Seating Depth Question (ogive to lands)


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MarkA
June 15, 2012, 10:42 PM
I had great luck finding an accurate load in my .308 with the 168gr SMK. This load was 2.825" COAL (base to tip); shot .292moa @ 200 yards a few weeks ago.

I recently started playing with the 175gr SMK, and have also been playing with the 165gr Nosler Ballistic Tip. I've been semi-concerned with keeping my cartridges at the 2.825" max because that's just shy of magazine length. My NBT hunting load is solid; 1.5" or better groups at 200 yards. The 175 SMK? Well, let's just say it's not shooting as good as the 168 yet and it's got me a bit bewildered (it's a 1:12" barrel... by all accounts it should be right there in the sweet spot).

I'm not overly worried about mag length, but before it's even suggested... know that the throat on my rifle is too deep to seat to the lands, or even .03 off.. just ain't gonna happen. The NBTs the longest of the three bullets, and when touching the lands, I only seat at about 1/3 neck depth. The other two's base to ogive measurements are similar / shorter. I'm not too hip on the brass just barely hanging on to the bullets, and based on the 168gr SMK performance... not necessary.

I was thinking about it this morning, so at lunch I went to the local store and bought a bullet comparator. Does it seem logical to assume that seating these 175SMK and 165NBT the same distance off the lands as the 168SMK (ogive) will yield the best results? Or is this just going to be another variable to play with?

If this logic is sound, the the NBT would be 2.9" COAL, and the 175SMK would be 2.848" COAL, keeping the jump to the lands the same.

Opinions? Go. :)

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243winxb
June 16, 2012, 09:42 AM
Years ago when Remington & Dupont were one, loading data & ammo showed they found the COL by where the bullet base was seated. Take the longest bullet for the caliber (308 win.) seat so round fits the magazing. Find where the base of the bullet (full diameter,not boat tail) is. Seat the rest of the shorter bullets with there bases in the case at the same spot. Then work up loads. Note that factory ammo does not cover the full range & weight within a caliber that is available to the handloader. The perfect setup would be to me, bullet base at the neck shoulder junction. The ogive would be .010" of the rifling. Not easy to do with factory chambers.

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