required starting charge weight without sacrificing accuracy. I am by far,not the most intelligent person on this planet but,I definitely know what I am doing when it comes to reloading for my equipment. If I am to give you any information regarding  a particular load,rest assured that I will never give you any falsehoods......

The bottom non-firearm is the one I got on the mail.

Interestingly, fired cases from both my 7.62 barreled rifles measure 1.628 inch headspace using the comparator.   One was built at Long Branch, and the other by DCRA Armourer and member of the Canada shooting team Dave Reynolds.   I’ve got a copy of a DCRA news letter he wrote giving instructions on how to rechamber a .303 barrel to 7.62, he used a .308 Win chamber reamer.  Some of his rifles are probably still around. 

I’ve not seen 7.62x51 dies.  I’m using Lee .308 Win dies, the collet neck sizer and keep the brass separated for the two rifles.  I like the collet die as it eliminates the need to lube the case necks.

Accuracy with the Canada service ammunition was not exciting to say the least, 3 to 4 inch 10 shot groups at 100 yds.  Handloads with a 168 match bullet dropped this to 1.5 to 2 inches, sometimes less.

Ive come to realize that with the amount of shooting I do, reduced loads will help reduce wear on the rifle and on me.  As long as accuracy is acceptable to me, the lower the chamber pressure, the better.  

Which is right here in Scottsdale Az sells once fired Lake City brass. It is slightly time consuming because of the thickness of the brass when you start the resizing process but well worth the time spent once you are done.

Once you get past all the hype & misunderstandings its simple. 7.62 is a military spec version of .308 Win. (actually not they are both developed from the same parent round , the .300 Savage

The reloading dies are identical (which should tell you something)!

Yes the brass is thicker, but thats not really the point. Its thicker to withstand the greater firing & cycling stress of fully automatic weapons.

Because the outside dimensions are fixed so it'll chamber the thickness takes up internal, not external volume. Now because of this the charge weight has to be reduced (typically 1.5~2.0Gr for any given load) to keep the pressures to the same levels.

Most of the confusion comes from the 2 rounds using a different & non-interchangeable measuring system & data!

It like saying which is colder 32℉ or 0℃!