Seating depth

[jpruitt]

I have a 6.5/284 Broughton 8 twist barrel shooting Berger 140gr. Match bullets, CCI BR2 primers in Lapua brass and pushed with 49gr of H4831sc powder. Looking for thoughts on seating depth. How far off the lands, as of now it is .005 off, does it need more jump. Looking to tighten up my group more. Thanks in advance for any help. Jim

 

[tenring]

You will never know unless you try it. I run Sierra 142 MK's at .005" jam.

 

[Shynloco]

You will never know unless you try it. I run Sierra 142 MK's at .005" jam.

+1, Gotta try because each rifle is different. I too use Sierra 142's and I'm .010 off jam and still testing.

Alex

 

[lpreddick]

i suspect seating depth and neck tension could be the two most significant variables that determine accuracy. there are, obviously, those that will disagree, no problem. if you can get enough cases with the same neck tension, and you can precisely measure seating depth, start .005 off lands and move bullet .005 in groups of 3-5, ex .005, .010, .015, .020 and .025 off lands and you may see the group not only tighten but it moves. h4831 should work but sometimes this process doesn't and you can change powder (faster or slower burning) and things might improve. i once did the above and found small groups that weren't there months later and i suspect neck tension variation over time...some disagree here. most accuracy nuts, me included, seat bullets into the lands and then vary powder charge by .2-.5 gr depending on the cartridge and usu find the small holes. welcome and good luck.

 

[Shynloco]

i suspect seating depth and neck tension could be the two most significant variables that determine accuracy. there are, obviously, those that will disagree, no problem. if you can get enough cases with the same neck tension, and you can precisely measure seating depth, start .005 off lands and move bullet .005 in groups of 3-5, ex .005, .010, .015, .020 and .025 off lands and you may see the group not only tighten but it moves. h4831 should work but sometimes this process doesn't and you can change powder (faster or slower burning) and things might improve. i once did the above and found small groups that weren't there months later and i suspect neck tension variation over time...some disagree here. most accuracy nuts, me included, seat bullets into the lands and then vary powder charge by .2-.5 gr depending on the cartridge and usu find the small holes. welcome and good luck.

I certainly do not disagree with anything ipreddick writes. I would just caution that the part about accuracy nuts seating into the lands and then vary the powder charge by .2 - .5, doesn't always result in "the small holes." I went through that process with my .308 Benchgun shooting 168 gr HPBT's and was looking to see if I could get a tighter group that were already in the .300 - .400 range at 100 yds. And what I discovered by moving the seating depth .005 at a time and changing the powder loads by .02 in each direction, was that the closer I got to jam, the bigger the groups no matter the powder load change. Now I did this exercise over a lengthy period of time to make sure I wasn't missing anything. I ended up (with that particular barrel) finding that it flat needed that .020 jump and when I got to jam, the rifle shot absolutely the worse it has in over 20 years.

My point - do not lock into one particular setting or even powder until you start seeing remarkable improvements. Obviously you will note the groups getting smaller when you are on the right track. I should mention that when I put on a new Match Grade barrel, that same rifle made a remarkable improvement when I changed to another powder from what she used to shoot .300 -.400 sized 5 shot groups. That barrel was the same twist (same bulllets too) as was the previous one but that only showed me what difference a barrel switch can make, let alone finding the sweet load in each. Good luck in your search.

Alex

 

[jpruitt]

Couple of things I think I need to try, one is, as right now it's running 2837fps with 49gr of H4831sc. I think I need to get it to around 2925fps or maybe a little more. There is no signs of high pressure on the necks or primers. The bullets are seated .005 off lands and will start pulling them off more and see how groups do. The neck on a loaded round measured .294 and I ordered a .292 neck bushing so sould b good there. Wish me luck, will let u know. Thanks again for the help Jim