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Internal ballistics frustration.

 
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  #8  
Old 12-21-2006, 07:21 PM
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Re: Internal ballistics frustration.

Take a fired and a sized case and lay them on their sides on a flat table. Roll each and see if either is lopsided. Different test for same issue - take two fired and set them on a table on their bases so the bases touch. slowly rotate one and see if the distance between the shoulders changes. Do the same with two sized cases.

Now then if you wish to be more precise, set up your runout gauge to check for the runout of the base.

All of this is to check whether your chamber is aligned and your bolt face and lugs are bearing properly. If they are not then you wind up with a gun that fires new brass fine but then your dies cannot get the bases squared up and you wind up with a goofy headspace/ misalignment problem.

Your problem sounds similar ( but not exactly) to the problem I experience with the one lug Savage 340 and high pressure loads. Bolt flexes and I get warped bases on the brass.
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Old 12-21-2006, 07:27 PM
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Re: Internal ballistics frustration.

Thanks BB,

I will look at them further for squarness. Knowing my smith pretty well though, and the work he does, I highly doubt anything is not square. Unless of course a problem developed scince its being built.
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  #10  
Old 12-21-2006, 09:16 PM
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Re: Internal ballistics frustration.

Michael,

I have that same problem with my 300 Win Mag. You probably running your load pretty hot. Did you try different brass?. In the case of my 300 which is a factory rifle and sloppy chambering to both, I can only reload the brass 3 times before experiencing difficulty in chambering the round. I only necksized the brass though because if I do fullsizing, the brass were so streached out that the shoulder will collapsed and ruined the brass. In either case, I can't use the brass anyway. The brass is Federal Gold Medal. I now switched to Winchester and I'm able to reused the brass up to 6 times before discarding them. The rifle is very accurate so I leave it alone. In your case, I suspect that the culprit is the shoulder. Try partial resizing only using full lenght sizing die and see what happen.
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Old 12-21-2006, 10:19 PM
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Re: Internal ballistics frustration.

[ QUOTE ]
Not sure how to officially check that, but they appear to be OK.

I may have figured out at least part of this problem. Tell me what your opinions are of this.

New case from base to shoulder (using a comparator) is 3.990" Fired cases are 3.993" I had been bumping the shoulder back to 3.988" Do you think that has anything to do with this? Should I be setting the shoulder back any at all or less than I have been? I have already reset my die to bump it back only 0.001". Will see if that helps any.

[/ QUOTE ]


Sorry to ask the obvious,but are the cases within the trim to length spec?
Bumping the shoulder back .005" does sound excessive.With the gun Im shooting I usually bump the shoulder back .001 after every other firing.I turn also and have checked my neck wall thickness after firing the cases six times with no increase in the thickness and these are RP cases.The only dimension Ive had increase is the OAL of the case itself.Be interesting to see what you find.
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Old 12-21-2006, 11:45 PM
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Re: Internal ballistics frustration.

[ QUOTE ]
are the cases within the trim to length spec?


[/ QUOTE ]

That is a good question. Yes. I also run them through a shoulder style trimmer during the prep for each re-load.

Also, without using a runout guage, the case bases appear to be square.
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  #13  
Old 12-21-2006, 11:51 PM
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Re: Internal ballistics frustration.

Initial tho't was a "carbon ring"??

Case stretching? Will fired round accept a bullet?

Strange. Keep us posted, ok.....
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  #14  
Old 12-22-2006, 12:56 AM
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Re: Internal ballistics frustration.

Meichele,

Several things come to mine.

First off, is this a standard spec 300 RUM chamber meaning is it consistant with the standard 300 RUM case dimensions. I realize it may be tighter but has there been any modifications to shoulder diameter in any way, even slight?

Second, This is in no way a shot at maker of the rifle, just something else to check out. Take a fired case and hold it up to a light and look down the body of the case. Is it perfectly straight or can you see any expansion bands around the body of the case.

Anytime you chamber a barrel to a min taper design you run the risk if turning a chip and ringing the chamber. This can vary from extremely mild rings that are generally polished out in the final chamber polish to more siginificant rings that can be seen of you hold the case up and look down the line of the case body.

This happens from time to time even if you do your best to prevent this it can happen with a min taper chamber design. The 300 RUM has a bit of taper but not alot.

So your probably wondering why this would cause this. If you have a ring in the chamber it would do have sticky extraction from the very first shot and with all shots right, not nessecarily.

The reason is because on the first shot, the brass is very elastic, it stretches under the chamber pressure but as soon as the pressure is released the case will contract back down a significant amount allowing the case to eject easily.

That said, the heat from that first firing has begun to temper the brass case and it is now not as elastic as it was on the first firing. On the next loading, the case expands again under pressure but this time does not contract down as much as the first firing, bolt lift can get a bit stiffer and extraction can get a bit sticker.

THe case now has become even more tempered and has lost more of its elastic properties because of the heat from firing and work hardening from FL sizing.

You load it again and shoot and this time it expands but does not contract back down and the case is basically stuck in the chamber and needs to be tapped out.

Another situation that can cause this is if the bore is not polished properly. Again this is due to the same reasons as the above example. The case gets harder and does not contract as easily and with a rough chamber it gets tight to extract.

One more area to look at is the diameter of the brass head in comparision to your chamber. In a min spec chamber, many times you will find a lot of brass that is a couple thou larger in diameter at the head then the average and when you chamber those rounds, the first couple may work alright but as the case head expands slightly under pressure, it gets larger in diameter and can wedge in the chamber if the fit is to tight.

I have encounters your problem with my Allen Magnums when building the first few test rifles. Several things were the problem, main one was to little taper in case design which resulted in modifying the reamer to correct the problem. Also, a ring in the chamber, Yes I did this until I learned that I really needed to take alot of time and take very shallow reamer cuts between cleaning to prevent this.

Some will polish a chamber with 400 grit paper. I do not feel this is enough for a large diameter, long case such as the 300 RUM. I go to at least 600 grit polish. The larger surface area of the case and chamber require a finer finish for easy extraction, especially with a min taper case design.

Just some more things to look at.

It does seem odd that you need to set the shoulder back that much to chamber easily with a sized case. Do you have even bolt lug contact and has your receiver been accurized and trued?

Let us know, what you come up with, very intersting.

Kirby Allen(50)
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