6BR Pressure Found Low Charge

[gunsandgunsmithing]

.268 nk 68 grain bt. Old brass still showing sine wave.

That's a sweet group Larry and I won'tr disagree but I'm not sure how much value the sine wave on necks really has. If I post a group or several that are the same or even smaller, but with cases that don't show the sine much at all, does that mean anything? I can but my point is not that but I used to look for that pattern a lot and I can't say that I can conclude much from it on target. So, I'm not sure what it's telling us.

I don't get it nearly as much with 6 Grendel Lapua brass as I'm use to seeing with most others. I suspect it has something to do with the neck being a tad shorter than ppc or br necks but I'm not sure. All I know is two things. I've had tight neck and no turn versions of 6 Grendels and I can't see the sine on either and the target tells me there is no difference on paper. Not saying it doesn't have some value. Again, I lived by the same rule for a long time. This is strictly experience doing it both ways and not seeing it matter on the target. I've always heard that the sine will correspond with the lugs too...a 2 lug has two dips, a 3 lug has three dips and they line up with the lugs. I can't confirm that either as I've never found it to be consistently true.

I'd like to know what it's telling me for sure but whatever it is is small and I can't say that I can shoot the difference either way. The guns/bbls have good records so it's not like they are non-competitive either way. To the contrary, they've been very competitive both ways. So, I don't know.

 

[Larry Koch]

I would say the sine wave is another observation tool. I can say with confidence on new 6 ppc brass if you’re not getting a sine wave. Your brass is to thick, or too thin, at the neck. Yes 2 waves on 2 lug, and 3 on a 3 lug Bolt.

 

[MikeG08]

I put a camera down my bore before cleaning and my carbon ring is easily double the size it normally is.
I’m thinking trimming the case shorter may not have been the culprit. In going to load a few at the 1.550” mark I was doing before and see if it improves.

 

[Kracken McGraw]

So when I say jam I’m using Erik Cortina’s method for finding the stuck bullet jam point.
Prep case as normal, seat very long, apply imperial wax to the ojive and chamber the round so that the barrel is seating the bullet further down the neck. I repeated this twice and took the longer of the 2. They were within .002” of each other.
I believe this method likely goes deeper in than just touching?
I have the Hornady tool but not a BR case to measure.

It’s been a while since I watched Erik’s video, but doesn’t he back off .020 or something after he finds hard jam?

 

[MikeG08]

It’s been a while since I watched Erik’s video, but doesn’t he back off .020 or something after he finds hard jam?

Yes he’ll start testing seating depth working away from .020” off jam

 

[Dolomite]

I put a camera down my bore before cleaning and my carbon ring is easily double the size it normally is.
I’m thinking trimming the case shorter may not have been the culprit. In going to load a few at the 1.550” mark I was doing before and see if it improves.

Did you shoot with or without the suppressor?

 

[MikeG08]

Did you shoot with or without the suppressor?

With suppressor, I don’t want to change more than one variable at a time. Also I shoot at my house frequently so I use supressor as a courtesy to my neighbors.

 

[MikeG08]

268 loaded round neck dia. is going into a .269 - .271 neck

So your neck is at least .271 and maybe a hair bigger. That gives you at least .003 clearance. You should be fine on clearance. We need to see a couple pics of the head end of those cases that were hard to open. Need to see them with the fired primer still in them. A good close-up with good focus.

Frank

The neck also has two carbon rings.

 

[JFrank]

Those look like rub marks on the shoulder just below the neck shoulder junction.

 

[MikeG08]

Regarding the dented necks, just use your fingertip to keep the front of the case from pushing out until the bolt is entirely open. Then that fired brass will suitable to check for the bullet slipping cleanly out.

I'd call the smith who chambered this barrel and find out what the reamer neck diameter is. I have a .271 neck and that's as small as I'd ever consider a 'no-turn' neck because some brass is too thick for that. .272-.273 is far more common.

I did this and it worked great. I tested the fired brass and the bullets drop in at out with just gravity. The dent was causing the resistance.

 

[MikeG08]

Those look like rub marks on the shoulder just below the neck shoulder junction.

I think that’s from the comparator I was using both SAC and Area 419 for checking shoulder bump.

 

[MikeG08]

I’ve never seen this before. Just noticed when I was finishing up cleaning my rifle. Could that be sharpie? I did mark the cases for separating which I haven’t done before.

 

[Dolomite]

I'm not sure if the comparators would leave that heavy mark on the shoulder and it looks pretty high too close to the neck. Wouldn't the comparator measure closure to the middle of the shoulder?

Just curious are these signs of case head separation? or is it chamber marks?

Maybe you can post a video of the chamber by the neck shoulder junction and towards the base where these lines could be coming from.

I'm wondering if you have high spots in the chamber causing the rub marks on the brass. When the brass expands it grabs the high spots and makes for a heavy bolt lift. I could be wrong, someone with more experience might be able to help on any marks or high spots in the chamber.

 

[Dolomite]

That round ghost spot is interesting to say the least. Has that always been there from the beginning?

Did the bolt face come like that? looks like the coating was removed. I wonder if your bolt is out of spec?

What action are you using?

 

[MikeG08]

That round ghost spot is interesting to say the least. Has that always been there from the beginning?

Did the bolt face come like that? looks like the coating was removed. I wonder if your bolt is out of spec?

What action are you using?

View attachment 1494893

I was looking at that spot and thinking the same thing. I’m not sure if it was like that or not. It’s an Impact Precision 737 that I just got back from Speedy blueprinting.

 

[MikeG08]

I'm not sure if the comparators would leave that heavy mark on the shoulder and it looks pretty high too close to the neck. Wouldn't the comparator measure closure to the middle of the shoulder?

Just curious are these signs of case head separation? or is it chamber marks?

Maybe you can post a video of the chamber by the neck shoulder junction and towards the base where these lines could be coming from.

I'm wondering if you have high spots in the chamber causing the rub marks on the brass. When the brass expands it grabs the high spots and makes for a heavy bolt lift. I could be wrong, someone with more experience might be able to help on any marks or high spots in the chamber.


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View attachment 1494899

Those are from the comparator. Here are 2 pieces of virgin brass. I did the comparator on the one on the right and you can see the line. I spin them around to get them flush.

 

[G Liles]

Maybe you can post a video of the chamber by the neck shoulder junction and towards the base where these lines could be coming from

I would like to see a video of the chamber also. If possible.

 

[MikeG08]

I would like to see a video of the chamber also. If possible.

Here is a video of my bore camera. I normally have a bore scope in when I run the camera. I noticed a carbon ring right at the neck/shoulder junction.

 

[TaperPin]

I was looking at that spot and thinking the same thing. I’m not sure if it was like that or not. It’s an Impact Precision 737 that I just got back from Speedy blueprinting.

That circle looks like the size of the end mill used on the bolt face - just a ghost of the last moment before it left the bolt face.

 

[spike-55]

I'm not sure if the comparators would leave that heavy mark on the shoulder and it looks pretty high too close to the neck. Wouldn't the comparator measure closure to the middle of the shoulder?

Just curious are these signs of case head separation? or is it chamber marks?

Maybe you can post a video of the chamber by the neck shoulder junction and towards the base where these lines could be coming from.

I'm wondering if you have high spots in the chamber causing the rub marks on the brass. When the brass expands it grabs the high spots and makes for a heavy bolt lift. I could be wrong, someone with more experience might be able to help on any marks or high spots in the chamber.


View attachment 1494890


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View attachment 1494899

Have you used go/no go gauges on the chamber???? Those faint marks at the .200 line could be the start of case separation??? Are you having any issues with primer strikes??? What does your shoulder bump gauge show on fired brass for spec versus virgin brass??? Can you check the concentric measurement of the brass at the rear of the case versus virgin brass all the way to the .200 line??? Just to make sure your smith cut your chamber concentric?? Your primers don't show to be pressured and there's not that much of a carbon ring in that barrel to cause a hard bolt lift like you describe. Loaded round is .268 and fired round is .271 so you should be ok on neck clearance. Sized rounds load with no resistance??
stan