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Old June 19, 2013, 12:33 PM   #13
Bart B.
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Join Date: February 15, 2009
Posts: 8,927
Eppie, if those pictures were exactly where 30 caliber bullets fit in case necks as thick as pictured, I would agree with you. But those pictures show neck wall thickness spread many times greater than 1/25th bore diameter as they're about 1/6th bore diameter. Therefore, they're a poor examle of showing bullet offset in the neck with normal neck wall thicknesses and variations.

Besides, seating bullets so they gently jam into the lands when chambered takes care of a thousandth or two of off center bullet diameters at the leade-ogive contact point anyway. How much off center from the outside diameter is a 30 caliber bullet in a case neck with a .001" spread in neck wall thickness anyway?

And even with exact neck wall thickness all the way around the neck, unless fired bottleneck cases headspacing on their shoulders are full length sized reducing neck diameter all the way back to the shoulder, all that exactness in neck wall thickness is lost. Case necks will otherwise not be perfectly centered on the case shoulder; case necks will float off center in the chamber neck when the round's fired.

Would you believe what I'm talking about if I showed you two 15-shot groups at 1000 yards, both under 5 inches, one with and the other without turned necks? None had any flash hole uniforming. One group was with new, unfired cases; the other with 2x fired full length sized cases with turned necks. And all 30 rounds went into about 6 inches; the most accurate long range benchrest rifles do not shoot any more accurate.

It's very normal that a 1/4" at 100 yards becomes 8" at 1,000 yards. Virtually all groups' subtention angles get bigger as range increases due to muzzle velocity, subtle air movements and the bullet's BC spread across each shot fired. It's usually about 10% increase for each 100 yards of range past the first 100 yards. Folks have known this for over a century. After all, all groups are zero inch/MOA at the muzzle, aren't they?

The exception's positive compensation caused by bullets having a big muzzle velocity spread leaving as the muzzle axis is on its way up to its greatest angle. Slower ones leave later at higher angles than the faster ones leaving sooner agt the lower angles. Accuracy at long range was better than at mid range. This was proved over a century ago and is a long known issue with British SMLE's with cordite charged .303 ammo. M14NM's also exhibited a small amount of it with M118 7.62 NATO match ammo.

Last edited by Bart B.; June 19, 2013 at 02:46 PM.
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