Headspace and case stretch

[johnnyk]

Just recently installed new Pac Nor barrel in .300WinMag. Using new Norma brass which measures on average 2.2620" using Hornady HS Gauge. My fired brass is averaging 2.2760". Is this about right or does it seem too much of a stretch? I can always try and tighten the barrel/headspace up if need be.

 

[dok7mm]

Two ways to prevent excessive case stretch on new undersized brass are to jam the bullet into the lands or by putting a false shoulder on your cases.

 

[sedancowboy]

That sound about right if my math is right you are expanding the case about .014. All Belted mag brass are undersized by a good amount. The belt controls the head space on belted mag for the first firing then you can use the shoulder to head space from now on. If you only bump the shoulder back .002 or so you will get good case life.

 

[ar10ar15man]

when starting out on a new cartridge, look up the SAAMI spec's on the chamber and the case/ammo. it will help with questions like this.
and yes new brass is typically on the small side of the ammo spec.

Just recently installed new Pac Nor barrel in .300WinMag. Using new Norma brass which measures on average 2.2620" using Hornady HS Gauge. My fired brass is averaging 2.2760". Is this about right or does it seem too much of a stretch? I can always try and tighten the barrel/headspace up if need be.

 

[lancetkenyon]

I have found about .014"-.020" growth in base to shoulder datum in virgin 7RM & .300WM. When FL sizing, just bump them back .002" like any other case.

 

[johnnyk]

Thanks guys. Not new to reloading or .300WinMag, just checking.

 

[AZShooter]

.140" seems like excessive amount of stretch. I would remeasure, must be a mistake. That kind of stretch IMO would have created a case separation.

 

[sedancowboy]

That sound about right if my math is right you are expanding the case about .014. All Belted mag brass are undersized by a good amount. The belt controls the head space on belted mag for the first firing then you can use the shoulder to head space from now on. If you only bump the shoulder back .002 or so you will get good case life.

.140" seems like excessive amount of stretch. I would remeasure, must be a mistake. That kind of stretch IMO would have created a case separation.

Reread the numbers? There are a lot of difference in .140 and .014.

 

[Lefty G-gear]

ON BELTED MAGNUMS - SOMETIMES IT'S BETTER TO NECK-SIZE ONLY.
ESPECIALLY IF THERE IS A BIT TO MUCH HEAD SPACE.
TOO MUCH EXCESSIVE HEAD SPACE CAN BE DANGEROUS = CASE BLOWS-UP GUN AND YOU!
BUY A SET OF 3 HEAD SPACE GAUGES FOR CALIBER = ABOUT $100 AT PTG, et al.
THEN YOU WILL KNOW WHERE YOU ARE AT!

IF THE BOLT CAN CLOSE ON THE 'FIELD GAUGE' THEN IT IS TOO LONG AND DANGEROUS! - I'VE SEEN THIS IN 2 RIFLES!
http://pacifictoolandgauge.com/683-saami-headspace-gauge-sets-go-no-go-field-nopix-

THROW EACH OF THESE IN AND TRY TO CLOSE THE BOLT.

IT SHOULD CLOSE ON THE GO GAUGE. (IF IT CLOSES ON THIS ONE ONLY - IT IS BEST.)

IT MAY CLOSE ON THE LONGER NO-GO GAUGE - AND BE OK. (BUT, BETTER IF BOLT DOESN'T CLOSE. = NICE TIGHT SAFE CHAMBER + LONGER BRASS LIFE.)

IT SHOULD NEVER CLOSE ON THE FIELD GAUGE! (BRASS MAY RUPTURE - BEING TOO LONG OF A CHAMBER!)

WHETHER YOU ARE BUYING A NEW OR USED RIFLE - TAKE A SET OF THESE AND CHECK IT

OR A CASE GAUGE ON A FIRED CASE OF THE USED GUN.

 

[Lefty G-gear]

OR USE A CASE GAUGE AND MEASURE.
DROP FIRED BRASS CASE IN ONE OF THESE.
https://lewilson.com/gages/

300 WM
https://www.midsouthshooterssupply....on-adjustable-case-gage-300-winchester-magnum

HORNADY CASE GAUGE FOR .300 WM $17.48
https://www.midsouthshooterssupply....300-winchester-mag-(point308)-cartridge-gauge

 

[bigedp51]

You can't judge case shoulder location on new rimmed and belted cases.

Below is a new unfired .303 British case in a Wilson case gauge. And the case would drop further into the gauge but the rim is stopping it from moving further. The Enfield rifle this case will be fired in will not close on a NO-GO gauge but still has .009 head gap clearance due to the case rim thickness.

Below the case belt manufacturing headspace can vary .008 and the chamber headspace can vary .007. And the NO-GO gauge is .224 so you can see the case belt will make the biggest difference in head clearance.

You can take a new unfired case and measure from the case mouth to the base of the case. And then take a fired spent primer and start the primer into the primer pocket with just your fingers.

Now chamber the case and let the bolt face seat the primer and extract the case. Then measure the case from the case mouth to the base of the primer and subtract the first case measurement from the second. And this will be your head clearance or air space between the rear of the case and the bolt face. And this will be how far the case will have to stretch on the first firing to contact the bolt face. Meaning the case shoulder location means nothing until the case is fired. And what is important is your head clearance with new cases and not the shoulder location.

 

[entoptics]

Just recently installed new Pac Nor barrel in .300WinMag. Using new Norma brass which measures on average 2.2620" using Hornady HS Gauge. My fired brass is averaging 2.2760". Is this about right or does it seem too much of a stretch? I can always try and tighten the barrel/headspace up if need be.

I have a fairly tight headspace on my Shilen barrel, and new unfired Sig and Norma brass stretched about 0.01" on first firing. The OEM barrel on the Savage had had an excessively long chamber, though the belt headspace was fine. For it, new brass would stretch 0.016.

After first firing, as mentioned by others, I simply bump the shoulder about 0.002" and everything is fine and dandy from there on out.

 

[AZShooter]

I must apologize for my mathematical error. Here I am telling the OP to remeasure when it should have been me doing the math problem again! The difference IS .014" which is not a concern with a belted magnum.

 

[freddiej]

Okay some of what I was going to talk about with you has been illustrated in another post. So here goes. I hope I do not offend too many people.
first, is the brass resized after your first firing? If so, then this is very normal. 223 brass sometimes stretches 0.030" after it's first firing and resize.
Second it could be excessive head space. This is if you have not resized the cases and they are just fired and cleaned. If this is over 0.010" it can cause case head separation. Even the best brass will start excessively stretching at 0.012"-0.013" excessive head space. This leads to case head separation after one or two firings.
To check for excessive head space. As illustrated above, resize the case, insert a primer, put it in your rifle and shoot off out the back door, then find your calibers and measure the primer protrusion versus the case in the chamber. that higher number minus the case alone number is the excessive head space. to correct for this problem you now take a feeler gauge of 0.001" to 0.003" thinner and set your sizing die to give you a small amount of excessive head space so the rifle is reliable. okay let me explain that, use the feeler gauge in between your shell plate/case holder and the bottom of your sizing die at the top of the stroke of your press as per normal setting. Your excessive headspace problem is solved. Stretching will be at a minimum from here on out.