High Pressure Signs? 300WM

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GONE BAD

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Minuteman
May 8, 2013
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#1
What's going on here?
Took my brand new 300WM out to just get some round through it.
Factory Hornady whitetail hunter 150gr . Primers are really flat but no ejector swipes. Also shot 6 handloads, (3)@73grs 7828ssc under 200gr Berger hybrid target, 215 federal magnum primers. Also (3)@ 73.5.
They ALL looked like this.

20210503_182452.jpg


Any suggestions?
Thanks
 
#4
So what should I look for as far as max loads? Ejector swipes, heavy bolt lift?
Disregard flat primers?
Run a magneto speed through COW workup?
My boys both run 300WM's and their primers don't look anything like these.
20210504_084000.jpg
 
#5
I measure the case web right above the belt before firing and after firing then compare the two. Any expansion above .002” indicates pressure.

Physical signs on the case head include ejector/extractor impressions, swipes, cratered primers. Sticky bolt lift is also a typical sign of too much pressure.

Recoil will also be noticeably more pronounced if your load has too much pressure (say compared to a factory load or mild hand load) but typically you see the other stuff first.
 
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#6
GONE BAD said:
So what should I look for as far as max loads? Ejector swipes, heavy bolt lift?
Disregard flat primers?
Run a magneto speed through COW workup?
My boys both run 300WM's and their primers don't look anything like these.View attachment 7622916
Click to expand...
All of the above.........no , don't disregard flat primers......BUT some rifles flatten primers regardless of load, I have one of them . Use magneto speed with ,published load data as a ( somewhat ) guide.......hard bolt lift and ejector marks are a sure sign of high pressure, back off powder if these acure
 
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#8
In the first post, the primers are sorta flat but there isn't a single mark on the base of any of those rounds. After a flat primer, usually next is primer cratered. Then some marks on the base where the ejector will mark it up and along with this I have noticed increased effort to lift the bolt. Back off a little if you are marking up the base or cratering the primer. Flat primer is near the max, no doubt.
Sometimes you will get a pierced primer and that always scares the hell out me!
 
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#9
Also, if you measure the case web growth at .002, it's way hot. Using the Case Head Expansion method of estimating pressure, .0005-.0007 (notice one more decimal place over) is used to estimate 55-65K psi. If you have a chronograph, the velocity is also a good guide.
 
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#11
NH4X said:
All of the above.........no , don't disregard flat primers......BUT some rifles flatten primers regardless of load, I have one of them . Use magneto speed with ,published load data as a ( somewhat ) guide.......hard bolt lift and ejector marks are a sure sign of high pressure, back off powder if these acure
Click to expand...


The funny thing is, there is no catering.
The firing pin strike looks perfect.
I don't know about the factory loads (I've had Hornady factory ammo run pretty stout right out the gate. Like right now with my 6 ARC).

The handloads are a different story and as far as book specs they are low to mid loads depending on which book I use 🤔
 
#13
Flat primers can also happen when there's a little headspace (could be below max allowable) with light loads. The expansion doesn't happen fast enough for the cartridge brass to grab against the chamber walls (or maybe the chamber walls were polished too smooth, or are oiled). Anyway, the primer can still flatten in this case and be giving a false pressure sign.
 
#14
GONE BAD said:
The funny thing is, there is no catering.
The firing pin strike looks perfect.
I don't know about the factory loads (I've had Hornady factory ammo run pretty stout right out the gate. Like right now with my 6 ARC).

The handloads are a different story and as far as book specs they are low to mid loads depending on which book I use 🤔
Click to expand...
Yep I've seen some "hot " whitetail hunter ammo ,too . I just got done looking at load data from Hodgdon .....yep middle load.......I'd strap on the magneto and see what the 73 and 73.5 gr. loads are running.......and go from there..
 
#15
skl1 said:
Also, if you measure the case web growth at .002, it's way hot. Using the Case Head Expansion method of estimating pressure, .0005-.0007 (notice one more decimal place over) is used to estimate 55-65K psi. If you have a chronograph, the velocity is also a good guide.
Click to expand...


Are you saying. 002 more that unfired brass or book number spec?
Fired brass is about .006 less than book
But .002 larger than new unfired brass.
 
#17
GONE BAD said:
Are you saying. 002 more that unfired brass or book number spec?
Fired brass is about .006 less than book
But .002 larger than new unfired brass.
Click to expand...
What he’s talking about would essentially be measured across the belt. You also need to measure each case before its fired, then again after, and you need to be able to measure to the .0001. Also this can only be done on virgin brass. Once its been fired, it cant be measured in this way again and be accurate. If you measure above the belt, you’ll just be measuring how much the case expanded to meet the chamber, minus some springback.
 
#18
skl1 said:
Flat primers can also happen when there's a little headspace (could be below max allowable) with light loads. The expansion doesn't happen fast enough for the cartridge brass to grab against the chamber walls (or maybe the chamber walls were polished too smooth, or are oiled). Anyway, the primer can still flatten in this case and be giving a false pressure sign.
Click to expand...


I've read this before about false pressure sign.

I'm going to clean the chamber real good and make sure there is no lubricants in the chamber.
Thanks
 
#19
918v said:
If your headspace is on the long end of the range then your primers will flatten out more than if it were on the short end. Ejector marks and hard bolt lift are better pressure signs.
Click to expand...

Well I know the headspace from base to belt is .003 and the base to a data line on shoulder is .008
Hmm, to much ?
 
#20
Supersubes said:
What he’s talking about would essentially be measured across the belt. You also need to measure each case before its fired, then again after, and you need to be able to measure to the .0001. Also this can only be done on virgin brass. Once its been fired, it cant be measured in this way again and be accurate. If you measure above the belt, you’ll just be measuring how much the case expanded to meet the chamber, minus some springback.
Click to expand...


Well starting to understand this.
I'll make another trip to the range with the factory ammo and check this out.
Thanks
 
#21
What is the measurement for fired brass and unfired brass? Base to datum line? Usually a 300wm chamber is about .01 or so longer than the shoulder on new brass, because it was designed to headspace off the belt. So a lot times new brass will show some signs like flat primers and light ejector marks with no swipe on the first firing. Once you start headspacing off the shoulder with .001-.004 shoulder bump, the flat primers, and sometimes very light ejector marks, with no swipe. Will go away.

Book numbers are more or less irrelevant when measuring brass. You are best to size the brass for your chamber.
 
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#22
Thanks everyone for all the info.
As of right now no numbers meet or exceed any specs I've read.
So now I will make a trip out again and check some numbers in real time and run with my magneto speed.
 
#23
GONE BAD said:
Well I know the headspace from base to belt is .003 and the base to a data line on shoulder is .008
Hmm, to much ?
Click to expand...

I’m not understanding your numbers.

When the FP hits the primer it drives the case forward until the belt bottoms out against the belt recess in your chamber. The case then expands and hugs the chamber. The primer then backs out and stops against the breechface causing it to mushroom. The case then stretches to the rear and seats over the primer. The longer the headspace the more the primer backs out and mushrooms.

Put some masking tape on the bottom of a new case and add layers until it’s hard to close the bolt. Then measure the difference and that will show you how much the case is moving in the chamber. If it’s truly .003” than you have too little in my opinion. I bet you have a lot more than that.
 
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#24
skl1 said:
Flat primers can also happen when there's a little headspace (could be below max allowable) with light loads. The expansion doesn't happen fast enough for the cartridge brass to grab against the chamber walls (or maybe the chamber walls were polished too smooth, or are oiled). Anyway, the primer can still flatten in this case and be giving a false pressure sign.
Click to expand...

That is incorrect.
 
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#26
918v said:
I’m not understanding your numbers.

When the FP hits the primer it drives the case forward until the belt bottoms out against the belt recess in your chamber. The case then expands and hugs the chamber. The primer then backs out and stops against the breechface causing it to mushroom. The case then stretches to the rear and seats over the primer. The longer the headspace the more the primer backs out and mushrooms.

Put some masking tape on the bottom of a new case and add layers until it’s hard to close the bolt. Then measure the difference and that will show you how much the case is moving in the chamber. If it’s truly .003” than you have too little in my opinion. I bet you have a lot more than that.
Click to expand...

Bolt closes on 1 piece of tape .
Bolt will not close on 2.
Not masking tape.
Clear packing tape .
2 layers = .004
So more than .002 <.004. I think .003
 
#28
I will load a few loads starting at mim powder charge and see what happens.
The belt makes this very confusing.
I will just neck size these 26 cases as they chamber fine. Probably 1 or 2 thousands short of overall chamber length and go to a regular LRP

In reality the wm case is no different than a rimmed case. So the bolt face could have contact with cartridge base but overall length could be anything within reason. No?
 
#29
After you fire the brass in your chamber, you can do a partial resize. What I mean by that is more than just neck size, just enough to bump the shoulder back 0.002". Ignore that dumb belt after those rounds have been formed to your chamber and treat the case as you would any other non-belted case. It will cause you other problems a little later on in your case life.
I have had 300WINMAG for about 40 years and have been reloading for it about that long. For about 25 of those years, I loaded light and fast. I would rarely get to 5 reloadings on a piece of brass. About 15 years ago, I really started paying attention to BC and moved to the heavies. I only shoot the 210 VLD now, in both the target and the hunting variety. That 210 leaves the barrel about 2950 as compared to the 150-170's leaving near 3250. At 400 yards, the 210 has retained more of that velocity!! Case life also increased.
About 10 or so years ago, I found out about these: https://www.larrywillis.com/ I recommend you get one. I use it after 3 reloadings. I have brass with 10-12 reloads on them now!
 
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#30
Also to be honest, there may have been oil in the chamber that I did not remove as I left some in the bore because I didn't think I would be shooting it for quite some time!
I never leave lubricant in my barrels.
May have this time?
 
#31
GONE BAD said:
I will load a few loads starting at mim powder charge and see what happens.
The belt makes this very confusing.
I will just neck size these 26 cases as they chamber fine. Probably 1 or 2 thousands short of overall chamber length and go to a regular LRP

In reality the wm case is no different than a rimmed case. So the bolt face could have contact with cartridge base but overall length could be anything within reason. No?
Click to expand...

If you bump the shoulder too much you’ll reduce case life jus like with any other bottle neck cartridge. Bump .002-003” and you’ll be fine.

I suspect your friends rifles have more generous chambers. Yours allows for .014” shoulder growth. Mine .025”. Also if you measure their base diameter vs yours, bet theirs will be bigger. I’ve seen some tight WM chambers whose cases drop right into mine without sizing.
 
#32
GONE BAD said:
Also to be honest, there may have been oil in the chamber that I did not remove as I left some in the bore because I didn't think I would be shooting it for quite some time!
I never leave lubricant in my barrels.
May have this time?
Click to expand...

That would not cause flat primers on all the brass. Did the cases eject oily?
 
Last edited:
#38
Looks like a healthy charge towards the top end. I wouldn't go much past that, but wouldn't have reservations about shooting it, either.

Primer crater, Pierced primer, primer blow-by (black ring around the primer), ejector swipes, stiff primary extraction (bolt lift), loose primer pockets, and swelling in the extractor groove are pretty solid indicators to look for.

Primers that have no visible corner that smoosh flat on the head are also no bueno. Yours definitely have a lot of the radius taken out, but I'd bet they pressure test fine.

Soft, warmish, HOT
1620774777223.png
 
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#39
918v said:
That is incorrect.
Click to expand...
918v,

Don't want to be in a urinating contest with you, but you can get flattened primers with a little headspace at light/normal pressures. This might not be the case for the OP, but this does happen. More often with softer primers. I thought maybe I'd picked up an internet legend and made it a fact in my mind (happens sometimes), but they talk about it in the Speer manual section on reading pressure signs.

I may have misunderstood what you were pointing out as incorrect, and if so, I apologize.
 
#40
skl1 said:
918v,

Don't want to be in a urinating contest with you, but you can get flattened primers with a little headspace at light/normal pressures. This might not be the case for the OP, but this does happen. More often with softer primers. I thought maybe I'd picked up an internet legend and made it a fact in my mind (happens sometimes), but they talk about it in the Speer manual section on reading pressure signs.

I may have misunderstood what you were pointing out as incorrect, and if so, I apologize.
Click to expand...

A little headspace meaning too much clearance or too little?

Oil in the chamber will get mopped out within three rounds fired. All his cases had flattened primers although in my opinion they were not too flat.
 
#41
It has headspace, meaning clearance (maybe within spec or maybe too much), which allows the primer to back out and then get smashed. Leaving it looking flat (though generally not super flat with low pressure). But as you say, the OP's primers don't look super flat.

Agreed that oil wouldn't likely affect more that a few shots.

Anyway, no harm no foul here. Hope there's no offense taken. Thanks for trying to help OP.
 
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#42
Ledzep said:
Looks like a healthy charge towards the top end. I wouldn't go much past that, but wouldn't have reservations about shooting it, either.

Primer crater, Pierced primer, primer blow-by (black ring around the primer), ejector swipes, stiff primary extraction (bolt lift), loose primer pockets, and swelling in the extractor groove are pretty solid indicators to look for.

Primers that have no visible corner that smoosh flat on the head are also no bueno. Yours definitely have a lot of the radius taken out, but I'd bet they pressure test fine.

Soft, warmish, HOT
View attachment 7623406
Click to expand...
Good pics of primers
AF8B574A-E8C3-47D6-A008-4D7B8351E47D.jpeg


Reading primers is helpful but they will lie to you.
Combined with brass and extraction you can get a pretty good picture.
 
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#43
skl1 said:
It has headspace, meaning clearance (maybe within spec or maybe too much), which allows the primer to back out and then get smashed. Leaving it looking flat (though generally not super flat with low pressure). But as you say, the OP's primers don't look super flat.

Agreed that oil wouldn't likely affect more that a few shots.

Anyway, no harm no foul here. Hope there's no offense taken. Thanks for trying to help OP.
Click to expand...

You’re way too kind.
 
#45
Ledzep, I'm starting to think that too.
I'll drop back to starting load and see how things look.
The problem is that it wasn't just my loads, it was the factory ammo too.

I do know Hornady can be a little warm sometimes too.

I'm not afraid to shoot it but if that is max for this gun then I'll just have to start with a lot lower charges.
 
#46
Like I said, I wouldn't sweat it. It looks like you're "there" but not over the line IMO. I have had 6.5 Creedmoors that have primers that look just like that well within published load data and the velocities matched up with the book, which is usually a pretty good indication that pressure is in line with reality. No worries.

Maybe softer primer cups, maybe a little bit of head space etc... The corner radius is a good thing to watch, but isn't the end-all-be-all for excessive pressure. Craters are kind of the same way. M700's are almost guaranteed to have them unless the bolt is bushed. Other guns don't have them until you get hot. Just depends.
 
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#48
Thanks to all that shared imput.
I'm going to start load development down a couple of grains and see where that goes. With supplies being what they are its kinda the worst time to do this.
Especially when you're limited to less options .
Anyway I'll carry on. Thanks
 
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#49
GONE BAD said:
Thanks to all that shared imput.
I'm going to start load development down a couple of grains and see where that goes. With supplies being what they are its kinda the worst time to do this.
Especially when you're limited to less options .
Anyway I'll carry on. Thanks
Click to expand...
Maybe I read the thread fast but no mention of a short throat? Would explain high pressure signs with most normal loads. Someone trying to get a few extra chambers without changing reamers
 
#50
Bio said:
Maybe I read the thread fast but no mention of a short throat? Would explain high pressure signs with most normal loads. Someone trying to get a few extra chambers without changing reamers
Click to expand...

Yeah, could be but the case mouths seemed to look normal 🤔

The cases do seem to grow quite a bit.
I'll watch for this . If it is something along that line I'm sure it will show up at a minimum charge?
 
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