6mm creedmoor seating

[ddp43]

I'm new here. I've been reloading for a while. I just got a browning xbolt pro LR in 6mm creedmoor. I've put about 200 rounds through it trying to find something that will group sub moa consistently. When I first bought the gun I measured the distance to lands at 2.290. Im now getting 2.340, which I dont know how this is possible. Theres no way to seat anywhere near the lands. I measured factory federal ammo with 105 burgers at 2.120 cbto. Ive been loading anywhere from 2.200-2.250 cbto. I've tried nosler 105 rdf bullets, and 103 eldx. I've tried h4350, rl16, h4831sc, h1000. Nothing has gave satisfactory results. Whats concerning is what seems like very fast throat errosion.
Does anyone have any insight on the throat errosion that may be occurring? Also what are some of your cbto lengths you are loading, and distance to lands on your rifles?
Thanks

 

[Dthomas3523]

I'm new here. I've been reloading for a while. I just got a browning xbolt pro LR in 6mm creedmoor. I've put about 200 rounds through it trying to find something that will group sub moa consistently. When I first bought the gun I measured the distance to lands at 2.290. Im now getting 2.340, which I dont know how this is possible. Theres no way to seat anywhere near the lands. I measured factory federal ammo with 105 burgers at 2.120 cbto. Ive been loading anywhere from 2.200-2.250 cbto. I've tried nosler 105 rdf bullets, and 103 eldx. I've tried h4350, rl16, h4831sc, h1000. Nothing has gave satisfactory results. Whats concerning is what seems like very fast throat errosion.
Does anyone have any insight on the throat errosion that may be occurring? Also what are some of your cbto lengths you are loading, and distance to lands on your rifles?
Thanks

How are you measuring your lands? The first and easiest answer is an inconsistency in your measurements.

Even if you for some reason have some sort of long freebore try this:

Seat your bullet where the bottom of the bearing surface is just above the neck/shoulder junction. Load your preferred powder near the charge weight that will get your your desired speed. Load 1 grain up and down from there in .2 increments. 5 shot groups.

Shoot them over a chrono without shooting groups (no need for groups yet). Find the node that has the closest velocity and also a consistent acceptable ES in each charge weight in that node. Once you identify the node, use the middle charge weight.

Now, use that weight and load 3 shot groups. Start with the bullet seated as described above and keep moving further out in .003 or .005 increments. Do not use a magneto or anything on your barrel. Shoot the groups.

Eventually you will see a seating depth node appear. It will have large groups, they get smaller and smaller, and then get larger again. Where they are small is your seating node. Load on the long side of it. If you kept going, you would see this pattern repeat like a sound wave. Up and down up and down. There are several seating depth nodes you can find. It’s basically a barrel harmonics deal.

You can make *any* charge weight shoot via seating depth. It might take you a while depending on the barrel; but the groups will shrink.

 

[ddp43]

I have measured using 2 methods. First was with a disassembled bolt. Second with a hornady oal gauge. Getting the same measurement both ways. I have reloaded for many of my own rifles, and many people I know, and worked up loads for these over the last 15-20 years. I've never ran into this issue

 

[Dthomas3523]

Have you borescoped it to see how it looks in the chamber and into the lands?

 

[ddp43]

No I have not. I dont have a bore scope

 

[Dthomas3523]

I’m gonna move this thread to the loading section. Probably pick up more responses there.

 

[NW Valkyrie]

1) Re: throat erosion:
Most 6mm cartridges see ~ .001-.006 inches of erosion every 100 rounds depending on cartridge capacity and pressure. You have lost .005 in 200, for an average of less than .003 per 100. No problems here.

2) Re: length to lands:
It's hard to compare CBTO since we don't know what tool you are using. My Sinclair comparator may give a different value than your hornady version (just an example). Hornady specs for the 6 Creed call for a long freebore compared to many cartridges. Jumping a bullet .050 is not a problem either.

3) Re: accuracy
Sometimes we want a barrel to shoot a particular bullet well. Sometimes the barrel prefers something else
You've tried two bullets for your handloads, but it may be as simple as your rifle doesn't like either.

Some barrels will shoot anything. Some are picky. It's part of the game. I would limit the powder chasing for now and stick to known combos for the creed. Quality brass, CCI 450's, 38-41 grains 4350 and Berger 105 hybrids will get it done in a lot of rifles.

Shoot some Hornady match factory ammo too.

 

[Dthomas3523]

1) Re: throat erosion:
Most 6mm cartridges see ~ .001-.006 inches of erosion every 100 rounds depending on cartridge capacity and pressure. You have lost .005 in 200, for an average of less than .003 per 100. No problems here.

2) Re: length to lands:
It's hard to compare CBTO since we don't know what tool you are using. My Sinclair comparator may give a different value than your hornady version (just an example). Hornady specs for the 6 Creed call for a long freebore compared to many cartridges. Jumping a bullet .050 is not a problem either.

3) Re: accuracy
Sometimes we want a barrel to shoot a particular bullet well. Sometimes the barrel prefers something else
You've tried two bullets for your handloads, but it may be as simple as your rifle doesn't like either.

Some barrels will shoot anything. Some are picky. It's part of the game. I would limit the powder chasing for now and stick to known combos for the creed. Quality brass, CCI 450's, 38-41 grains 4350 and Berger 105 hybrids will get it done in a lot of rifles.

Shoot some Hornady match factory ammo too.

His measurements show .050 erosion.......not .005

 

[NW Valkyrie]

Sorry, my bad math. Yes, .050" is excessive.

My suggestion would be to purchase/borrow a borescope. I had a barrel that exhibited similar behavior- big change in distance to the lands over a short round count and less than expected accuracy. Using the borescope it was easy to see the freebore section still had rifling/tooling marks remaining that needed to be cleaned out.

 

[ddp43]

Thanks for the insight. I know a gunsmith that might be able to have a look at the rifling and see whats going on in there.
I will also try some 110gr sierra matchkings. I can't get my hands on any berger hybrids at the moment. I'll keep doing some testing and see what I can come up with