6mm GT chamber question

[StreetCred]

Welp. . . Things did not go well Saturday. Shot 2 rounds and both had pierced primers. There's something up with my receiver, bolt, firing pin, all of it, none of it. . . I don't know what the issue is. Had this problem with the previous barrel. Took it to a local gunsmith and they checked a few things - headspace is good, firing pin protrusion is a thou or two long. He measured a new Tikka and that was a thou longer than mine.

 

[Coleredding00]

Welp. . . Things did not go well Saturday. Shot 2 rounds and both had pierced primers. There's something up with my receiver, bolt, firing pin, all of it, none of it. . . I don't know what the issue is. Had this problem with the previous barrel. Took it to a local gunsmith and they checked a few things - headspace is good, firing pin protrusion is a thou or two long. He measured a new Tikka and that was a thou longer than mine.

Not sure in the tikka platform, but i know ive read that rem 700 actions need to have the firing pin hole bushed for the same reason. Too much clearance lets the primer flow too much.

Also, what primers are you using? CCI 450 is usually the go to, the cup is a little thicker.

 

[43.6N]

Welp. . . Things did not go well Saturday. Shot 2 rounds and both had pierced primers. There's something up with my receiver, bolt, firing pin, all of it, none of it. . . I don't know what the issue is. Had this problem with the previous barrel. Took it to a local gunsmith and they checked a few things - headspace is good, firing pin protrusion is a thou or two long. He measured a new Tikka and that was a thou longer than mine.

Can you take a picture of your bolt face and firing pin head?

Once a primer gets pierced, the hot gas can etch the bolt face and pin making pierced primers more likely to occur.

If you have a second tikka bolt, swapping it out can help isolate the problem to the bolt/pin assembly.

 

[StreetCred]

Not sure in the tikka platform, but i know ive read that rem 700 actions need to have the firing pin hole bushed for the same reason. Too much clearance lets the primer flow too much.

Also, what primers are you using? CCI 450 is usually the go to, the cup is a little thicker.

I had this same issue with the previous 6.5CM barrel using Winchester SR primers. Then I found out how horrible those were so I switched to CCI's. That's what I used Saturday.

One thing I though of today is the suppressor. Could that be causing enough back pressure to puncture the primer if the firing pin is just a hair too long? I didn't have any issue with the factory barrel. It wasn't threaded so I never used a suppressor with it.

 

[Lawnboi]

I had this same issue with the previous 6.5CM barrel using Winchester SR primers. Then I found out how horrible those were so I switched to CCI's. That's what I used Saturday.

One thing I though of today is the suppressor. Could that be causing enough back pressure to puncture the primer if the firing pin is just a hair too long? I didn't have any issue with the factory barrel. It wasn't threaded so I never used a suppressor with it.

How much powder?

 

[Coleredding00]

I had this same issue with the previous 6.5CM barrel using Winchester SR primers. Then I found out how horrible those were so I switched to CCI's. That's what I used Saturday.

One thing I though of today is the suppressor. Could that be causing enough back pressure to puncture the primer if the firing pin is just a hair too long? I didn't have any issue with the factory barrel. It wasn't threaded so I never used a suppressor with it.

I doubt its the suppressor, the primer is likely pierced at peak pressure shortly after detonation, long before the suppressor would have an influence.

 

[StreetCred]

How much powder?

38.0gr H4350 with a 105gr BTHP.

 

[wind gypsy]

I had this same issue with the previous 6.5CM barrel using Winchester SR primers. Then I found out how horrible those were so I switched to CCI's. That's what I used Saturday.

One thing I though of today is the suppressor. Could that be causing enough back pressure to puncture the primer if the firing pin is just a hair too long? I didn't have any issue with the factory barrel. It wasn't threaded so I never used a suppressor with it.

What CCIs? 400 and 450s/BR4/41s have a significant difference in cup thickness.

Like others said, prior pierced primers might have damaged the pin/bolt face and made it more prone to future pierced primers. To top it off, it sounds like you started with a hot charge that might be overpressure.

 

[StreetCred]

What CCIs? 400 and 450s/BR4/41s have a significant difference in cup thickness.

Like others said, prior pierced primers might have damaged the pin/bolt face and made it more prone to future pierced primers. To top it off, it sounds like you started with a hot charge that might be overpressure.

I'm using the standard CCI 400's. I spoke with another gunsmith and he said he's got another customer with the same issue. Different rifle and caliber. He switched from the 400's to the 450's and the problem went away.

I'm waiting for the gunsmith to look at my rifle. I think at the minimum I'll need to replace the firing pin or have it shortened a little. There is some minor pitting in the bolt face. Nothing protrudes and all the holes in the primers are in the firing pin dent.

I probably should back off on the charge a bit.

 

[Lawnboi]

Cci 400 are thin cup primers, not suitable for max pressure loads in standard bolt face tikkas especially.

Get some 450s, br4, 41s or rem 7 1/2s. I’d also start a little lower looking at velocity and signs of pressure.

 

[StreetCred]

Cci 400 are thin cup primers, not suitable for max pressure loads in standard bolt face tikkas especially.

Get some 450s, br4, 41s or rem 7 1/2s. I’d also start a little lower looking at velocity and signs of pressure.

I know every rifle is different. My MPA in 6.5CM uses the same CCI 400's with no issues with just as spicy loads. Would love to know the exact reason they get pierced in the Tikka.

 

[Lawnboi]

I know every rifle is different. My MPA in 6.5CM uses the same CCI 400's with no issues with just as spicy loads. Would love to know the exact reason they get pierced in the Tikka.

They are alittle sloppier on pin firing pin fit. I think I they would benefit from a bushed firing pin but I choose just to use thicker primers.

 

[43.6N]

I'm using the standard CCI 400's. I spoke with another gunsmith and he said he's got another customer with the same issue. Different rifle and caliber. He switched from the 400's to the 450's and the problem went away.

I'm waiting for the gunsmith to look at my rifle. I think at the minimum I'll need to replace the firing pin or have it shortened a little. There is some minor pitting in the bolt face. Nothing protrudes and all the holes in the primers are in the firing pin dent.

I probably should back off on the charge a bit.

Thanks for the picture.
Your bolt face is definitely etched. You can see a ring of removed material that matches perfectly with a small primer circumference. I would put money on the pin head also having etching marks.

All it takes is for one of these etching marks to have a sharp edge in contact with the primer during peak pressure. If it is creating a stress riser in the primer cup, that will be the failure point.

Also, if the firing pin shaft or bolt pin channel has etching, and the fit is even more sloppy than standard, any movement of the pin side to side will stress the primer cup potentially creating a tearing failure.

Solutions:
1. If the smith has told you he can take 1-2 thousandths off the protrusion, this will also resurface the pin head removing any etching.
2. Bushing the bolt face will remove any slip/slop side to side. It will also resurface the etched bolt face.
3. CCI 400’s are notorious for thin cups and pierced primer events. They are measurably thinner than cci 450’s, br4, or fed gmm srp. Going with a thicker primer cup now, would likely fix the issue enough to avoid the gunsmithing solutions, but would only be a bandaid over the underlying problem.

The reason I would suspect the MPA does not have issues is because the pin to bolt fit is still precise and there are no etching marks to stress the primer cups. But all it takes is one pierced primer to etch the contact points between bolt and primer, and you could then start seeing repeat pierced primers on that gun.

 

[wind gypsy]

Less than top notch pin fit to bolt face, etched BF/FP, CCI 400s which are notorious for popping, and a load that is probably over pressure = asking for pierced primers.

I dont have a 6 GT but have seen enough load data of guys using 35.5 - 37.5 grains h4350 with bullets that should build less pressure than a 105 BTHP.

 

[StreetCred]

Thanks for the picture.
Your bolt face is definitely etched. You can see a ring of removed material that matches perfectly with a small primer circumference. I would put money on the pin head also having etching marks.

All it takes is for one of these etching marks to have a sharp edge in contact with the primer during peak pressure. If it is creating a stress riser in the primer cup, that will be the failure point.

Also, if the firing pin shaft or bolt pin channel has etching, and the fit is even more sloppy than standard, any movement of the pin side to side will stress the primer cup potentially creating a tearing failure.

Solutions:
1. If the smith has told you he can take 1-2 thousandths off the protrusion, this will also resurface the pin head removing any etching.
2. Bushing the bolt face will remove any slip/slop side to side. It will also resurface the etched bolt face.
3. CCI 400’s are notorious for thin cups and pierced primer events. They are measurably thinner than cci 450’s, br4, or fed gmm srp. Going with a thicker primer cup now, would likely fix the issue enough to avoid the gunsmithing solutions, but would only be a bandaid over the underlying problem.

The reason I would suspect the MPA does not have issues is because the pin to bolt fit is still precise and there are no etching marks to stress the primer cups. But all it takes is one pierced primer to etch the contact points between bolt and primer, and you could then start seeing repeat pierced primers on that gun.

All good information. See what the smith has to say. Might be a couple of weeks until he can get to it.

 

[Decker9]

I haven’t read every comment, but do you anneal? Last weekend’s match a fellow with a dasher was blowing primers with his normal load, but did just start annealing. Consensus was he annealed the whole case far too much.

I’m not as up to snuff on this stuff but just something I thought I’d share incase you are annealing.

 

[StreetCred]

I haven’t read every comment, but do you anneal? Last weekend’s match a fellow with a dasher was blowing primers with his normal load, but did just start annealing. Consensus was he annealed the whole case far too much.

I’m not as up to snuff on this stuff but just something I thought I’d share incase you are annealing.

I do anneal but these are brand new Alpha cases. Can tell they've been annealed already. I just used a mandrel to make sure the neck was sized right and deburred/chamfered them.