Chamber Dimensions

[Engineering101]

Dumb question: When dealing with rifle cartridges that headspace off the shoulder, where on the SAAMI chamber drawing does it show how far into the barrel the chamber is cut?

Put another way - what prevents someone from reaming a chamber that is say 0.25" shorter than it should be thus exposing 0.25" more of the brass without support from the chamber resulting, I presume, in a ruptured case when the rifle is fired?

I am not talking about a situation where the headspace is incorrect but rather a situation where the chamber is not cut deep enough into the barrel to fully support the brass cartridge.

 

[orwapitihunter]

In your example the headspace measurement would be .250" short of the proper headspace measurement, and it would be incorrect. If you look at a SAAMI chamber drawing it will show the headspace length.

 

[Edd]

The SAAMI chamber drawing doesn't actually show the chamber depth. It shows the distance from the bolt face to the datum line. The person cutting the chamber has to allow for some clearance between the bolt and the back of the barrel tenon. To chamber a barrel .250" short and have the headspace correct would require having the barrel tenon .250" short also. What keeps someone from doing this is the same thing that keeps you from doing anything else incorrectly. You have to know the correct way to do it.

 

[Engineering101]

Edd

So there is no SAAMI control of this parameter and it is up to the person doing the gunsmithing to "do it right" - whatever that means since there are no specifications involved other than headspace. Wow. That is a surprise.

The reason that I bring this up is I have a rifle (properly headspaced) where it appears the case sticks out of the chamber by 0.030" farther than it "should". The results in obvious stretching of the case just before it goes into the chamber - so much so that the case looks like it is belted (and it isn't).

In any event a trip back to the gunsmith appears to be in order.

Thanks for the info.

 

[Edd]

What cartridge do you have and what work was done to the rifle?

 

[Engineering101]

I had a barrel chambered for 26 Nosler and installed on a vintage Sako action.

 

[Engineering101]

It seems like there would be no negatives to cutting the chamber farther into the barrel - as long as the bolt didn't hit the back of the barrel - right?

 

[kcebcj]

Read your post and it perked my interest as I'm building a 300 WM on a Weatherby Mark 5 receiver and I was talking with my smith about fudging the chamber some or staying with SAMI specs.

I posted the chamber drawings below. I don't care if you are building on a Winchester or Weatherby receiver if you keep with SAMI specs the chambers will be exactly the same. The difference is the end of the barrel and how the bolt and receiver interact with it. I'm sitting here looking at a Winchester and a Weatherby barrel and they are very different as to where the chamber starts in relation to the end of the barrel so it would be impossible for a SAMI to list those specs unless they did a drawing for every receiver barrel combination out there.

SAMI gives the smith the exact chamber numbers and all the dimensions he needs to set the head space properly when fitting the barrel to the receiver and the bolt. The rest is left up to the expertise of the rifle smith to get it right. The next time I see Dick I'm going to ask him about this and I bet he has a note book full of numbers just for things like this.Just another reason to pick your smith carefully.

 

[bigngreen]

It seems like there would be no negatives to cutting the chamber farther into the barrel - as long as the bolt didn't hit the back of the barrel - right?

No, if your issue is that to much of the cartridge is unsupported but your at spec for chamber depth on a gauge then it's the barrel tenon that is out of spec and needs to be set back which will mean you'll also need to cut the chamber deeper once the barrel is correctly fitted to the action and bolt.

Was this a new barrel or did you have a belted magnum barrel rechambered, if I recall right if you rechamber a belted mag with a RUM size case it will leave a small remnant of the original chamber at the belt.

 

[J E Custom]

Edd

So there is no SAAMI control of this parameter and it is up to the person doing the gunsmithing to "do it right" - whatever that means since there are no specifications involved other than headspace. Wow. That is a surprise.

The reason that I bring this up is I have a rifle (properly headspaced) where it appears the case sticks out of the chamber by 0.030" farther than it "should". The results in obvious stretching of the case just before it goes into the chamber - so much so that the case looks like it is belted (and it isn't).

In any event a trip back to the gunsmith appears to be in order.

Thanks for the info.

This dimension is normally left up to the rifle manufacture based on the action and barrel fit and type . It is called the Barrel shank and allows the cartridge to be supported properly and still be able to eject the cartridge.

The smith should copy these dimension within a few thousandths.

There are many barrel shank designs and if you look for the one your rifle needs you can compare it to the correct drawing.

The Link I included should have your barrel shank drawing. If not tell me what action and brand
of rifle you have and I will try to find it.

http://www.homegunsmith.com/cgi-bin/ib3/iB_html/uploads/post-71-40666-BSD_small.pdf

Hope this answers your question, because the barrel shank needs to be correct.

J E CUSTOM

 

[IdahoCTD]

Was this a new barrel or did you have a belted magnum barrel rechambered, if I recall right if you rechamber a belted mag with a RUM size case it will leave a small remnant of the original chamber at the belt.

The belt is .532 on a belted magnum and the base of a RUM case is .550". It would have to be a considerably over sized chamber to not clean up. A bigger concern of mine would be weather the neck will fully clean up with a rechamber.

 

[Engineering101]

As per usual there is more to this than I knew or thought about so I am glad I brought it up. Thanks guys for the education.

This was not a rechamber. I used a new 6.5mm Kreiger bull sporter barrel on a Sako L61R action (which was a 7mm RM before modification).

The case "belt" from stretching occurs 0.273" from the bolt face which, just eyeballing the case, seems like a bit too much to leave unsupported. (Those of you who have a 26 Nosler - how far from the bolt face does your chamber end?)

The barrel shank on the new barrel probably closely matches the factory shank that was removed so that does not seem to be an issue. The problem, if there is a problem, is that the smith didn't run the reamer far enough into the barrel. That appears to be the one area where it is left up to the smith.

At this point I should probably fess up that I have been "stretching" the truth a little (pun intended). The case that has the "belt" was one where the load detonated on me (due to little IMR7828SSC used) and I had to pound the bolt open with a hammer.

To say it was proof tested might be an understatement. Nevertheless it did bring this issue to light. This stretching also occurs but to a much lesser extent with "hot" loads as well. If I used my normal hunting load (140 grain Accubond at 3,300 fsp) I can't see stretching with the naked eye though you can see where the chamber ends as the case is still squished against the chamber walls and that leaves a "finish" on the case.

Bottom line - I'm going back to my smith and ask him to push the reamer in another 0.030" or so. Whatever SAAMI says or doesn't, that should fix the issue.

 

[bigngreen]

The belt is .532 on a belted magnum and the base of a RUM case is .550". It would have to be a considerably over sized chamber to not clean up. A bigger concern of mine would be weather the neck will fully clean up with a rechamber.

Yep, may bad!

 

[bigngreen]

Bottom line - I'm going back to my smith and ask him to push the reamer in another 0.030" or so. Whatever SAAMI says or doesn't, that should fix the issue.

If you just run a reamer in you'll just be blowing the shoulder forward more and change nothing at the back of the case, your chamber is already at spec or you would not have gotten factory brass in it, running the reamer in deeper will only make things worse for you!!

It kinda sounds like you've just pushed it so hard you have expanded the case head, do you have a picture you can post of a case?