.284win neck turn question

[Topshot]

I have a .284 win with a .312" chamber neck diameter. I use 6.5-284 Lapua brass and have turned it to 0.3095".
This seems to de giving large ES and vertical grouping at 500 yards.
Can anyone offer advice on just how much brass to turn off my necks to cure the problem?

[boomtube]

"This seems to de giving large ES and vertical grouping at 500 yards."

"Seems" to? Why do you attribute a large velocity ES to neck turning?

[JeffVN]

+1 why do you think your neck clearance - pretty tight - is affecting your load results?

Jeffvn

[WAMBO]

Here's some sound advice from someone in the know, got this from the 6mmBr.com site.

Jerry Tierney, a past NBRSA 1000-yard champion and .284 Win “guru”, has observed that overly tight chamber neck tolerances can cause accuracy problems. Jerry says “some guys who were running minimal neck clearance .284 chambers weren’t getting the accuracy they expected. If you open up the neck to allow more clearance, say .0015-.002″ per side, that seems to solve the problems. I can’t tell you exactly why–maybe it allows a ‘cleaner’ bullet release–but easing the neck clearance has helped many .284 shooters get better results.”

Found it under the 7mm Cartridge guide:

7mm Cartridge Guide within AccurateShooter.com

[Mikecr]

I don't see how neck clearance(tight or loose) would prove to make any difference.
I'd first be considering seating depth, primer/strike, load, component consistency, and overall gun design.

[Topshot]

I read that article in 6mmBR thats why I am asking this question.

The rifle is new with only 50 rounds down the tube. Its got a 28.5" heavy barrel on a Lawton 7500 single shot action, Jewell trigger.

Run it in for 20 then started group testing. 5 shot groups will put 4 into less than 3" at 500 yards with a random shot thrown 10" high or low. Chronograph readings show about 100 fps with the odd round.

Soot on the fired cases stops at a ring near the base of the neck so maybe donut issues? Load is 53gn H4831sc with CCI Benchrest primers. 175gn Sierra SMK seated to kiss the lands. Fired brass necks come out at .310"

[Mikecr]

Well you likely turned away the donut caused from upsizing, right?
The sooting could be from excess trimming(too much clearance from chamber end). This can also increase ES.

Why kissing the lands?
Doesn't sound like you tested for best seating yet. Try moving into, or clear of the lands & see if your crazy flier disappears. Be mindful that this area greatly affects pressure one way or the other, so load adjustment would be in order.