I have been reloading for my 17 Mach IV and I have found that the primers are backing out and showing signs of over pressure. I have done everything that I possibly can to figure out the problem some say that I need to turn the inside or outside of the neck on the case. However when I check the outside diamiter of the case mouth it is at or below what the specs in my book (new hornady) says. I run the shell into the neck sizer and then measure the mouth opening and it is a little smaller that the bullet diameter, I am sure that this is common to allow the shell to hold the bullet firmly.

A question that I have is how much smaller with the actual case mouth be after running through the neck sizer? I am wondering if the case is gripping to tightly on the bullet. It does not seem to slight in as smooth as it should.

Also I have not gotten the accuracy that I should out of this gun. It is a new Cooper.

I hope someone can help I am baffled.

Rich, if your primers are backing out it usually means that your cases are too short and you've got excessive headspace. If you're full length resizing the cases, your die might be adjusted down too far and you're creating the excessive headspace. Fire a non-maximium load in your rifle then resize the cases just to where you can feel the snug fit when the bolt handle rotates the last bit. This should result in a case that fits your rifle chamber exactly and leaves no room for the primers to back out.

I have been using only the neck sizing die and have fired not max pressure loads. My gun is at home so I will have to check to see what the manu. date is but I just bought it so it should be pretty new.

Rich,

One more thing: what is the manufactured date of your Cooper? It should be on the test target they include with the rifle.

Thanks,
Kory