.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.

 

[Bart B]

Two things that can cause irregular velocity are weak firing pin springs and insufficient firing pin protrusion from the bolt face. I don't know what your action's firing pin spring's rating is supposed to be but firing pins need to stick out .060" to .065" from the bolt face.

You might try IMR4350 powder and see if that helps. Slower powders sometimes have higher velocity spreads in cases the size of a .284.

 

[BillR]

I was having a similar problem with my .25-06AI tight neck. Drove me nuts for a while. I finally moved up 1.5 grains in half grain increments and noticed two things. 1 the ES got smaller and 2 the soot ring went away. Not sure that is your problem but it took care of mine. I was using Retumbo powder which is considerably slower so you might not have to go up that much but it will also depend on where your pressure signs show up. Sometimes a faster powder helps too. I also do not like less than .0015 clearance per side either. If you get some soot build up it can actually make it too tight. I will also fire a couple shots through the gun just to get things settled in on a clean bore prior to shooting for group or over the chronograph.

 

[JeffVN]

Your rig is nearly identical to the one my brother built. He and I shoot frequently, so I have shot it a ton. He runs H4831SC, Berger 180 vlds, but uses Fed. 210Ms for primers.

I have no problem with the burn rate of the powder, given how well my brothers runs. Nor do I have a problem with the neck clearance, as my custom chambers run the same 0.0015 per side. I am not a big fan of trying to seat "touching" the lands. For me its either jump them or jam them, no in-between.

Since you didn't post a picture of your fired brass, I'll go ahead and ask - did you turn the necks all the way into the shoulder? if not you have a doughnut on the back end of the neck. If you are seating down into that doughtnut, it will create problems.

+1 - once you settle on a load, then you tweak the seating depth (both in and out) to see if you can hone it even further. For me, with my .313 neck (.310 loaded rounds) 7WSM shooting BErger 180 VLDs, there is a huge accuracy change between "touching" (1+MOA) to 0.015 jammed (bugholes). My brother's .284 reacts the same.

if you've never jammed before, make sure you reduce the load and work up; its a different world.

Jeffvn

 

[Topshot]

Today, I seated the bullets 20 thou off the lands in brass that had an extra 1 thou turned off the necks (right down into the shoulder). Charge was bumped up to 54gn and the difference was amazing.
The rounds chambered and extracted easy, and the velocity ES was back to normal levels. Speed was 2770 fps and accuracy for 5 shots was 1/2" so one of these things worked.

 

[BillR]

Probably a combination of more than one thing. The hair jerker is which one but then why worry as it sounds like the problem has gone away.

 

[rscott5028]

Inconsistent neck wall thickness may contribute to MV ES/SD.

Turn the least amount possible (80-90% cleanup) unless you have a tight necked chamber which you don't have.

Any more than necessary is counter productive.

-- richard