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Old November 9, 2011, 02:14 PM   #3
Join Date: March 4, 2005
Location: Ohio
Posts: 13,722

It depends on what you are loading to shoot? Since you mention 600 yards, I assume it is Service Rifle or Match Rifle NMC shooting that interests you. On the other hand, you are using Ballistic Tips meant for hunting, so maybe not. All I can offer is what a computer simulation says a half grain of difference would do.

So, in QuickLOAD, loading with Reloader 15 to 55,000 psi, if I increase the bullet weight 0.5 grains, the velocity drops from 2760 fps to 2758 fps, and pressure goes up 140 psi. This is less than the normal shot-to-shot variation pressure and velocity that is normal in loads due to other factors. So, as far as interior ballistics (what happens inside the gun barrel), this just isn't significant.

For exterior ballistics, assuming the same external dimensions, ballistic coefficient will vary directly with weight. So the starting velocity for the lighter case with a BC of .4750 at 2760 fps verses a BC of .4164 with a starting velocity of 2758 adds up to a drop difference at 600 yards of and increase of 0.1 inches for the heavier bullet. A 10 MPH crosswind will move the heavier, slower bullet about 0.1" less at 600 yards.

So, that's as close as my calculations can come. I expect you're probably not going to worry much about half a grain after that.

As to the primers, if you go to match primers (you typically want something harder in gas guns with floating firing pins, but they're often best in bolt guns) the Federal 210M is a good one that I've used for years. Federal recommends for a large rifle primer that you seat until the anvil touches the bottom of the primer pocket, then push it in 0.003" further to set the bridge. For a small rifle primer, such as in the Lapua Palma .308 cases, that number is 0.002". That typically winds you up with the primer 0.004" below flush with the case head. You can use the primer seater on the Forster Co-ax press to set that distance below flush. You can use the K&M priming tool with gauge to measure it for each case.

If you can't do either of the above, err on the side of seating rifle primers too hard rather than too light. Erring too hard risks a misfire from cracked mix, while erring too light risks misfire from the firing pin having to complete seating, which absorbs a lot of its energy. But seating too light has another possible consequence; it can produce delayed firing by tens of milliseconds, and that allows shooter muscle contraction and trigger overtravel slam and other odds and ends of vibration to abnormally influence the position of the muzzle at bullet exit. It can also lower peak pressure by giving the bullet a slower start. So the consequences of an under-seated high primer are worse than for a hard seated primer. One of the authors of the Precision Shooting Reloading Guide said seating hard was the only way he could consistently get rifles down to under 10 fps velocity spread. So it's not a bad thing. Just don't crush them dead.
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