Chamber and Saami- shoulder bump problem?

[ucsfl05]

Hi there,
In need of some help.

Put on a new proof prefit. Was hard, if not impossible to close bolt on my full length sized ammo. The prefit chamber would accept virgin brass (without bullet).
Also, cannot chamber once fired brass from the same chamber. Pics from case gage:
Notice the protrusion of head and retracted neck.

What’s going on here? What dimension is off?
Thanks

 

[ucsfl05]

 

[Doom]

Probably the area near the base has expanded and not been resized. It’s the area shown on the SAAMI drawing 0.2008” from the cartridge base. Check the diameter in that area and slightly towards the case mouth. It’s possible it’s the shoulder. You can check the shoulder setback with a comparator.

 

[918v]

What does FL sized mean? Every die manufacturer is different. Redding bumps the shoulder more than Forster, for example. So we need more information.

 

[B y r o n]

So what where the results when you checked the prefit with headspace gages? Did it close on the go gage?

A lot of new brass is a couple of thousandth less than SAAMI spec, what is the dimension of your loaded rounds?

 

[ucsfl05]

I don’t have go/no-go gauges. (I know, I know).
FWIW, I put on another identical proof prefit and found the same brass issues.
Hornady sizing die.
Can someone pls post the saami case drawing?

 

[spife7980]

What was your actual a shoulder and base measurements of the fired and sized pieces?

You don’t need saami anything, you need to toss that dumb gauge and actually measure your brass relationship between your chamber and your die.


Also, how could we provide you a drawing (that’s publicly available to you on the saami website anyways) for a chambering that you haven’t even bothered to name?

 

[918v]

I don’t have go/no-go gauges. (I know, I know).
FWIW, I put on another identical proof prefit and found the same brass issues.
Hornady sizing die.
Can someone pls post the saami case drawing?

Did you cam over the shell holder when you sized the brass?

 

[ucsfl05]

Did you cam over the shell holder when you sized the brass?

Yes

 

[ucsfl05]

What was your actual a shoulder and base measurements of the fired and sized pieces?

You don’t need saami anything, you need to toss that dumb gauge and actually measure your brass relationship between your chamber and your die.


Also, how could we provide you a drawing (that’s publicly available to you on the saami website anyways) for a chambering that you haven’t even bothered to name?

It’s a 6gt.
How do I measure the shoulder? Do I use a comparator and check the difference or do I measure at the base of the neck?

 

[ucsfl05]

What was your actual a shoulder and base measurements of the fired and sized pieces?

You don’t need saami anything, you need to toss that dumb gauge and actually measure your brass relationship between your chamber and your die.


Also, how could we provide you a drawing (that’s publicly available to you on the saami website anyways) for a chambering that you haven’t even bothered to name?

And so you know, 6GT is not listed.

 

[Im2bent]

Show a pic of an unfired complete loaded round that you reloaded. All we can see is your case gauge. A side view showing the neck shoulder area.

 

[spife7980]

It’s a 6gt.
How do I measure the shoulder? Do I use a comparator and check the difference or do I measure at the base of the neck?

Yes, a comparator. The issue with the plunk gauge is that you don’t know if it’s the shoulder or the base jamming you up.

Measure the base diameter, the shoulder diameter, the neck diameter with calipers and then the shoulder datum with calipers&comparator for both fired and sized brass. You answer is in the numbers.

I don’t have a 6gt to share my numbers with that. But it’s relative to your chamber that you need to worry about the fitment of.

 

[Feniks Technologies]

And so you know, 6GT is not listed.

If you Google “6gt saami” it’s the first result.

 

[Feniks Technologies]

It’s a 6gt.
How do I measure the shoulder? Do I use a comparator and check the difference or do I measure at the base of the neck?

You can use the Wilson case gauge and a dial or digital indicator and a stand. Like a concentricity stand/base.

You can also buy the Wilson micrometer top for the case gauge.

Will do the same thing a comparator and calipers will do.

 

[918v]

Yes

Then get a Redding die

 

[918v]

Or sand the bottom of yo Hornady die

 

[Dolomite]

Rechambering a fired case may not be a good measurement. Use the headspace comparator on the fired case and bump the shoulder back till the bolt handle drops on a sized case. Check out wheeler accuracy videos he explains the step by step.

If the sized case still doesn’t chamber use a marker and color the brass around the neck, shoulder body junction and the base of the sized brass. This may show where the brass is being hung up in the chamber. If there is no problem chambering a sized case shoot the brass at least 2-3x to get the right shoulder bump.

Maybe your not adjusting the die enough to bump the shoulder and size the base? That’s where the headspace comparator comes in handy. Remove the bolt firing pin and ejector and chamber the case. If the bolt close is heavy bump the shoulder more till the bolt handle drops.

You have to find your headspace measurement as the smith could have chambered your barrel short, meaning your headspace maybe close to the measurements of a virgin case.

 

[ucsfl05]

Yes, a comparator. The issue with the plunk gauge is that you don’t know if it’s the shoulder or the base jamming you up.

Measure the base diameter, the shoulder diameter, the neck diameter with calipers and then the shoulder datum with calipers&comparator for both fired and sized brass. You answer is in the numbers.

I don’t have a 6gt to share my numbers with that. But it’s relative to your chamber that you need to worry about the fitment o

Thanks! This datum- is that also cartridge specific?

 

[ucsfl05]

Thank you to everyone so far.

 

[spife7980]

Thanks! This datum- is that also cartridge specific?

No, a datum is just a geometric plane that we use to intersect the case, in the case of a bottle neck rifle cases with coned shoulders we use a hole that settles down on the shoulders at the same level around consistently and repeatably.

In most cases for the non mag standard short action chamberings you’ll see a C-.375” or D-.400” hole referenced as the diameter of the hole they use in their datum plane in the comparator.
You can use whatever you want though that fits on the shoulders to get your cases before and after.

 

[Feniks Technologies]

You can get datum gauges (basically a hole like posted above) and then you can get shoulder comparators that are cut for shoulder angle.

A datum gauge can be used to get absolute measurements that can be compared to a drawing or spec. But, with it comes a sharper edge and therefore much easier to get inconsistent measurements without being fairly well practiced used them.

Then there are shoulder bump comparators. They are much easier to use. They are however, relative measurements. As in you can’t give that measurement to someone else or compare against a print.

 

[ucsfl05]

No, a datum is just a geometric plane that we use to intersect the case, in the case of a bottle neck rifle cases with coned shoulders we use a hole that settles down on the shoulders at the same level around consistently and repeatably.

In most cases for the non mag standard short action chamberings you’ll see a C-.375” or D-.400” hole referenced as the diameter of the hole they use in their datum plane in the comparator.
You can use whatever you want though that fits on the shoulders to get your cases before and after.

View attachment 7835123

View attachment 7835124

You are awesome man. Thank you!!!

 

[Doom]

I don’t have go/no-go gauges. (I know, I know).
FWIW, I put on another identical proof prefit and found the same brass issues.
Hornady sizing die.
Can someone pls post the saami case drawing?

There is an issue here. Reloading dies assume certain dimensions. They ultimately will size a case to meet and not exceed SAAMI maximum cartridge dimensions. The actual dimensions will vary, from die to die and manufacturer to manufacturer. But all of these dimensions are based on the minimum chamber dimensions. Similarly the case gauge is based on either max cartridge or min chamber. Installing a prefit barrel without benefit of headspace gauges means you cannot be sure that your chamber matches up to the same dimension specs as the gauge and sizing die or that a FL sized case will fit your chamber.

Head spacing a barrel is also a safety issue, too much and you experience case separation. No competent gunsmith would ever install a barrel without a go gauge as a minimum.

 

[r.tenorio671]

...as far as "case gauges" go, I prefer the Sheridan Slotted case gauges, can see in an instant if it's shoulder bump, case web sizing or trim length that is an issue on an empty casing. If one is sticking to absolute SAAMI spec, it can also determine if bullet seating for individual bullets is compliant in regards to the seating depth COAL or if bullet ogive type is contacting the leade/lands. Unfortunately Sheridan doesn't have 6GT listed, but a quick email can get the info on when it may become available, especially as it's a SAAMI spec and increasing in popularity. The enclosed type of gauge "hides" too much IMHO and you can't tell which one is the cause, i.e., shoulder, case web or bullet...or a combination.

...in the meantime, measuring a fired casing with something like the Hornady LnL HS tool to determine the shoulder bump after firing and calipers to measure case web expansion after firing to compare against sized cases would give you numbers to work with to adjust your sizing die accordingly. Using same Hornady like tool and comparator inserts you can determine amount of seating depth for whatever bullet you use and if its jammed or off the lands an X amount.

 

[ucsfl05]

Ok so I got the Forster comparator and measured from the shoulder datum point using a .375 size hole.

The measurement that is supposed to be 1.364 measures 1.3475 on my virgin alpha brass using my mitutuyos.

Brass longer than this doesn’t chamber.

What’s going on here?

Thanks

 

[B y r o n]

It is simple, minimum chamber is 1.3607" and maximum is 1.3707". Your virgin brass measures under the minimum chamber so it chambers. Your loaded rounds exceed whatever your chamber measures which is still unknow at this point.

 

[Im2bent]

So the new virgin brass that is 1.3475 at the shoulder fits but anything longer does not? Sounds like your chamber is short?

 

[ucsfl05]

It is simple, minimum chamber is 1.3607" and maximum is 1.3707". Your virgin brass measures under the minimum chamber so it chambers. Your loaded rounds exceed whatever your chamber measures which is still unknow at this point.

Sorry for being dense but where are you extrapolating those numbers from?
Wouldn’t max be 1.364 and minimum be 1.357?

 

[ucsfl05]

Sorry for being dense but where are you extrapolating those numbers from?
Wouldn’t max be 1.364 and minimum be 1.357?

I see- you are looking at chamber not case. Thanks

 

[ucsfl05]

It is simple, minimum chamber is 1.3607" and maximum is 1.3707". Your virgin brass measures under the minimum chamber so it chambers. Your loaded rounds exceed whatever your chamber measures which is still unknow at this point.

So how do I measure the chamber? Lead cast? Fired case?

 

[Doom]

Op,
How did you set the headspace on the barrel when you installed it without a go gauge?

 

[r.tenorio671]

...1.) Your virgin brass measures

View attachment 7837923

Ok so I got the Forster comparator and measured from the shoulder datum point using a .375 size hole.

The measurement that is supposed to be 1.364 measures 1.3475 on my virgin alpha brass using my mitutuyos.

Brass longer than this doesn’t chamber.

What’s going on here?

Thanks

1. Your "virgin brass" measures 1.3475" at the datum; that brass is 0.00025" SHORTER than the minimum SAAMI spec's allowance of -0.007" variation. That virgin brass should chamber in any SAAMI spec chamber. Once it is fired it will blow out the shoulder; you need to now measure that fired case to determine how far it blew the shoulder out to, that will be close to the MAX that YOUR chamber was reamed to.

2. When you set your sizing die up, you want to bump the shoulder back far enough to be UNDER the fired cases measurement of the now blown out shoulder.

3. You also need to be mindful of the fired cases overall case length, the "1.725 - 0.020" numbers in the event your fired case experiences "brass flow" in the case neck area. You need to trim the cases to remain within that spec range of 1.725 max, 1.705 min. You can do a cerro casting of your chamber or the expedient method is take a fired, sized casing and mark it up around the case mouth/neck with a sharpie and gently insert it into the chamber, seating fully. Remove the casing and see if there are any marks around the case mouth that would indicate the casing is now too long despite having the correct shoulder bump. If it is, you would have to trim that casing to below that mark. It would be unusual for virgin brass to grow that much on it's 1st firing, but not impossible, especially if the reamer was "short" in the neck dimension, over worn, lathe operator error or an anomaly with the brass.

4. Being too long in the shoulder or too long in the case length individually, or a combination of the two can prevent proper chambering of a loaded round. Improper seating depth of a bullet can also prevent proper chambering if the ogive of the bullet is contacting the lands too early. Typically tangent ogives make contact earlier than secant ogives when COAL is the same between the two ogive types.

 

[Haney]

Headspace gauges are cheap . Looks like you had a "crush fit" when you used brass instead of steel as a gauge .

 

[Doom]

Headspace gauges are cheap . Looks like you had a "crush fit" when you used brass instead of steel as a gauge .

I didn’t see where he said he did this but that’s the likely method he used, with new brass.

OP also needs to set his calipers at 0.471” and make sure no part of the fired brass other than rim exceeds that diameter.

As for measuring actual headspace of the chamber from fired brass with a comparator, this doesn’t work. The comparator only measures the approximate dimension of the case. It is not machined to be precise, it is meant to be repeatable. All of this comparison to SAAMI is BS without knowing that the headspace was set correctly.

I don’t see where OP has compared his fired vs unfired brass to see if he is setting the shoulder back.

 

[Haney]

I didn’t see where he said he did this but that’s the likely method he used, with new brass.

OP also needs to set his calipers at 0.473” and make sure no part of the fired brass exceeds that diameter.

As for measuring actual headspace of the chamber from fired brass with a comparator, this doesn’t work. The comparator only measures the approximate dimension of the case. It is not machined to be precise, it is meant to be repeatable. All of this comparison to SAAMI is BS without knowing that the headspace was set correctly.

I don’t see where OP has compared his fired vs unfired brass to see if he is setting the shoulder back.

 

[Doom]

Haney,
Didn’t take it that way, but if he is using resized brass to set headspace ....!

 

[Haney]

Haney,
Didn’t take it that way, but if he is using resized brass to set headspace ....!

OK, maybe I am misunderstanding or reading too much into it ? I have done it with brass while waiting for my gauges to arrive . After checking with gauges I was short . Steel gives a much better read for me , brass is easily manipulated into a crush fit .

 

[Doom]

Who knows? Using brass to reset a barrel is doable but as you noted it will crush. Filling it with epoxy can be done to help prevent this. Using new brass is guaranteed to result in a short chamber. You can also used FLS brass from another barrel IF you know the fired dimension in question and maintain it.

Prefit is really a misnomer. It a prechambered barrel. It still requires proper head spacing.

 

[ucsfl05]

Headspace gauges are cheap . Looks like you had a "crush fit" when you used brass instead of steel as a gauge .

Where to buy?

 

[Haney]

Where to buy?

6GT Headspace Gauges - Pacific Tool and Gauge

6mm BRX Headspace Gauges at Pacific Tool and Gauge. Made in White City Oregon USA. Making Firearm Parts and Chamber Reamers Since 1957.

pacifictoolandgauge.com

 

[Haney]

Rental also by the looks of it .

6mm GT - 4D Reamer Rentals

6mm GT, chamber reamer for rent.

4drentals.com

 

[2aBaC̶a̶]

Your barrel needs to be unscrewed a smidgen. put some tape on the ass of a case and reset your barrel a few thou longer. easy peesy. Imo, its better to be on the longer side than the short. dies are meant to take a case to the short end off saami at most and will almost always size a case out of a chamber headspaced on the longer end. even chambers set on the no go guage are still acceptable in most cases and not dangerous until you flirt with a field guage.

 

[Haney]

Your barrel needs to be unscrewed a smidgen. put some tape on the ass of a case and reset your barrel a few thou longer. easy peesy. Imo, its better to be on the longer side than the short. dies are meant to take a case to the short end off saami at most and will almost always size a case out of a chamber headspaced on the longer end. even chambers set on the no go guage are still acceptable in most cases and not dangerous until you flirt with a field guage.

Tape and brass, two soft materials . Those are two avoidable variables . If he goes too long he has case-head separation issues , maybe sizing issues etc. . This is a 6 GT , precision is the name of the game right out of the gate . A simple steel gauge takes all of the guess work out .

 

[2aBaC̶a̶]

Tape and brass, two soft materials . Those are two avoidable variables . If he goes too long he has case-head separation issues , maybe sizing issues etc. . This is a 6 GT , precision is the name of the game right out of the gate . A simple steel gauge takes all of the guess work out .

he could have the same problem with a go guage. plus if you screw the barrel in lightly and dont ham fisted gorilla tighten it down on the brass and tape your good. not likely to get case head separations unless hes waaaaay long. you dont have to be exact, its a ballpark. like I said earlier being on the longer side has its pros.

 

[Haney]

he could have the same problem with a go guage. plus if you screw the barrel in lightly and dont ham fisted gorilla tighten it down on the brass and tape your good. not likely to get case head separations unless hes waaaaay long. you dont have to be exact, its a ballpark. like I said earlier being on the longer side has its pros.

You sound sloppy. In my opinion brass/tape is an ignorant approach . It's a 6 GT for precision , not shooting 223 range brass .

 

[2aBaC̶a̶]

You sound sloppy. In my opinion brass/tape is an ignorant approach . It's a 6 GT for precision , not shooting 223 range brass .

To each his own. all the barrels Ive ever set were precision. and I can control exactly where the headspace is between min and max using some tape. it may be a redeck rocket science approach but it gets the job done. and I dont need to buy a bunch of guages for every caliber.

So tell me, do you just tighten the barrel down on a go guage and call it good?

 

[Haney]

To each his own. all the barrels Ive ever set were precision. and I can control exactly where the headspace is between min and max using some tape. it may be a redeck rocket science approach but it gets the job done. and I dont need to buy a bunch of guages for every caliber.

So tell me, do you just tighten the barrel down on a go guage and call it good?

Sorry, you don't seem teachable . Wasted enough time on you . Don't let facts cloud your math .

 

[2aBaC̶a̶]

Sorry, you don't seem teachable . Wasted enough time on you . Don't let facts cloud your math .

lol, not surprising.

 

[spife7980]

@Haney @2aBaCa
How are yall going to argue over how a third party set a barrel then when you dont even know if its a barrel nut prefit or a shouldered prefit? All we know is that proof chambered it, we dont have any particulars to even begin to argue yalls points as it pertains to the OP.

So how do I measure the chamber? Lead cast? Fired case?

You can do a chamber cast but its not at all necessary. Your fired case will be close enough. If you suspect that it might not have swelled up all the way on the first then load it up again and let it go bang once more. The fact that your fired brass is tough to chamber tells me that its chamber sized enough to at least impinge on the chamber.

The measurement that is supposed to be 1.364 measures 1.3475 on my virgin alpha brass using my mitutuyos.

So you just like you got a 1.347-1.348 number on your virgin case using the .375 datum comparator, you get that measurement for your fired case.
Lets say your 1.347 brass blows out to 1.352 on a fired case- you screw your sizing die in or out until the brass comes out of the die and measures 1.350.
If your fired brass measures 1.370 on a fired case- you adjust your die until the sized brass comes out of it at 1.368.

If doesnt matter what your calipers comparator reading actually is. We just want to adjust it so that after sizing the brass reads .002 less than the fired before it went into the die.
.002 is enough sizing to ensure smooth chambering and operation of the rifle without sizing it excessively and causing brass to wear out prematurely.