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7prc headspace issue!

Hybridhunternm

Hybridhunternm

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Nov 1, 2018
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#1
Ok so I bought a prefit tikka proof barrel in 7prc. I bought Manson go and no go gauges to just check as I’ve read some issues online. Well go gauge dropped no issue but so did the no go gauge. Couldn’t find a Manson field gauge so I bought a Forster field gauge and bolt will not close. Thinking it’s good to go but I was interested in the measurements so got the calipers out and the Forster field gauge says 1.9080 but measures 2.015…the Manson garages don’t have any measurements but the no go is 2.036 and the go is 2.032…

All of this just isn’t making sense…it doesn’t close on the shorter gauge..wondering if maybe I need to order the Forster go and no go and check everything again? Just want to make sure I’m safe out there! I’ve put together quite a few prefit barrels and have never come across something like this.
 
#2
Not meaning anything, but did you check zero on your calipers before each measurement? If so, then start measuring gauge diameters of each one, starting at the case(gauge) web and then just before the taper transition.

Also measure OAL of each gauge.
 
#3
My best guess is you aren't using a comparator to measure those gauges so your measurements basically mean nothing.

SAAMI spec for headspace on a 7 PRC is 1.900-1.910" (using a .420 dia datum on the shoulder)
 
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#6
Personally, I’d size some cases to nominal and fire some test rounds. That would tell the real story. And please, dear god, if you don’t have a comparator already, get one.

www.hornady.com

Lock‑N‑Load® Headspace Comparator & Anvil Base Kit - Hornady Manufacturing, Inc

Lock‑N‑Load® Headspace Comparator & Anvil Base Kit Lock-N-Load® Headspace Comparator The Lock-N-Load® Headspace Comparator extends brass life, improves accuracy, and enhances safety. The gauge measures...
www.hornady.com www.hornady.com
 
#7
lash said:
Personally, I’d size some cases to nominal and fire some test rounds. That would tell the real story. And please, dear god, if you don’t have a comparator already, get one.

www.hornady.com

Lock‑N‑Load® Headspace Comparator & Anvil Base Kit - Hornady Manufacturing, Inc

Lock‑N‑Load® Headspace Comparator & Anvil Base Kit Lock-N-Load® Headspace Comparator The Lock-N-Load® Headspace Comparator extends brass life, improves accuracy, and enhances safety. The gauge measures...
www.hornady.com www.hornady.com
Click to expand...
Just posted measurements with the comparator
 
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#9
Ya some times those Comparator holes aren't exactly the right size. At least your measurements follow the order of the gauges.
Based on ur measurements and what your telling me it sounds like the rifle is fine.
 
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#10
Hybridhunternm said:
Measurements just retaken with comparator using the .420 bushing from hornady

No go is 1.893
Go is 1.890
Field is 1.897
Click to expand...
Take a piece of scotch tape, put it on the Manson No-Go gauge to make it roughly 1.895" (measured with the comparator; average thickness of scotch tape is roughly 0.002"). This will end up being in between the Manson No Go and the Forster Field.

If it closes, you're probably at 1.896", if it doesn't close, you're at 1.894".
 
#11
Hybridhunternm said:
My factory Hornady ammo is 1.885 using same comparator so that would seem to line up that everything should be fine I am hoping
Click to expand...
That’s what you should expect from factory ammo. Essentially nominal.
Taylorbok said:
Ya some times those Comparator holes aren't exactly the right size. At least your measurements follow the order of the gauges.
Based on ur measurements and what your telling me it sounds like the rifle is fine.
Click to expand...
I concur.
 
#12
Evintos said:
Take a piece of scotch tape, put it on the Manson No-Go gauge to make it roughly 1.895" (measured with the comparator; average thickness of scotch tape is roughly 0.002"). This will end up being in between the Manson No Go and the Forster Field.

If it closes, you're probably at 1.896", if it doesn't close, you're at 1.894".
Click to expand...


It will not close with scotch tape on the no go
 
#13
Hybridhunternm said:
It will not close with scotch tape on the no go
Click to expand...
Record the info somewhere in a maintenance log book or something. If you handload, once your brass is fully fire formed to your chamber, expect it to measure 1.894" with the comparator. Use the number as the reference point to determine whether or not you need to bump the shoulders back or in the case of a non-fully fire formed case, simply need to squeeze the case body diameter down for reliable chambering.
 
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#14
Last edited:
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#16
If it did not close on a field gauge just go load a few "medium" pressure rounds and see how it does. Pay attention on how it opens and inspect the brass afterwards at the web.
Story time
I have had only one custom rifle have a bad headspace and it wasn't with my primary gunsmith who I should of went with anyways. It was with my 1903A1 aka M1941 sniper build. I had someone else build it who was supposed to be the expert on building these reproductions. Well flash forward I get the rifle back go to shoot it with CMP 30-06 ammo that uses lapua brass and once fired the bolt is really hard to lift. Then flash forward to its first reload and I have a case head separation. (I only bump shoulders .003".). I inspected all the once fired brass after that and they had signs of imminent casehead separation that you could feel and see with a borescope. I take it to my primary gunsmith and the bolt easily closes on a field gauge. Luckily his father had extra 1903 bolts laying around and one of the bolts allowed the gun to headspace correctly. Which i was happy about because I didn't want to purchase another 1903 Krieger barrel. We polished the chamber for good measure and it has performed flawlessly since.

I said all that to pretty much say, if your headspace is a problem, you'll know quickly and with the field gauge not closing its safe to test it.
 
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#17
JR1200W3 said:
What was the shoulder length growth in that one firing? If you can fireform BR brass into Dasher brass with bullet jam without immediate, imminent case head separation, it must have been a lot.
Click to expand...
The key is jamming the bullet but you also need enough neck tension to hold it firmly against the bolt face. I did a wildcat where I was moving shoulder ahead .030". At the start I had an issue where I didn't have adequate neck tension and the plunger ejector was enough to push the case onto the bullet creating excessive headspace and I nearly had case head separation on a few cases.
 
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#18
JR1200W3 said:
What was the shoulder length growth in that one firing? If you can fireform BR brass into Dasher brass with bullet jam without immediate, imminent case head separation, it must have been a lot.
Click to expand...
Sorry for the late reply. Had to stop by Scheels to get ADG 7 PRC brass of all things after work. 😂

1.901” to 1.915” after firing. Photos attached of the inside and out.


Shane first polished the chamber to see if that would stop the heavy bolt lift and that didn’t work. We were using some m72 to test it. After that we went and found a new bolt his dad had laying around.
 

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#21
Wannashootit said:
That's a problem...
Click to expand...
Yup. I won’t be recommending that smith personally after this. He offered a 1941 dated replacement barrel (not star gauges) and tried to convince us the old barrels shoot better than a new Krieger. Didn’t buy into that one bit. Since getting the new bolt it’s been shooting great, but all that Lapua brass is trashed.
 
#22
JR1200W3 said:
That doesn't make any sense. You already have excessive headspace with a bullet jam. The brass has to stretch and modify to fill that space. The stretching of the case wall is what causes casehead separation. There is so much space when fireforming non-ackley cartridges like Dasher that it also comes out of the shoulder and neck. What no one can say is how much comes out of the shoulder and neck vs out of the case body above the web when forming Dasher brass.

When you fail to have neck tension to effect bullet jam and the firing pin pushes the case forward, you just have light strikes. No one reports, almost casehead separation. I would suspect you had something else contributing that you weren't aware of.
Click to expand...
When the case is held against the BF it blows the shoulder out when the case is up against the shoulder it try's to stretch in the wall which causes the issue. I would have thought I'd have light strikes also but they all went bang, I fired about 5 cases before I had a good look and noticed. I went home and measured the BTO of a chambered unfired round, it was indeed pushing it back. I ended up forming false shoulder and that solved my problems. That brass now has 6 firings on it.
 
#23
JR1200W3 said:
Ya, 14 thou is a lot. I'm a little surprised, but not too surprised. I've seen virgin brass from the manufacturer that was 6 thou under. I've in the past accidently over sized brass down 6 or 7 thou past the fired length impatiently setting up a sizing die. I'm sure we've all made that mistake at least once. My original lot of fireformed Dasher brass from 2018 probably has 30 firings on it across 3 barrels. I've never once had a casehead separation. It really makes me think there's a little something else going on that contributes to casehead separation that most of us don't recognize. We just attribute it to case growth in excess headspace conditions.
Click to expand...
My virgin Starline brass for my 1903A4 sits at 1.902” and when fired is 1.903-1.904. Once we put his dad’s bolt in and fired the gun brass was only growing .004” if I remember correctly.
I still have the old bolt and can go back and test things, but the m72 had the same physical signs of imminent case head separation as the Lapua brass did after its first firing as well as similar growth. The m72 fired in the gun after the bolt swap didn’t show the same signs.

If I’m remembering my 1903 info correctly the back of a case on 1903s is unsupported, which is where this case head separation happened. So excessive headspace, a chamber that needed a polish job and an unsupported case could very well do it.

As far as resizing goes I’m pretty OCD when it comes to getting .002”-.003” of bump which is why run the Area419 case holders and have slowly moved my sizing dies to SAC modular dies. So between the .001” sac shims and the 419 case holder bases that go up in .002” increments I get dialed in pretty easily.
 
#24
@JR1200W3
You had near casehead separation on a fired, fireformed case because the bullet slipped in the neck and didn't hold tension? So many questions....
-No they were virgin cases that were being formed, (not sure how you got confused there)

If you were shooting them and they were forming as desired, what would make you think to inspect them?
-They were forming but they were nearly tearing the case apart, see picture. (Not sure where you got confused here either)

If they were firing and forming, how would you know the case jumped forward?
-See Picture of once fired case, the others weren't nearly as noticeable

How would you know that the bullet slipping in the neck is what's causing "near casehead separation"?
-I supposed it's not certain but it's what I deduced when I measured the shorter BTO and rectify the issue with a false shoulder to form cases properly

How do you know you had near casehead separation? You inspected them after? But now you say they haven't separated and have 6 firings on them?
-See picture. The cases that got false shoulder treatment now have 6 firings. those original 5 pieces of brass went in the bin.

IMG_7707.jpg


If you don't want to believe that's fine and isn't really relevant to this thread.
 
#25
JR1200W3 said:
Bamm. That's the issue.

I almost typed this out to explain the difference between casehead separation from excessive clearance between the bolt nose and breech causing unsupported case heads vs casehead separation from excessive headspace. Two slightly different things, related, but sometimes mutually exclusive.

If someone chambers a barrel wrong to the point they have excessive bolt nose clearance which leaves too much of the casehead unsupported, it often times comes with excessive headspace. People talk about excessive headspace causing a gun to blow up. This situation is where they get explosions inside the action. Because the case wall separates and blows out at the breech.

But a barrel can have excessive headspace and also NOT have excessive bolt nose clearance. If someone keeps resizing the same case to excessive headspace... eventually they will have casehead separation inside the chamber where the explosion is still captured safely and they just get a stuck case body.

These two things are related but not always. And I think this is where some of the misconceptions come from.

You're just lucky the case separated when you were sizing it instead of during the act of firing.
Click to expand...
Makes sense, I appreciate the info!

Oh and I found the picture of a piece of brass that made me stop and go to Shane. Just a cunt hair away from completely separating. I got lucky.
 

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#27
JR1200W3 said:
I'll be honest, I don't really understand some of what you're saying. Some of it just doesn't make sense.

You had near casehead separation on a fired, fireformed case because the bullet slipped in the neck and didn't hold tension? So many questions....

If you were shooting them and they were forming as desired, what would make you think to inspect them?

If they were firing and forming, how would you know the case jumped forward?

How would you know that the bullet slipping in the neck is what's causing "near casehead separation"?

How do you know you had near casehead separation? You inspected them after? But now you say they haven't separated and have 6 firings on them?

It just sounds like you're making a lot of assumptions unless it's just something that's easier to explain in person vs on text.
Click to expand...
As I've always understood it, internal ballistics are:

Firing pin pushes the case forward- it'll seat on the shoulder, belt or rim depending on the design
Once the clearance is removed, the primer detonates and the case blows out to form to the chamber.
Case is held in place by 60 some thou of pressure against the chamber wall, this leaves only the gap between the boltface and base of the cartridge- the headspace- which is closed as the brass flows to fill that gap. This also re-seats the primer which was slighty backed-out at ignition.

Case pressure abates and allows for extraction.
 
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