help with 375 h&h mag reloads

Trying to come up with an explanation of the shoulder/neck groove that could also contribute to the high pressure issue, I may have stumbled onto a possibility. First of all, I'll assume that your rifle has a standard commercially produced chamber of proper dimensions. That will be the good news but if the chamber is short throated, that could contribute to the issue. Next, what is the history of the brass you have been using? Is it essentially new or has it been reloaded a number of times, perhaps excessively? If the latter the cases may have elongated to the point where the chambered round results in the case mouth contacting the mating surface at the end of the chamber. Upon firing, should the shoulder move forward, the case neck would not be able to move with it and perhaps result in the groove you are referring to as the shoulder attempts to force the neck forward also. And that might conceivably contribute to the high pressure. So the possible solution here might be to check the length of your cases and trim if needed to prevent case mouths from contacting the mating surface at chamber's end.

I guess that is the reason why we trim brass. And since most reloaders do not allow their brass to elongate to the point of contributing to high pressure, and the resulting shoulder/neck groove if that is related, it is not immediately obvious how the results would display when excessively case elongation is allowed to happen. A photo of the shoulder/neck groove would be desirable but it seems to be well enough explained.

The Lyman manual shows the .375 case length as 2.850 with trim to length as 2.840.
 
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All cases are trimmed 2.840 exact an it's new brass.I am getting the groove around neck after shooting .the gun went to smith an checked out in excellent condition ???????now what I'm running outta options
 
Yes I guess we are all running out of options by now. Did the smith have any comments or suggestions as to the cause of the grooves or anything else related? I assume he was able to verify that the chamber was not short throated which could result in the same effects as overly stretched brass. What rifle have you been using, I don't recall that it has been mentioned. This is a difficult situation that should have an answer.
 
I have two model 700 Remington .375s with standard contour and length barrels so I agree your rifle is top of the line. Did your smith have anything to add about the situation? Especially about the shoulder/neck grooves around the fired cases?
 
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So the groove runs around the case in a circle, right? Is it a mark on the brass or actually pressed in? Fine like a phonograph record or deep like a crimp groove? Could you get a picture of it?
 
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