Questions about run-out and seating depth

[roberti11]

I am no expert so I am putting this post up to attract opinions from those of you who have been around precision shooting a while.
After reading several posts about loading bullets ‘close to the lands’ I tried to do this with my Berger 155gn bullets. My rifle is a Rem700VS in .308, it has had the bolt lapped but no other work performed. The twist is 1/10 and the chamber is a standard commercial chamber. I use Lapua brass, and a Forster co-ax press with a Forster micrometer seating die.
For Berger 155gn bullets the OAL to touch the lands is 3.003, so I loaded these 10 thou less at 2.993. When I shot these I found that they grouped quite poorly ~0.8inch at 100 yards. I backed them down to an OAL of 2.9003 (100 thou off the lands) and the groups improved to ~0.5 inch at 100 yards. After I acquired a concentricity gauge I found that the rounds loaded to 2.993 simply had poor run-out. I assume that there was too little bullet in the case to allow for reliable alignment of the axis of the bullet with the axis of the case (ie good run-out). I probably can get away with an OAL of 2.950 and still maintain good run-out but that will be for me to solve next time I go to the range.

Next I worked up a load of Berger 175 gn bullets. This time I worked the bullets back slowly from a maximum OAL of 3.019 (touching the lands) until I got seating with reproducible run-out of ~ 1 thou. In this case it came to an OAL of 2.980 (40 thou back from the lands). This seating performs considerably better than a seating of 2.800 (200 thou back from the lands) as attested by the attached pictures (the only real difference between the two groups is seating depth).

Can I assume that my inability to get bullets within 5-10 thou of the lands is related to the fact that my commercial chamber is simply too long? Or is there some other factor that keeps me from getting reliable run-out when I load the bullets too long?

Is there a formula for optimum minimum seating depth? It seems that the two loads that I came to by trial an error have ~10% of the bullet in the case. If I have much less than this I start to get poor runout, and poor groups.

 

[maynard]

Some rifles like the jump, some shoot better touching the lands, some like the bullet forced into the lands.
This of course depends on the bullet having very little or no run-out.
Try seating the bullet then rotating the case 180 and cycle the press again. Sometimes this will staighten the run-out.
You are going about it the correct way. Find the load that works best then play with the seating depth to fine tune it.

 

[alberta tactical rifle]

Not all rifles perform best with the bullets touching or even nearly touching the rifling, experimentation will be the answer.
Having a concentric chamfer on your brass helps, and as mentioned turning the case as seating is happening will help, I turn the cases in 1/3 increments while seating on my match ammo, despite using a Redding match seater die. My Neco runout gage says this gives me under .001 run out
You may want to get your headspace checked as well, lapping lugs removes metal, thereby increasing headspace, sometimes you get unluckt with a new rifle, where the chamber is loose to begin with, the when the lugs are lapped you have excessive headspace occur, be worth checking just to be safe.
Also before I panicked I would try the loads at further distance, many of the VLDs and ULDs do not fully stabilize until beyond a 100 yards, you may find all in the same hole at 200.