Chamber cast to determine headspace?

[Mk42gunner]

For decades the answer to anyone having questions as to what cartridge their rifle is has been to do a chamber cast with cerrosafe. After using it for the first time, I have a question.

Just how do you determine headspace with a chamber cast in something you can't fill to the breechface? I am thinking of bolt actions or most rotating bolt repeaters. There is space between the rear of the barrel and the boltface.

I would not want to fill the locking lug recess with melted cerrosafe then try t get it out.

Just filling the barrel portion will get you most of the dimensions, but not the overall length of the chamber, unless I have missed something in the thousands of times I have read about it.

My best guess so far is to sacrifice a case of the appropriate head size and drill a large hole in the base. take out any spring loaded ejector and remove the extractor, then push the modified case in the chamber with the bolt leaving it there while filling with cerrosafe.

Sound reasonable or am I thinking again?

Robert

[Fitz]

A chamber cast will tell you your neck dimensions , your throat dimensions , your freebore dimensions , and your bore dimensions but for your headspace you need to measure your fired brass , and with the chamber cast read the directions on the time to take your measurements after it cools it grows a couple of thousands .

[samari46]

Take one of your cases that has been fired in your rifle, seat a fired primer just so it will stay in place. Place case in chamber and close the bolt. Remove case and use the tail end of a set of dial calipers to measure the distance between the top of the primer and case head. Should get you close to actual headspace numbers. Hope this helps. Frank

[M-Tecs]

Headspace is a defined data point about halfway up the shoulder. There are various methods to measure actual headspace with direct measurements. Using an actual headspace gauge is one. Another is something like an RCBS Precision Mic. It will measure a fired case minus spring back for an actual measurement.

More commonly used are indirect measurement methods like the Hornaday case comparator. You can use it to set shoulder bump the same as the RCBS Precision Mic but is does not give you an actual headspace measurement unless it set using an actual headspace gauge.

I've never tried using cerrosafe for headspace but I did use my heat gun to remove a mess for a buddy that did try it.

Too the OP if you do get a complete chamber cast how are you going to measure it for an actual headspace measurement?

[John Taylor]

A cheap way to check headspace is using a new factory round and masking tape. First check to see if the bolt will close on the round. If it does then add a layer of masking tape to the base (This adds about .004"). If the bolt closes easy you have too much headspace. You can keep adding tape to the base to see how much headspace you have, peel off all the layers and check with a micrometer.
I had an 1894 Winchester come in with .025" headspace and it was still being used. The customer was concerned because the primer was backing out when fired. The 30-30 usually does not have enough pressure to stretch the brass. Someplace above the 42,000 psi of the 30-30 the brass will stretch to fill the chamber.

[racepres]

Why Check?? On a Wildcat??
Fireform and go..Headspace will be Perfect!!!...
obviously neck size only after that!!!

[waksupi]

As hand loaders, this should be of no concern to us. Seat the bullet out to meet the lands, fire form, and you have perfect brass for your chamber.


racepres beat me to it!

[stubshaft]

They about covered it, nothing for me to add.

[Mk42gunner]

As I suspected, I was thinking again. Thanks for your input.

I am familiar with using actual headspace gauges and the tape method, I was just curious about the blanket "use cerrosafe" recommendation.

Robert

[scooterwrench]

A cheap way to check headspace is using a new factory round and masking tape. First check to see if the bolt will close on the round. If it does then add a layer of masking tape to the base (This adds about .004"). If the bolt closes easy you have too much headspace.

This is the method I use for setting up headspace on new builds using one layer of tape and adjusting barrel depth until I can just close the bolt with light tension. I tend to be concerned when I can easily close a bolt with four layers.

[M-Tecs]

This is the method I use for setting up headspace on new builds using one layer of tape and adjusting barrel depth until I can just close the bolt with light tension. I tend to be concerned when I can easily close a bolt with four layers.

I've only used a factory case and tape a couple of times when I didn't have a headspace gauge. I do use layers of scotch tape on a go headspace gauge to inspect factory chambers. Used in conjunction with an RCBS Precision Mic. you can get actual dimensions with a case and tape.

[waksupi]

This is the method I use for setting up headspace on new builds using one layer of tape and adjusting barrel depth until I can just close the bolt with light tension. I tend to be concerned when I can easily close a bolt with four layers.

I would add, remove the mainspring from the bolt first. It gives you better feel of what you are doing.

[scooterwrench]

I would add, remove the mainspring from the bolt first. It gives you better feel of what you are doing.

Oh yeah, bolt needs to be completely stripped especially the firing pin if working with live ammo.