Need help with a mauser marked 7mm mag barrel

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paw66

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Hey everyone i have a custom mauser with a parker hale trigger. The barrel is marked 7mm mag.
A 7mm rem mag cartridge drops right into the chamber and has about 0.135" headspace.
A 7mm weatherby magnum sits about flush with the chamber.
I made a chamber cast out of 2 part epoxy not perfect i know but it has to be close. It seems to be the same shape as the 7mm rem mag but the cast is longer.

So i dont know what to do. I dont know what cartridge it could be. Anyone have any ideas?

I thought about rebarreling in a more common cartridge but i am having trouble finding an affordable barrel.
 
Something is being lost in the message. Headspace is the "extra" distance between a reference point in the chamber to the bolt face. For many cartridges, the difference between GO gauge and NOGO gauge is .004" to .006". Both the 7mm Rem Mag and the 7mm Weatherby cartridge "headspace" on the front edge of the belt, and use the same headspace GO and NOGO gauges. You are saying you have .135" between the bolt face and the back of the cartridge when you place a 7mm Rem Mag cartridge in the chamber?? And then a 7mm Weatherby case goes in flush to that same chamber?? That is not "computing" !!
Edit - a thought just occurred - a "custom" rifle - maybe that is a 7mm Ultra Mag chamber??
 
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Ya i have 0.135" space between the base of the cartridge and bolt face. The weatherby is 0.050" longer and is about flush with the chamber. I am using motorcycle valve shins to measure the distance between base and bolt face. Is a mauser action strong enough to handle a 7mm ultra mag?
 
If you go here: ht tps://saami.org/wp-content/uploads/2018/01/Z299-4_ANSI-SAAMI_CFR.pdf (take out the space in the "ht tps" and paste link in your browser), you will see SAAMI cartridge and chamber specs. The 7mm Ultra Mag has same over all cartridge length as 375 H&H (3.600"), and Mauser 98's have been converted to accept cartridges that long, although "magnum" length 98's were preferred. On page 24 of above document SAAMI list Maximum Average Pressure of 7mm Ultra Mag at 65,000 psi. They show a MAP of 61,000 for 7mm Rem Mag, and 62,000 psi for 375 H&H.
Page 242 has cartridge and chamber drawings for 7mm Ultra Mag. That is what you compare your chamber cast to - diameters, shoulder angles, throat, etc. You will also find most of the other big 7mm's shown. Big hint is that the Ultra Mag does not have a belt - should be apparent on your chamber cast.
Let us know what you discover.
 
Do the best you can to figure out what it's chambered in.
Sort of like a high school project.

THEN take it in to a competent gunsmith for evaluation.

See how close your homework gets you.
Could be cheaper in the long run.
 
Thanks potashminer for the link. The chamber drawings are very descriptive. There is a belt on this cartridge and after carefully measuring my chamber cast it measures out to be a 7mm shooting times westerner. Thank you again potashminer for the link and insight. It was great help.

Now just gotta find some ammo.
 
The 7mm Weatherby, the 7mm Remington Magnum, and the 7mm STW all are standard belted magnums and all use the same headspace gauge.

I don't understand how you are measuring 'headspace'. In the case of these 3 cartridges the headspace measured off the belt is the same.

You should have this inspected by a qualified 'smith' before attempting to fire it.
 
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And, if a 7mm Rem case drops deeper into the chamber than any of the others, that means the chamber is effed up and is over-sized. The belt is accomplishing nothing.
 
You might want to measure that 7mm Rem Mag and 7mm Weatherby case you used and compare them to their SAAMI drawing. Would not be entirely unexpected to see that it is undersize - especially the belt diameters. Might go a ways to explain why they drop in to different depths - both should be stopped by the belt recess in the chamber, if the recess and the cartridge belts are to spec.
 
Potashminer said:
You might want to measure that 7mm Rem Mag and 7mm Weatherby case you used and compare them to their SAAMI drawing. Would not be entirely unexpected to see that it is undersize - especially the belt diameters. Might go a ways to explain why they drop in to different depths - both should be stopped by the belt recess in the chamber, if the recess and the cartridge belts are to spec.
Click to expand...

If the chamber is cut for a 7mm STW, both a 7mm Rem Mag and a 7mm Wby Mag will fit in at the same headspace as a 7mm STW.... there are no great differences in belt diameters on brass and the 'belt thicknesses' won't be out more than a few thou... not a visible difference to a naked eye.

If a 7mm STW seems to fit and the other cases fall in deeper, it is chambered for something larger than a 7mm STW.

It would be foolish to fire this rifle until the chambering is determined.

The OP needs to respond now.
 
When i wrote that first comment i was frustrated. dropped right in is an exageration. So basically i was using a case with the bullet and powder pulled. I tried to chamber it to see if i could pop the primer. Didn't pop. I dont have go no go gauges so i was measuring headspace with motorcycle valve shims. When i closed the bolt it with the shim it shoved the case into the chamber. I then had to take a cleaning rod and pop the case back out of the chamber. Thanks everyone for your concern. Thanks Grizz I ordered up some 7mm stw from cabelas last night. Thanks again potashminer i didnt think to measure the case. How you seen a lot of deviation in other cases?
 
Well, a few things there. Whenever I check headspace, the extractor and bolt face ejector are removed from the bolt, and I use very light "one finger" on the bolt handle to try to feel when the gauge touches its stop. I can't imagine pushing something into the chamber so tight that a cleaning rod is needed - whatever that is, that is not "checking headspace". Just so that you know for next time.
Deviation in cases - in part because of this thread I measured up some brass last evening. I just received a brand new bag of Winchester 338 Win Mag. Of the 10 belts that I measured, all vary between .527" and .528" diameter - not perfectly concentric, and .527" is SAAMI minimum for this belt, however, even that minimum is significantly larger than the .5136" dimension for the chamber immediately in front of the belt. That is why I had thought 7mm Ultra Mag - the rear of it's chamber is larger than .550" and a belted case, even maximum SAAAMI size, would "drop right in".
 
guntech said:
If the chamber is cut for a 7mm STW, both a 7mm Rem Mag and a 7mm Wby Mag will fit in at the same headspace as a 7mm STW.... there are no great differences in belt diameters on brass and the 'belt thicknesses' won't be out more than a few thou... not a visible difference to a naked eye.

If a 7mm STW seems to fit and the other cases fall in deeper, it is chambered for something larger than a 7mm STW.

It would be foolish to fire this rifle until the chambering is determined.

The OP needs to respond now.
Click to expand...

This is a question, not a recommendation but...

What are your thoughts about fire forming say something like a 7mmRM at say 50% powder with paper wad to determine chamber?

Again, no one follow this as a recommendation.
 
50% load of slow powder will likely cost some fingers. Not a good idea, at all. I do fireform 7mm Rem Mag brass into 458 Win Mag by using 12 grains of Unique and the rest of the case full of Cream of Wheat cereal.

Thoroughly clean the chamber and bore. Put the barrel vertical in a padded vise, chamber end up. Plug the bore by inserting a patch into the bore ahead of the chamber. Warm barrel up with a heat gun / hair dryer - get it warm/hot to the touch - 100 to 125 degrees Fahrenheit or so. Melt Cerrosafe alloy in a metal pouring cup. (Old timers apparently used sulphur instead of Cerrosafe). Pour melted Cerrosafe into the chamber until it is full. Allow Cerrosafe to set - it will shrink slightly. Push the casting out with a cleaning rod from the muzzle end. Set it on the bench and allow it to "cure" for one hour - it will swell back to exact size in 60 minutes. Use a micrometer and take measurements. Compare to SAAMI drawings.
 
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paw66 said:
When i wrote that first comment i was frustrated. dropped right in is an exageration. So basically i was using a case with the bullet and powder pulled. I tried to chamber it to see if i could pop the primer. Didn't pop. I dont have go no go gauges so i was measuring headspace with motorcycle valve shims. When i closed the bolt it with the shim it shoved the case into the chamber. I then had to take a cleaning rod and pop the case back out of the chamber. Thanks everyone for your concern. Thanks Grizz I ordered up some 7mm stw from cabelas last night. Thanks again potashminer i didnt think to measure the case. How you seen a lot of deviation in other cases?
Click to expand...

So what do you intend to load and fire in that rifle?

It is not correctly chambered and head spaced for 7mm STW.
 
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Thanks potashminer i will use less force next time i check the headspace. I definitely used too much force. I was looking into the cerrosafe and sulfur. I think the cerrosafe would be easier to use.
 
like guntech said all belted magnums head space on the belt. So whether you put 7mm rem mag 7mm weatherby or 7mm stw in the chamber the primer should go pop. If the 7 rem pushed further it is not chambered for a belted cartridge of that size. If the stw case sits back and fires it is head spacing off the shoulder but the chamber is still to large and it is likely chambered for a 7 RUM. You obviously don't know whats going on so take it to someone that does.
 
Leeper said:
And, if a 7mm Rem case drops deeper into the chamber than any of the others, that means the chamber is effed up and is over-sized. The belt is accomplishing nothing.
Click to expand...

guntech said:
If the chamber is cut for a 7mm STW, both a 7mm Rem Mag and a 7mm Wby Mag will fit in at the same headspace as a 7mm STW.... there are no great differences in belt diameters on brass and the 'belt thicknesses' won't be out more than a few thou... not a visible difference to a naked eye.

If a 7mm STW seems to fit and the other cases fall in deeper, it is chambered for something larger than a 7mm STW.

It would be foolish to fire this rifle until the chambering is determined.

The OP needs to respond now.
Click to expand...

guntech said:
So what do you intend to load and fire in that rifle?

It is not correctly chambered and head spaced for 7mm STW.
Click to expand...

Both of the above qualified gunsmiths are giving you good advice telling you not to proceed as you are planning.

The rifle has obviously been rechambered to a cartridge larger in base diameter than the belt of any of the cartridges you are considering.

Ted
 
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