Counterbore question

[jeffsvice]

I had a M-700 opened to PPC some time ago and sako type extractor installed. With warm loads, I get ridge just ahead of the web on the case - kindof like a mini-belt on a mag. The bolt counterbore is 0.165" deep - I am assuming this too much, leaving too much case unsupported by the chamber... am I correct and is there a remedy? Also the boltface was opened to 0.465" - I'm thinking that is a little too much as well... but don't think that's a real problem.

 

[jeffsvice]

I had a M-700 opened to PPC some time ago and sako type extractor installed. With warm loads, I get ridge just ahead of the web on the case - kindof like a mini-belt on a mag. The bolt counterbore is 0.165" deep - I am assuming this too much, leaving too much case unsupported by the chamber... am I correct and is there a remedy? Also the boltface was opened to 0.465" - I'm thinking that is a little too much as well... but don't think that's a real problem.

by the way, I should also point out this is a small bore wildcat on a very small, shortened case, and pressures can get high in a hurry. Just want to determine if this ridge is an unsafe situation or a common malady with high intensity cartridges.

Besides, I shoot much more consistently when I'm confident the gasses will go out the muzzle instead of down the magazine well -

 

[Cheechako]

Jeff

The bolt face opened to .465" should not be a problem since many PPC bolts can accomodate the 308W case head. However, the nominal counterbore depth dimension is only .145" and the additional .020" could indeed be the culprit. With a SAKO extractor I would be especially leery of using "warm" loads.

Another reason for the "belt" on fired cases could be that the chamber is oversize for the brass you are using. Such case head expansion is typical of factory chambers. Not necessarily dangerous but it will probably lead to loose primer pockets eventually.

I'd say some serious dimension checking is in order.

Is there some reason the counterbore was made excessively deep? The only correct fix is to set the barrel back and recut the chamber and counterbore. Assuming that this is a custom job throughout, for a wildcat cartridge, loaded to the max. I would take measurements of the brass you are using and make the counterbore close to the actual dimension of the rim and extractor groove combined. That way you will have the maximum case head support that is possible.


JMHO

Ray

 

[h-bar]

The counterbore can be deeper because of something else also. If the barrel stub/tenon is made slightly longer it will make the counterbore have to be deeper. I would check the spacing between the bolt and barrel by placing incremental shims in the counterbore and trying to close the bolt. That will definitely show you what the claearance is. One way you can see if the barrel tenon is long without taking the barrel off is to look into the vent hole and if the barrel is covering it in any amount the tenon is prabaly long.

 

[Mike Bryant]

Jeff, measure what the distance is from the bolt face to the end of the bolt nose. When Bob Brackney wrote his article in Rifle magazine back in the early 80's, he recommended reducing that dimension on the bolt to .125" when installing a Sako extractor and reducing the counterbore depth in the barrel to match. At that time, he recommended with the balloon head Sako cases that were being used, that the gap between the end of bolt nose and barrel counterbore be set at .001 to no more than .005". It's not quite so critical now with Lapua cases as the web is thicker than the old Sako brass. If the rifle is already barreled when the Sako style extractor is installed, reducing the counterbore depth in the bolt to .125" won't do anything unless the barrel is also set back.

.165" counterbore seems like quite a bit too much to me, but I'd also look to see if there is much of a bevel cut into the beginning of the chamber. Too much counterbore and/or too much of a bevel cut at the beginning of the chamber can cause the case to develop a belt like you are describing.

The correction to this would be to set up the bolt with .125" bolt face depth and reduce the gap between the barrel and the bolt nose to a minimum clearance. I'd suggest having the gunsmith that chambered your rifle do this as when the barrel is set back to reduce the clearance between the bolt face and counterbore of the barrel, the chamber will need to be deepened with the reamer that he used to chamber the barrel.

 

[jeffsvice]

Many thanks

For the input/guidance. I had the reamer ground so I have it - guess I'll be sending it all back.

Ever have someone tell you, "If you want to do that, you need to do this..." and you don't pay them any attention...

When someone (aka skilled gunsmith) tells you that altering an otherwise fine Remmy action is, perhaps not, maybe not, probably not, (you get the idea) the best way to go for a particular high pressure round, you could save yourself some headaches and buy the custom.

Looking at the front of this poor bolt, it has more holes and slots than I'll ever be comfortable with -

Alas, I'm committed now.

What would we learn without a little first hand trial and error and what is fun about learning from other's mistakes... I like to OWN my errors....

 

[Jay Idaho]

If the front of the Sako extractor is very near the front of the bolt snout, I doubt if you can reduce the depth of the counterbore a satisfactory amount. The only reason that I could see for the Sako install would be to reduce the counterbore to .125, as Mike outlined.
Some things do not have a simple, acceptable "fix". This barrel might be a good candidate to become a 6BR, the counterbore is almost large enough to accept that casehead. Reducing the counterbore depth as much as possible during the rework, of course.
Or a new bolt from Dave Kiff, for a PPC, might be an option. Kiff may have some good suggestions, also.

 

[Boyd Allen]

The unsupported length is the depth that the bolt face is recessed from the front of the bolt, plus the clearance between the bolt nose and the back of the chamber, PLUS the depth of any chamfer or radius that has been cut at the back of the chamber to help feeding. If you have a belt after firing (before sizing), it sounds to me like you have an unsafe condition. With the barrel off, you can accurately measure down to the back of the chamber, and the head of a fired case, do the math, and know how much case is unsupported. The depth of the chamfer will probably have to be determined by eyeball, but with the barrel off, this will be a lot easier. If you want to shoot this caliber, and you decide that you can't from where you are to a safe condition with your current bolt, you can still order a new bolt body from Pacific Precision for $125. He offers many options, so that you can have exactly what you want/need.