flat/blown primer within recommended specs

[mk443]

I have been working out a load for my rifle and when I go by my manual/hodgen specs and get in the mid/high range I'm flattening primers and at the high range I actaully blew a primer. Here is what is recommended:
7mm STW, 180 berger VLD, Retumbo, 24 in barrel, 1-10 twist, federal magnum primer, brass 2.840
COAL=3.655 Grs: 69.3-73.8

Here is my set up:
7mm STW 180 berger Hybrid, Retumbo, 27.5 in barrel, 1-9 twist, winchester magnum primer, brass 2.840
COAL= 3.91 just off the lands,

69-72 grains shoots great with 71 shooting very very well in a test group. 73 grains had slight flattening on the primers, a 74 gr load blew the primer.

Could the longer COAL and longer barrel and faster twist be causing the additional pressure? I'm planning on running the 71 gr load through the chrono to see what speeds its pushing out but to be honest I thought I would be able to push the load a little hotter since recommended specs usually are set lower. Any input would be great.


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[Barrelnut]

Could the longer COAL and longer barrel and faster twist be causing the additional pressure?

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Yes.

 

[Unofficial Gun Addict (UGA)]

Interesting issue and one not to take lightly. I was going to recommend chronoing, but you've already decided to do it which is a good thing. Another thing I would highly recommend is backing off your powder charge, as well as backing up your bullet. Some guns are fine with bullets jammed into the lands... others aren't, and extreme pressure spikes are one symptom of a gun that doesn't like bullets jammed into the lands. I'd back off .015 to .020 from the lands... and I'd make **** sure that my chamber measurements were spot on. I'd double and triple check using a overall length gauge, and a bullet in a dummy case with the neck just tight enough to hold the bullet tight, but enough so when you chamber it, it can be pushed back into the neck. Then I'd compare my gauge results with the bullet results.

By backing off the bullet and charge, you can then see what kind of primer response you are getting, as well as velocity which you can the compare against your load data & manual.

Be interested to hear back on how this goes or you. This might sound lame... because you likely already do this, but eye protection would be highly advised.

 

[mk443]

I agree that the chrono will give me a lot of info. I plan on shortening the length slightly and see if that has anything to do with the pressure. I removed my firing pin assembly last week and tried this method (see video) to determine when I was touching the lands and it worked great:

[ame=http://www.youtube.com/watch?v=Jv-D1mEI514&list=PLVuBaIw3GRtRmExIMmSTREXxn7_sg40aU]Bullet Seating Depth Method - YouTube[/ame]

As I said, 71 grains was right in the recommended zone and shot a great group with no pressure signs on the brass, so naturally I'm going to go down from there till I get it fine tuned. I was also told when I talked to Berger last week and they said the 180 Hybrids are so long and due to the shape have a lot more the bullet making contact with the rifling so they do create more back pressure, which is why I started low. I'm mainly just curious what the speed will be at this lower charge. Time will tell.

 

[RT2506]

Did not see any mention of what rifle you are shooting. If it is a custom chambered rifle most likely it is a nice and tight chamber and that is the reason for the high pressure. You will probably be getting the velocities of heaver loads.

 

[FearNoWind]

Recommended:
7mm STW, 180 berger VLD, Retumbo, 24 in barrel, 1-10 twist, federal magnum primer, brass 2.840
COAL=3.655 Grs: 69.3-73.8

My set up:
7mm STW 180 berger Hybrid, Retumbo, 27.5 in barrel, 1-9 twist, winchester magnum primer, brass 2.840
COAL= 3.91 just off the lands,

69-72 grains shoots great with 71 shooting very very well in a test group. 73 grains had slight flattening on the primers, a 74 gr load blew the primer.

Could the longer COAL and longer barrel and faster twist be causing the additional pressure?
gun)

There are more variables in your load than just COAL. The test data in the manual was not recorded with the bullet touching the lands. The absence of any jump in the OAL means the test data in the manual is invalid for your loading purposes. Also consider that the tests rifle(s) were most likely strict SAMMI factory specs. When we build our own rifles we sometimes play with the chambering specs - or so I'm told. A little tighter neck, some shoulder adjustments, ???
The increased twist rate will also contribute to higher pressures.
The Berger bullets have greater surface engagement with the rifling than many other bullets so it's difficult to compare the Berger 180 VLD (which is what I use in my .284) with other bullets of the same caliber.
As an added note, the Chrono won't tell you anything except muzzle velocity and the factors generally associated with MV (ES/SD/etc.) so while it may help you decide how efficient your loads are it can't do much else for you.
I'd suggest you back down the load, run the Chrono tests as you build the load and find the efficient node for your rifle. Then stop.
Pushing the bullet faster isn't necessarily a good thing, and the 7STW is notorious for short lived barrels. Great cartridge and deadly accurate, but a chamber throat eater-upper.

 

[mk443]

If weather holds my plan is to go down to 67 gr and work up to 71. I agree that the crono won't give me much info but it will let me know if I need to re think my bullet selection due to so many pressure variables. For what my shoulder goes through I don't want a slow shooting STW. The rifle is a build I've been planning for 5 years now (first full custom). Its a remington 700 action (handed down family action) that was machined out for the Wyatt center feed box for 4" rounds. I have a Benchmark #4 fluted that I got on here from chris in the group buy finished at 27.5. Its all sitting in a mcmillan game scout edge fill with ADL floorplate. My smith did a great job bedding the Wyatt to accomidate for the ADL. I have a rifle basix trigger set a 1.5 and a set of talley rings with a mark 4 4.5-14x50. I wanted a vertical grip, longer barreled rifle that didn't weigh as much as a normal match rifle.

 

[rooster740]

Could be a weak firing pin spring if everything else seams normal. I had this problem and it darn near drove me crazy!

 

[FearNoWind]

Remington 700 action
Benchmark #4 fluted 27.5
McMillan game scout
Rifle basix trigger
Talley rings
Mark 4 4.5-14x50

Reads more like a tactical rifle than a hunting rifle but I sure like the combination you've chosen. Not that I would hesitate to take it hunting ....

 

[Bravo 4]

What chu talkin bout Willis? That has hunting rifle all over it.

OP,
That is why you are suggested to start at the low end, each rifle can top out differently. My 7 mag couldn't run top book loads, it's loads were rather mild compared to the other two 7 mags I load for by several grains of powder. Some people say the manuals are rather anemic for legal reasons and such. I say maybe, but all it takes is that one rifle (like mine) to blow up in your face and everybody wants to start pointing fingers. When the problem is usually operator error.
Good luck.

 

[mk443]

Went to the range this morning and found that right at 70.5gr of Retumbo I shot two different 4 shot groups in the .4 range. I chrono'd the load at 2850 which seems a little on the slower end. I ran the 72 gr loads through I was in the 2920 range. I keep reading people running my set up in the 3100-3200 range but I don't see that be obtainable for me.

 

[FearNoWind]

My competition rifle isn't an STW, but it's a good shootin' .284 Winchester so we're in the same park. I shoot 180 Berger VLDs at 2750fps and, if I do my job right, it'll hold its own remarkably well at 1K with proper scope dope. Looks to me like you've got a good gun there. IMO pushing MV just for the sake of bigger numbers is a waste of time. If you can't find an accurate node above where you are then stay with what you have. It'll work out just fine for LRH.

 

[pods8]

That is why you are suggested to start at the low end, each rifle can top out differently.

Also the obvious I don't think was mentioned in my skimming, all components have variations from batch to batch. Even once you find a load combo that works well in your gun when you get the next lot of powder or such it may behave differently.

Chrono's are one clue to the puzzle when components change (if it was shooting 3000fps consistently before and now the new powder batch is 3050fps and not grouping well you may need to back down the charge to see if a new load at 3000fps groups well again, etc.).

Generally this is why folks like enough powder/primer/bullets/brass from the same lot for their foreseeable needs so once the load is dialed they can just run with it for a long while.

 

[Unofficial Gun Addict (UGA)]

Jeez.... If your shooting .4 I'd make a note of that load and hold onto it. If you crave more velocity and this configuration doesn't seem to get you there without pressure issues or accuracy, trying changing one thing and see what impact you get. I'd go primers first... See what happens...good or bad. Then powder. Even cases can make a difference so you could try another manufactures cases.

You could be chasing this for a while... So be prepared to spend some coin trying to get this dialed the way you want.

Personally... .4 groups is nothing to sneeze at and if you have a good BC bullet... You can put some serious hurt a long ways downrange as is.