Any ideas why my 1-8 twist 6.5mm barrel refuses to shoot 142's well

[bgordon]

Any ideas why my 1-8 twist 6.5mm barrel refuses to shoot 142\'s well

Just got a new barrel in 260 remington caliber. It is 27" finished length heavy contour 1-8 twist.
In the last month I have ran in excess of 500 rounds thru the barrel while doing some load development. The 120's work just fine but for some reason the 139, 140, & 142 grain bullets don't seem to shoot nearly as well.
The pattern on the various brands of 140's is pretty consistent. Out of 10 shots in a group 5 or 6 will be in a nice tight 1/2 MOA group with the rest being randomly placed flyers which open things up to between 1 & 2 MOA. It seems to stay consistent at all ranges out to 600 yards. 100 yard groups show the flyers as do 600 yard groups.
I have tried various powders, velocities, seating depth, and anything else I could think of to get the bullets to shoot well. I ran the shots across the chronograph to see if the loads might show a lot of velocity variation. No luck so far.
Any suggestions?

If I can get this problem licked the rifle will be my new 1000 yard prone match rifle.

 

[BountyHunter]

Re: Any ideas why my 1-8 twist 6.5mm barrel refuses to shoot 142\'s well

Bruce

Without more info it is hard to tell. Is it tight neck, what brass, seating in or off lands, moly etc?

As a general rule sierras shoot best just kissing lands or little off. The custom vlds such as berger, jlk, cartaruchi like to be into the lands as much as .025.

Never could get sierra 142s in moly to shoot consistent in my 6.5/06AI. Went back to plain and tightened right up.

Could be brass problem also. If you are not neck turning and using remington brass especially.

Try using the Audette or ladder method of load development with one bullet and powder and find the barrels sweet spot and fine tune from there. I normally leave sierras about .010 in for that test and once find powder load work it back out. With the VLDs, I jam seat, measure the mark, and seat at half that distance and shoot ladder and come back on seating later. Leaving long for test means you only have one way to go to find best spot and that is back.

Mine with 142 will group from 5-8" consistent at 1000. Working with the 140 JLK and it seems to have better potential to come under 5". However the 142 has a bearing surface that is .112 longer than the JLK, that causes problems in enough neck for the jlk with my throat. Mine was originally throated for 142s which have a longer bearing surface than VLds.

Good luck

 

[bgordon]

Re: Any ideas why my 1-8 twist 6.5mm barrel refuses to shoot 142\'s well

Ahhhhh, finally figured out what the problem is. When I prepped all the new brass I used a standard inside and outside deburring tool. You know, the type that has a 45 degree angle cut to clean up the inside and outside faces of the neck. I bought a 20 degree neck deburring tool from Sinclair and did a couple of turns on 50 pieces of brass then loaded it up with some VLD bullets.

Flyers disappeared!!!

Now the rifle is shooting 1/2 MOA with VLD bullets as well as with 120 grain Matchkings.
I went out to the range this morning and shot 40 of the 123 Lapua Scenar's loaded to kiss the lands with 48 grains of Reloader 19. Got 3124 fps average velocity with an extreme spread of 32 fps for 10 shots.
Shot 10 shots at 300 yards across the chronograph with a nice tight 1 1/4" vertical spread. The horizontal spread went 2 3/4" but the wind was gusting from 10 to 25 mph and switching directions.
Shot three 10 shot groups at 500 and let the barrel cool for 15 minutes between strings but did not set up the chronograph because it blew down in the wind at 300. None of the groups went over 1/2 moa vertical except for one low called flyer. The groups were not well centered on the target, but I attribute that to the wind switching around so greatly. All three of the 500 yard groups had less than 1 moa horizontal spread, so the high BC is super helpful in windy conditions.

It is funny how such a little thing can make so much difference. Next thing will be to retest the 140's and see if the problem disappears with them as well, even though I intend to use the Lapua 123's because they have nearly as good a BC as the 142 Matchkings but 200 fps additional velocity.