300WM - Factory ammo "overpressure" signs

[dsmhero]

Overview: Doing load development for Seekins Havak Pro Hunter PH2 in 300WM, brand new with maybe 60 rounds of factory ammo through it. Rifle owner gave me the once shot brass after he zeroed it. I noticed the once shot brass was quite a bit more difficult to resize than I am use to, to the point were I was getting small brass shavings near the belt. Never had this issue with my 300WM once shot brass.

Range: Did a work up of H4831SC from 68.7 to 70.5 with Nosler 200AB. At 68.9 had partial case head separation and a barely flattened primer @ 2752 fps. I immediately stopped there. I shot a factory Hornady ELD-X 200gr just to check since I've never shot this rifle before. Primer was extremely flat and a nice shiny ring above the belt. This leads me to believe the headspace is off by quite a bit, but wanted to double check with people more experienced than I.

Measurements:
Hornady ELD-X 200gr unfired factory round
Datam at shoulder: 2.256
Diameter just above belt: 0.506

Once shot
Datam at shoulder: 2.274
Diameter just above belt: 0.516

I attached the pictures of the once fired FACTORY round.

Any thoughts and advice? I appreciate your time.

 

[nt7332]

I would venture to guess that there is a nice carbon ring built up inducing pressure on factory rounds. I would clean the crap out of the throat area and full bore clean and try it again with factory ammo to set a good baseline.

 

[dsmhero]

I know the owner cleaned the barrel several times within those first 60 rounds. I have a bore scope, I'll see if there is anything noticeable.

I would venture to guess that there is a nice carbon ring built up inducing pressure on factory rounds. I would clean the crap out of the throat area and full bore clean and try it again with factory ammo to set a good baseline.

 

[ShtrRdy]

The 0.516" above the belt sounds rather large. I'll take a look at the SAAMI drawing and update this post.

Later ...... the chamber drawing says a MAX diameter of 0.5156" just above the belt.

 

[L.Sherm]

Don't be surprised if the factory Hornandy ammo is over pressure.
I had a guy bring me his 300WM with Hornandy 180 Precision ammo because it quite shooting.
Anyway to make a long story short the gun was fine it was the ammo when I chronoed it they were anywhere from 3000-3125fps then the guy said well the other day when I was shooting I got some that had hard bolt lift

 

[cdherman]

My only question to add would be: Are you SURE the brass he gave you is really once shot? Belted mag cases don't really resize well, all the way to the belt, and so they tend to display the event you describe (brass shavings) when they get too swollen. There is a special die that someone makes to fix this, and I have one, but the name escapes me....

Anyhow, it *could* be that your gun is fine, but the brass you were given has been fired a bunch more than the prior owner admitted to. FL resizing belted magnum cases will result in brass stretch in most guns and case head separation eventually too.

There's a reason for the love affair with all the non-belted magnums these days. I have yet to play with the collett resizing die that allows full body resizing of a belted case.

 

[Dean2]

First off, try a different Factory load in the gun. Hornady ammo being too hot is nothing new. Second, work up your load with new unfired brass and see what you get. Third, belted mag cases don't take well to being full length resized. Fired cases will chamber easily so neck size them and call it good. When you FL size, all that brass you squeeze back down has to go some place. Part of it grows the neck, thus the need for regular triming but some also gets pushed back towards the belt, eventually causing a donut just above the belt that makes the case hard to impossible to chamber unless you get the Larry Willis collet die. http://larrywillis.com/ This die works well but it is solving a problem you don't really need to have if you stick to neck sizing and use the brass only in one gun.

 

[MTbackwoods]

Are you resizing the brass back to virgin dimension? If so, it's not necessary. I FL size but I only do a .002" shoulder bump and I have zero issues

 

[SMK1000plus]

It seems your Hornady factory load brass is @ 0.18" (18 thousandths) under your datum measurement of your once-fired brass. Your once-fired brass datum is right-on for the SAMMI spec of 2.276" (2.196" to shoulder/body junction, plus 2.356" to shoulder/neck junction, equals 4.522", then ÷ 2 equals 2.276" to center/datum).

0.018" is a LOT of case stretch! Most of my 300 Win Mag. factory ammunition runs @ 0.004 to 0.006" under to the 2.276" datum of my once fired brass. It makes sense that it would show up just in front of the case belt, as the belt is actually setting the headspace and holds the case head end relatively stationary. The brass must flow forward to reach the chamber shoulder and the powder column would cause the brass to stretch behind it, that micro-second of initial ignition.

Try another Lot# ammunition or a different brand.

 

[entoptics]

0.018" of case stretch is EXTREME. Also, 0.516 above the belt is quite large, and none of the 4 or 5 belted magnum chambers I have dealt with, leave them that large, even with several firings of fairly saucy loads.

As mentioned above, that 0.018 is being taken up right in front of the belt, and will thin the cases significantly. Was the "60 rounds of factory ammo" a different lot/type? If so, it's kinda unlikely there were two bad lots, and instead, the rifle has an overly long, and perhaps fat, chamber.

 

[Dean2]

0.018" of case stretch is EXTREME. Also, 0.516 above the belt is quite large, and none of the 4 or 5 belted magnum chambers I have dealt with, leave them that large, even with several firings of fairly saucy loads.

As mentioned above, that 0.018 is being taken up right in front of the belt, and will thin the cases significantly. Was the "60 rounds of factory ammo" a different lot/type? If so, it's kinda unlikely there were two bad lots, and instead, the rifle has an overly long, and perhaps fat, chamber.

You could be right about the chamber, but firing three rounds of new brass with a load 1 grain above min will give him a perfect chamber cast. Once he measures those it will be easy to tell if it is chamber/headspace or too hot a factory load. I always figure, work from the simplest to the more complicated. Far more likely to be hot loads than a bad chamber on a fairly expensive semi custom rifle. That said, current measurements and the picture of the once fired brass does show some pretty big expansion and bad stretch on the case just ahead of the belt..

 

[entoptics]

You could be right about the chamber, but firing three rounds of new brass with a load 1 grain above min will give him a perfect chamber cast. Once he measures those it will be easy to tell if it is chamber/headspace or too hot a factory load. I always figure, work from the simplest to the more complicated. Far more likely to be hot loads than a bad chamber on a fairly expensive semi custom rifle. That said, current measurements and the picture of the once fired brass does show some pretty big expansion and bad stretch on the case just ahead of the belt..

I was under the impression this is not his rifle. Buying 3 pieces of new brass isn't generally an option, and unless I'm mistaken, he already has a "perfect" chamber cast. A hot load isn't gonna expand to bigger than the chamber (or if it does, there's usually some subtle signs of it, like searing hot gasses, shrapnel, blood, etc).

Anyway, in my experience, the "simplest" is what the OP already has. A piece of new brass expanded 0.018", had a pancaked primer, and incipient case head separation. To me, that's indicates that when things hit 60,000 psi or so, the front of that brass went forward more than it should, and the back went backward more than it should.

I suppose it is possible that the Hornady factory ammo is bad, with an incredibly short belt to shoulder, leading to a "false" sense of excess headspace, but you could figure that out by measuring some other new brass, without the need to fire 3 pieces of it 1 grain above minimum.

 

[dsmhero]

My only question to add would be: Are you SURE the brass he gave you is really once shot? Belted mag cases don't really resize well, all the way to the belt, and so they tend to display the event you describe (brass shavings) when they get too swollen. There is a special die that someone makes to fix this, and I have one, but the name escapes me....

Anyhow, it *could* be that your gun is fine, but the brass you were given has been fired a bunch more than the prior owner admitted to. FL resizing belted magnum cases will result in brass stretch in most guns and case head separation eventually too.

There's a reason for the love affair with all the non-belted magnums these days. I have yet to play with the collett resizing die that allows full body resizing of a belted case.

Yes I am positive the brass was only once shot, still kept in the original boxes.

Are you resizing the brass back to virgin dimension? If so, it's not necessary. I FL size but I only do a .002" shoulder bump and I have zero issues

That is my exact process as well, with no issues with my other 300WM rifle.

It seems your Hornady factory load brass is @ 0.18" (18 thousandths) under your datum measurement of your once-fired brass. Your once-fired brass datum is right-on for the SAMMI spec of 2.276" (2.196" to shoulder/body junction, plus 2.356" to shoulder/neck junction, equals 4.522", then ÷ 2 equals 2.276" to center/datum).

0.018" is a LOT of case stretch! Most of my 300 Win Mag. factory ammunition runs @ 0.004 to 0.006" under to the 2.276" datum of my once fired brass. It makes sense that it would show up just in front of the case belt, as the belt is actually setting the headspace and holds the case head end relatively stationary. The brass must flow forward to reach the chamber shoulder and the powder column would cause the brass to stretch behind it, that micro-second of initial ignition.

Try another Lot# ammunition or a different brand.

I will take a look at the lot numbers since the owner gave me the brass still in the original boxes. I will look into getting a different brand of factory to double check. I also have fresh Hornady brass I could use as well.

I appreciate all the help and responses. I will keep you updated.

 

[Beluebow]

Fat chamber with excessive headspace is my guess.......find a gunsmith with a go/no gauge.