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Pressure question?

 
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  #1  
Old 12-27-2006, 05:44 PM
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Pressure question?

Got a question about what is happening during load developement of my 300SAUM Sendero. The powder that shows great accuracy out of my factory rifle is Varget, shooting in the .6's or better with 59, 59.5 and 60 grains.

My question is that I am seeing some very slight cratering in the primers with 59.5 and 60 grains, not every shot, but usually on the 3rd shot of a 3 shot group, each shot fired back to back. No sticky bolt, or flattened primers. The first shot, from a cool bore, shows no sighn of pressure(no cratering) the second shows a slight amount, and the 3rd shows slightly more. Why does it do this? (chamber heat?)

This gun will only be used for whitetail hunting, where it will be shot seldom, and from a cool barrel. Will the slight cratering, with no other pressure sighns be okay? I also had this problem with a 7MM Ultra Mag, sent the bolt off to Holland and had the bolt bushined and firing pin turned to match. This solved the problem on the 7MMUM.

Thanks
RidgeRooster
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  #2  
Old 12-27-2006, 08:17 PM
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Re: Pressure question?

My first bet would be that their is tyo much clearance around the firing pin , the Holland replacement pin would probably fix that problem.

excessive pressure might be caused from chamber heat if you using a very temp sensitive powder and in that case any loads you work up in the winter will be hot in the warmer weather.
BUT , in you case you say that your not getting any flattened primers along with the cratering which leads me back to thinking that its a sloppy firing pin fit.
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Old 12-28-2006, 04:35 AM
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Re: Pressure question?

Thanks for the reply. I also suspect a sloppy firing pin fit, I had the same problem with my 7MMUM.
Do you think that shooting this gun, if this is the problem, will hurt anything?

RidgeRooster
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  #4  
Old 12-28-2006, 09:16 AM
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Re: Pressure question?

What JDJones said!

I always have a bit of a ridge around the firing pin on all of my rifles but one (military mauser). On that one I flatten the primers to fill the bevel around the firing pin hole. On the others there's no ridge around the edge of the primer when removed.

You should have no problem at all. Bolt lift and extraction resistance would be a bit of a problem causing reduced case life.
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  #5  
Old 12-28-2006, 10:07 AM
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Re: Pressure question?

No its not realy a safety issue unless your firing pin is over protruding in which case you might pierce a primer if they are soft but than can happen with any firing pin it it sticks out to far. A pierced primer will scare the hell out of you and probably get you face full of gas and grit (another reason to wear glasses!!) but generaly no serious damaged is caused to you or your gun.

This whole topic brings out some very good points that alot of guys should pay attention to especialy with a custom gun.
When a gun is built correctly , everything is very tight and true in most case the max allowable run out is.0002". Your chamber is likly cut glass smooth and a bit tighter than factory , the bolt and barrel mate the case perfectly , the firing pin has next to no gap to allow primer flow(cratering). All these things make seeing the general pressure signs a bit harder to detect , it'll take a good bit more pressure to cause sticky bolt lift , you primers may not crater at all only flatten out and some primeres are alot harder than others and won't flatten till the pressure is WAY to high.
So this brings us to our good friend the chrongraph , if your running loads out of you gun that are higher than the listed specs in load books then your probably in the high pressure range , if they are alot faster then your flirting with dissaster. Thats why I reccomend the chronograph and case expansion and life as the best pressure signs. Builders like Kirby that desgine their own rounds and don't realy know what kind of velocity to expect due to no previous data are in my oppinion very brave and I'm sure that they take their sweet time working up their loads an=d spend alot of time with a mic checking things.
Alot of folks don't relize that once your running a gun in the 65,000-70,000 psi range that it takes very little powder or change in seating depth to make a giant jump in pressure which can cost you your life or at a minimum some eye site and face. Anybody that ahs seen the pics of a gun that was over stressed and come apart knows what I'm talking about. I have seen a custom built Rem 700 in
22-250 , shoot great with a load that was well above max actualy out running some Swift loads and worked fine in the 40 deg spring but when it was shot the next time it was 90+ degs and the first round galled the bolt into the action and the reciever cracked length wise accross the top bewteen the scopebase holes. I would have liked to seen that load shot accross the chronograph just get an idea of the pressure.

Sorry to jack your post RidgeRooster , I just figured that this might pretain here
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  #6  
Old 12-28-2006, 10:43 AM
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Re: Pressure question?

If all other signs are OK, then don't worry.

I have a new Sendero-II and the primers show "gap cratering", which is different than pressure cratering.

Pressure cratering comes from the pressure pushing the pin back and pushing primer metal into the hole. This type is identified by sharp walls on the outside of the crater, and a sharp feeling as you draw your finger across the primer.

Gap cratering is caused by a firing pin hole that is much larger than the pin itself, so when the pin is down in the fired position, there is a ring of space around the pin... or "Gap".

When the round is fired, the pressure forces the metal into this space, but does NOT puch the firing pin back.

This kind of cratering is typified by craters that have a "soft" look, like the metal melted into the space... there are rarely sharp edges, and they appear even with light loads.

My new Sendero has this, plus two new XR-100s showed the same signes... even with light loads.

It is because Rem is drilling the bolt faces with drills that are too large.

You can have the bolt "bushed and redrilled" for a better fit, and I did that on one of the guns, but it was a waste of money ($70) and time... he didn't do a clean job, and I picked up 5 thou of head space in the process [img]/ubbthreads/images/graemlins/frown.gif[/img] [img]/ubbthreads/images/graemlins/frown.gif[/img].
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  #7  
Old 12-28-2006, 03:25 PM
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Re: Pressure question?

Thanks y'all!

RidgeRooster
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