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Threads here have mentioned this and I got my second experience yesterday. The first time was shooting Argentine 7.62 Nato in my Ishapore. The bullets went through the target sideways, but my OFV marked ammo performed perfectly. Yesterday, in finally sighting in a 1944 Long Branch converted to 7.62 by Cogswell & Harrison, it happened once more, with Pindad ammo marked AD 85. Again my OFV ammo functioned flawlessly. How can Nato spec ammo go so wrong? It can't be the rifles.
Have you slugged the barrel to determine if it is indeed 7.62? Pull some bullets and mic them to see if they are what they say they are.
Boat tail bullets do not upset or expand to the size of the bore and can leave the bore unstable, a second problem with boat tail bullets is that the crown must be perfect or the escaping gasses can push on the base of the bullet and cause it to tip and become unstable.
Check to see if the barrel is cord worn or if the crown is worn or damaged.
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You can check for burs or rough edges with a Q-Tip
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Two rifles giving similar results with the same ammo leads me to lean towards the ammo being at fault. Not surprising though. I've had surplus Portugese NATO spec ammo shed the jackets whenever fired from an M1A.
There are other reasons for ammo being surplused aside from overruns, obsolecence, or expiration dates. Some of it was never good to begin with or suffered some deterioration.
Also, it's possible that original tracer or AP bullets etc were replaced with ball for sale on the civilian market.
JB is dead right. The NATO spec standard is pretty damned wide I can tell you! Just look at NATO spec 9mm ammo that Britain purchased several years ago. Most of it is at the bottom of the North Sea!
Nate;
External ballistics can be lots of (expensive) fun, can't it?
JB and Peter are right: there is usually a good reason "surplus" stuff is surplus. Quite often it is because it is sub-standard.
The whole business of bullet stabilisation is tied up with many variables. Mr. Greenhill's formula, circa 1879, on its own, is only a part of the answer. From Hatcher's Notebook - pg. 556 "The twist required (in calibers) equals 150 divided by the length of the bullet (in calibers)."
For a start, his "constant" of 150 predates velocities over 2000fps and boat-tailed spitzers. The "constant" gets quite variable once you add high velocity and extreme streamlining.
Boat tails do nothing to improve accuracy. They are employed to reduce base drag, especially in the trans-sonic region, well down range. This is handy for long range machine gunning, less so for precision rifle shooting. Their trajectory is flatter, but there are other issues. Unless they are a very close fit to the groove diameter of the barrel, (ideally a whisker over sized), as Ed said, they will not upset and completely obturate the rifled bore.
Additionally, boat tails result in much reduced bearing surface. Some of the NATO spec projectiles I have seen only seem to have not much more than 8mm or so of parallel surface. Unless such a bullet is precisely aligned with suitably tight grooves, it will start down the barrel at a funny angle. If the crown is less than perfect, that angle will greatly increase and result in angular yaw (see Eds Pix) and thus either precession or nutation.
Precession means that it will ultimately settle down ("go to sleep" in the old parlance). This may take a few hundred yards. If it doesn't, then "keyholes all the way" is the result. Shooting SS109 ammo in a 1:12" M16 is a good demo of that. Also, the longer the ogive of the bullet, the further to the rear the centre of mass.
Lee Enfields have 1:10" twist barrels because that was what was required for the early, heavy low velocity round-nosed projectiles.
The advent of spitzers, pre WW1 inserted a large feline into the aviary.
The 174gn Mk7 projectile is the same length as the heavier bullet in the Mk6. This was essential because it had to feed and function in hundreds of thousands of rifles and machine guns. Hence the little aluminium filler at the front of the core. Mk8Z; likewise. Take a Mk7 projectile, add a boat tail, delete the filler and hey presto, a 180gn projectile of the same length as the Mk7. Take a truckload of ammo and a crate of spare barrels to the range and fine tune the firing tables that the boffins calculated.
So, Nate; get your barrels gauged for groove diameter and run a measuring stick over those projectiles. Then chronograph the ammo. You might find some interesting stuff.
Final general note: If you want best accuracy and you want them to last, DO NOT SHOOT BOAT TAILS IN YOUR ORIGINAL LEE ENFIELD BARRELS.
Bruce, I pulled a bullet from a OFV round and it was flat based. That could explain the problem. These rounds were bought many years ago. Thanks for the insight.
"...How can Nato spec ammo go so wrong?..." Argentina is not and never has been part of NATO.
Key holing is caused by undersized bullets or an oversized barrel.
Mr. Horton is right about boat-tail bullets, but a bad crown can affect any bullet, not just boat-tails. If release from the rifling is not perfect, the bullet will not be accurate, period.
Also, if there is any throat erosion or even too long a leade, the boat-tail with a shorter bearing surface, can tip and skid even before entering the rifling, affecting accuracy. This is again true of any bullet, but less erosion is required to cause the boat-tail to tip.
Also, a big +1 to Bruce for "Boat tails do nothing to improve accuracy." The purpose of the boat tail is to increase range, not improve accuracy, though the boat tail does reduce turbulence, the same as it does in a race car.
I would disagree to a point about ammo being surplused because it is bad. The more common reason is that an army has changed ammunition for whatever reason, and is selling off its stock of the old ammo. However, much of that ammunition has been in storage for years (armies buy huge quantities and if they don't fight a war, most of it just stays in depots) and may or may not be even functional, let alone accurate.
Jim
The Argie ball that fell into my possession has been pretty accurate but the specs are a bit off - seems a bit lower powered and the cases are just a tad longer than true NATO spec ball. Other than that it shoots fine for me. FNM Portuguese ball is very accurate for me but BF marked Portuguese 7.62 ball has indifferent accuracy. Go figure.
I had the muzzle crowned yesterday so I will find out how it effects my rifle and I'll report.