Belted Magnum Head Spacing

[woods]

Once a new case has been fired, if it's not FL resized (just neck sized) the once fired case will head space off the shoulder not the belt?

IMO, no

The reason is that the shoulder has not moved all the way to contact with the chamber so the belted case will headspace on the belt when the case moves all the way forward during firing and there will still be a gap between the case shoulder and the chamber shoulder

For example I reload for several belted cases and take measurements with the Hornady comparator installed on the calipers

These measurements are not "headspace" and are useful in comparing the brass movement from one measurement to the other

Example of measurements taken on belted magnums

264 win mag
new case - 2.109"
once fired - 2.136" still gap between case shoulder and chamber shoulder
twice fired - 2.1385" case fully expanded

300 win mag (one of many)
new case - 2.253"
once fired - 2.270" not fully expanded
twice fired - 2.272" almost fully expanded but still no "crush fit" when chambering before sizing
3 times fired - 2. 2725" full expanded with "crush fit" and case WILL headspace on belt and shoulder

338 win mag
new case - 2.097"
once fired - 2.120"
twice fired - 2.123"
3 times fired - 2.124" crush fit

Just some of the examples

Now if you had said

"Once a new case has been fired, if it's not FL resized (just neck sized) the case that has been fired until you have a crush fit, will the case head space off the shoulder not the belt?"

Then yes

IOW it is possible and preferable to headspace a belted magnum off the shoulder after the case is fully expanded and you have a crush fit

JMHO

 

[J E Custom]

When comparing a SAMMI chamber dimension Of most Belted cases to a Full length sized case there are major size differences so I never bump the shoulder unless I neck size only.

If you neck size only, the head space is close to .000 , only brass spring back makes it possible to chamber easy. I like to head space .002 thousandths based on the brand/dimension of the belt. Once fire formed, this head space is gone and the .002 thousandths ends up as clearance in front of the belt, so a slight bump may be necessary for chambering, so from this point on, the shoulder decides what the head space is not the belt. If you full length size the belt is responsible for the head space.

The case head to the front of the belt on a 7 Rem Mag SAMMI chamber is .220 minimum and .227 max. loaded cartridges must fall in this dimension. This clearance is set buy the gunsmith while cutting the chamber.

The cartridge dimension to the case neck junction are .009 shorter than the SAMMI chamber dimension when full length sized. so if you have add the differences in the case belt dimension and the chamber dimension to the differences to the full length sized case and the chamber dimension. It is possible to end up with up to ,017 thousandths of total case length differences.

With normal .002 thousandths head space and .002 shoulder bump and neck size only, you should end up with .002 to .004 total case length clearance.

If you full length size, you only increase the Shoulder to chamber dimension. So in my opinion it is best not to bump the shoulder if you elect to full size belted cases.

Also on the bulge in front of the belt after firing, This is caused by the chamber being to big for the SAMMI chamber dimension. Dies will size the cases back to the proper dimension, but the problem will still exist unless the chamber is re cut properly.

Just my preferred way to size belted cases.

J E CUSTOM

 

[DUNEZRUNNER]

When comparing a SAMMI chamber dimension Of most Belted cases to a Full length sized case there are major size differences so I never bump the shoulder unless I neck size only.

If you neck size only, the head space is close to .000 , only brass spring back makes it possible to chamber easy. I like to head space .002 thousandths based on the brand/dimension of the belt. Once fire formed, this head space is gone and the .002 thousandths ends up as clearance in front of the belt, so a slight bump may be necessary for chambering, so from this point on, the shoulder decides what the head space is not the belt. If you full length size the belt is responsible for the head space.

The case head to the front of the belt on a 7 Rem Mag SAMMI chamber is .220 minimum and .227 max. loaded cartridges must fall in this dimension. This clearance is set buy the gunsmith while cutting the chamber.

The cartridge dimension to the case neck junction are .009 shorter than the SAMMI chamber dimension when full length sized. so if you have add the differences in the case belt dimension and the chamber dimension to the differences to the full length sized case and the chamber dimension. It is possible to end up with up to ,017 thousandths of total case length differences.

With normal .002 thousandths head space and .002 shoulder bump and neck size only, you should end up with .002 to .004 total case length clearance.

If you full length size, you only increase the Shoulder to chamber dimension. So in my opinion it is best not to bump the shoulder if you elect to full size belted cases.

Also on the bulge in front of the belt after firing, This is caused by the chamber being to big for the SAMMI chamber dimension. Dies will size the cases back to the proper dimension, but the problem will still exist unless the chamber is re cut properly.

Just my preferred way to size belted cases.

J E CUSTOM

I am having a new barrel installed on my Model 70 and going to have them chamber it in .274 Win Mag. What should I say to my smith to have him set the gun up this way?

 

[J E Custom]

I am having a new barrel installed on my Model 70 and going to have them chamber it in .274 Win Mag. What should I say to my smith to have him set the gun up this way?

Do a SAMMI chamber and the recommended head space for the 264. That way you can shoot different factory loads, and after fire forming the first time you have the option of Neck sizing, partial sizing of the body to fit the chamber and head space off the shoulder or do a full size for SAMMI sized cases that head space off the belt.

Standard freebore and neck diameter is recommended for this cartridge. Don't over think it and you will be pleased with the SAMMI chamber.

J E CUSTOM

 

[DUNEZRUNNER]

Sounds good. Thank you