Hondo64d
WKR
I recently got a bunch of 178gr ELD-Xs that were shooting OK, but not as well as I thought they could. Tried a couple of powders and the results were largely the same, so before giving up on them, I decided to try a seating depth test. Load consisted of 41.6gr 8208 in once fired Alpha brass with BR2 primers. All groups were 10 round groups. I started with the bullet lightly touching the lands and went deeper in .025” increments, testing from touching to .075” off the lands.
Long ago, I read a post from Jon Barsness IIRC, that with tight freebore diameter chambers, he wasn’t convinced seating depth made a big difference in precision. From this test, I tend to agree. This rifle was chambered with a Bisley Palma reamer, with a freebore diameter only .0005” larger than bullet diameter, much tighter than a SAAMI spec .308 reamer. With many “modern” cartridge reamers, the SAAMI spec reamers have a much tighter freebore than most older cartridge designs. One example is the 6.5 Creedmoor. The theory is, and I agree, that the tighter freebore diameter keeps the bullet better aligned with the bore before the bullet engages the lands, which in turn provides better precision than a bullet engaging the lands slightly off axis.
Sample size is somewhat lacking here, but given the confidence interval of 10 shot sample sizes, I’m not convinced that seating depth made any significant difference in the precision of this load. I am going to go with the .050” off the lands load because the mean radius looks marginally better with that load, but again, the difference is not large, and sample sizes are not large enough to say with confidence that one depth is better than the other.
John
Long ago, I read a post from Jon Barsness IIRC, that with tight freebore diameter chambers, he wasn’t convinced seating depth made a big difference in precision. From this test, I tend to agree. This rifle was chambered with a Bisley Palma reamer, with a freebore diameter only .0005” larger than bullet diameter, much tighter than a SAAMI spec .308 reamer. With many “modern” cartridge reamers, the SAAMI spec reamers have a much tighter freebore than most older cartridge designs. One example is the 6.5 Creedmoor. The theory is, and I agree, that the tighter freebore diameter keeps the bullet better aligned with the bore before the bullet engages the lands, which in turn provides better precision than a bullet engaging the lands slightly off axis.
Sample size is somewhat lacking here, but given the confidence interval of 10 shot sample sizes, I’m not convinced that seating depth made any significant difference in the precision of this load. I am going to go with the .050” off the lands load because the mean radius looks marginally better with that load, but again, the difference is not large, and sample sizes are not large enough to say with confidence that one depth is better than the other.
John
