My milsurp alchemy. The results.

[Pond James Pond]

First the conditions. 300m was closed so I was not able to shoot there and had to make do with 100m instead. It was a warm day, lots of mirage, and a slight breeze.

So a recap of what this was all about. Basically I had access to some Brazilian mil-spec .308. I pulled a stack of them, measured the OAL and powder charge and then reassembled them. However, I reset the OAL to get me closer to the lands in my rifle. I also gave more homogeneity to the standard powder charge (43.8gn) as well as doing a little charge weight ladder starting at a weight 7.5% below the one above.

Given the number of cases I had I was able to do this with 7 shots per group. The charge weights came in at 42.6, 43.1, 43.6, 43.8 (the homogenised original) and 44. I also threw in 7 untouched rounds for comparison.

All were fired suppressed.

The results were on the one hand interesting yet on the other predictable. The predicatble side was that there was not some dramatic difference between the original cartridges and any of my home-tune jobs. The interesting thing was that at 43.8gn, 44gn and the untouched originals, I got some stiffer bolt turns. That to me suggests over-pressure of some kind.

Does 7.62 NATO have higher pressures than .308 Win as with 5.56 NATO and .223?

As a result I ditched the 7th 44gn cartridge.

For each group I measured MOA horizontally and vertically. They're listed in that order:

42.7gn: 1.6MOA/1.4MOA
43.1gn: 2.2MOA/2.1MOA (6 hits: one seemingly went straight through another bullet hit!!)
43.6gn: 1.5MOA/2.2MOA (drops to 1.0/1.5 if I ignore one outlier)
43.8gn: 1.7MOA/2.1MOA
44gn: 2.3MOA/2.7MOA (only six shots)
Original: 0.8MOA/2.8MOA

Of those, I'd have to discount the standards, 43.8 and 44gn due to sticky extraction, but 42.7gn looks promising. Amongst the smallest groups and also pretty uniform in shape. A call to a club member about the sticky extraction (he's shot them too) and it was suggested this might be because of slightly long cases from factory, perhaps due to the crimp? However, it still doesn't explain why it only happened at higher loads. Perhaps someone can clarify.

I can also say that shooting with a suppressor is way less tiring and recoil is indeed much less.

My take-home from this is that, whilst not ground-breaking, those rebuilt with 42.7gn of powder and a slightly longer OAL for my gun would definitely suit my needs from a practice round. They can also be reloaded, being boxer primed. This suddenly makes my .308 cheaper to shoot than my .38!!

A very informative outing.

 

[Pond James Pond]

Here are the 3 charge weights that gave me some incidence of sticky extraction:

 

[Pond James Pond]

Does 7.62 NATO have higher pressures than .308 Win as with 5.56 NATO and .223?

A call to a club member about the sticky extraction (he's shot them too) and it was suggested this might be because of slightly long cases from factory, perhaps due to the crimp? However, it still doesn't explain why it only happened at higher loads. Perhaps someone can clarify.

Can anyone comment on these two related points?

 

[Jimro]

Ok...

No, 7.62x51 and 308 Win do not have different chamber pressures when you use the same measuring methods. Individual loads may be higher or lower than others, and the milspec loads also have to meet "port pressure" standards where 308 Win doesn't.

5.56 and 223 are also not different except when they are. The Army recently increased chamber pressure for the M855A1 round, but the older M855 and M193 are within published 223 Rem pressure specs. That being said, bad storage conditions can significantly raise chamber pressures, so a lot of milsurp ammo was surplused because it is out of spec.

What you have to know is what chamber you have. How your rifle is chambered will have a larger impact on chamber pressure than the loading spec of the ammunition. In AR-15s shooting crap ammo a looser milspec chamber is a good thing for reliability and safety (not so much for accuracy though).

I believe that a lot of the 5.56/223 Rem problems come not from the chamber, but from secondary pressure spikes caused by slow burning surplussed powder pushing a too light projectile. Other people are absolutely convinced that this is wrong because rechambering to a 5.56 "loose" chamber will often cure the symptoms, but they can't explain why (and I think that the shallower entrance to rifling changes the action timing so that the secondary spike happens while the bolt is still actively engaged in the lugs, sealing the primer against the bolt face instead of popping it into the trigger mechanism).

Jimro