300WM - Factory ammo "overpressure" signs

SMK1000plus

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Dec 23, 2015
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284
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Fat chamber with excessive headspace is my guess.......find a gunsmith with a go/no gauge.

The issue with the conventional Belted Magnum Headspace Gauges is that they headspace off the belt. That measurement is probably correct. The distance to the shoulder tapper datum in the chamber wouldn't show with conventional belted magnum headspace gauges. According to his once fired brass, that dimension is correct by SAAMI specifications anyway. His chamber diameter is somewhat excessive just in front of the case belt but again, a headspace gauge won't really identify that either. His factory brass seems to be significantly under the proper length (-0.018") to the shoulder/datum, and his chamber is a bit too fat (too large in diameter just in front or the belt).

His once fired brass should reflect all the information he needs from his chamber, to compare with the actual SAAMI drawings/specifications for a proper 300 Winchester Magnum cartridge. A chamber casting would be of little to no benefit after having once fired brass from his chamber, to measure.
 

entoptics

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Jan 16, 2018
Messages
535
That is my exact process as well, with no issues with my other 300WM rifle.

I will take a look at the lot numbers since the owner gave me the brass still in the original boxes. I will look into getting a different brand of factory to double check. I also have fresh Hornady brass I could use as well.

I appreciate all the help and responses. I will keep you updated.
Have you tried the 200 ELDX factory load you mentioned in your rifle? If so, were there similar issues, or did it behave fine?

Does the fresh Hornady brass measure the same as the unfired factory brass from base to shoulder? Also, how does the base to shoulder from the Seekins fired brass compare to the base to shoulder from brass fired in your 300WM? That will at least give you two points of data to compare.

It's plausible that the chamber really is fine, but the bolt face is back too far. Case is stopping where it should when chambered, but when fired, it's moving back further than it should? A conventional no-go gauge would diagnose this problem I think.
 

dsmhero

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Jun 20, 2014
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MT
I have not shot the factory Hornady load in my rifle yet. I will look at the fresh cases in a few. I think you are correct in the head CLEARANCE being off(breech face to case head). That would account for the primer being force back and flattened without actual overpressure.

I had a gunsmith take a look at it. The GO gauge fit, but required ~0.013" worth of "shims" to find any resistance on bolt close. I am assuming this is corroborating the head clearance issue. I will be contact Seekins shortly. Please correct me if I am wrong.

Have you tried the 200 ELDX factory load you mentioned in your rifle? If so, were there similar issues, or did it behave fine?

Does the fresh Hornady brass measure the same as the unfired factory brass from base to shoulder? Also, how does the base to shoulder from the Seekins fired brass compare to the base to shoulder from brass fired in your 300WM? That will at least give you two points of data to compare.

It's plausible that the chamber really is fine, but the bolt face is back too far. Case is stopping where it should when chambered, but when fired, it's moving back further than it should? A conventional no-go gauge would diagnose this problem I think.
 

SMK1000plus

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Joined
Dec 23, 2015
Messages
284
Location
Jacksonville, FL
I have not shot the factory Hornady load in my rifle yet. I will look at the fresh cases in a few. I think you are correct in the head CLEARANCE being off(breech face to case head). That would account for the primer being force back and flattened without actual overpressure.

I had a gunsmith take a look at it. The GO gauge fit, but required ~0.013" worth of "shims" to find any resistance on bolt close. I am assuming this is corroborating the head clearance issue. I will be contact Seekins shortly. Please correct me if I am wrong.
0.013" (13 thousandths of an inch) to get the go gauge to engage, is WAY over on headspace! Seekins will have to set back the barrel and recut the chamber (which will shorten the barrel by the amount they have to set it back to correct the chamber and correctly rethread the barrel), replace the barrel, or they may opt to replace the entire rifle. I doubt they will set back the barrel but, I wouldn't accept a set back barrel anyway, as that will leave you with a shorter barrel than what you bought. Hopefully, they'll send you a new rifle with the correct chamber dimensions.
 

entoptics

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Joined
Jan 16, 2018
Messages
535
Hornady fresh brass datam - 2.255"
Once shot from MY rifle - 2.266"

The GO gauge fit, but required ~0.013" worth of "shims" to find any resistance on bolt close. I am assuming this is corroborating the head clearance issue. I will be contact Seekins shortly. Please correct me if I am wrong.
Yeah, I think you've found your issue. I set my Savage prefits up with a go-gauge and a piece of painters tape as a no go, and that's only about 0.003" thick if I remember right.

Keep us updated on how it plays out.
 

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