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.260 load pressure problem

 
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  #1  
Old 02-07-2011, 01:29 AM
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Join Date: Aug 2007
Location: Auckland
Posts: 18
.260 load pressure problem

Just thought I'd throw this one out to "the floor" as it were to get everyone elses opinion.

Due to unfortunate work commitments I had to send my custom .260 (shilen 1-8" twist on trued rem 700 in Hunters edge stock) to a local load development "specialist" to have a long range load worked out. A while later the rifle came back with the report that it was shooting 1/2 MOA with 140gn A-Max's.

Great! I take it hunting at the next opportunity and whack a deer with it. Problem is the pressure was so high that the brass extruded into the ejector hole and when the bolt was opened, shaved that said piece of brass locking the ejector in the bolt face.

The powder charge looks reasonable according to various manuals. One thing I noted was a projectile would not fit in the neck of a once fired case..... Something I have not seen before. The loads were developed in Lapua .308 brass necked down WITHOUT a neck turn. My rifle has a standard chamber.

My theory is that the necks are so thick that on ignition the neck has inadequate room to release the bullet causing a pressure spike. Loaded round neck dia is 0.297"

Anyone have any thoughts/ideas/comments?
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  #2  
Old 02-07-2011, 02:19 AM
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Join Date: Nov 2008
Location: Nelson, New Zealand
Posts: 289
Re: .260 load pressure problem

Quote:
Originally Posted by Gota6.5 View Post
Just thought I'd throw this one out to "the floor" as it were to get everyone elses opinion.

Due to unfortunate work commitments I had to send my custom .260 (shilen 1-8" twist on trued rem 700 in Hunters edge stock) to a local load development "specialist" to have a long range load worked out. A while later the rifle came back with the report that it was shooting 1/2 MOA with 140gn A-Max's.

Great! I take it hunting at the next opportunity and whack a deer with it. Problem is the pressure was so high that the brass extruded into the ejector hole and when the bolt was opened, shaved that said piece of brass locking the ejector in the bolt face.

The powder charge looks reasonable according to various manuals. One thing I noted was a projectile would not fit in the neck of a once fired case..... Something I have not seen before. The loads were developed in Lapua .308 brass necked down WITHOUT a neck turn. My rifle has a standard chamber.

My theory is that the necks are so thick that on ignition the neck has inadequate room to release the bullet causing a pressure spike. Loaded round neck dia is 0.297"

Anyone have any thoughts/ideas/comments?
You should go back to the load development guy.

I would think that the necks need turning, the bullet should fit into the neck after firing.

I use 308 Lapua brass in my 260 & I do neck turn them, it isn't hard when you have the correct gear
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  #3  
Old 02-07-2011, 07:34 AM
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Join Date: Nov 2010
Location: Helena, MT
Posts: 96
Re: .260 load pressure problem

If you have any fired brass from this chamber that has not yet been resized, measure the neck diameter and add .001. This will tell you how much expansion you are getting from the .297 neck diameter of your loaded rounds. I looked at the SAAMI dimensions for the .260 Rem chamber. It's .298. so your right near there. You might not be getting enough expansion. Good luck

-- gr8whyt
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Old 02-07-2011, 10:03 AM
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Join Date: Jul 2004
Location: Texas
Posts: 5,627
Re: .260 load pressure problem

Quote:
Originally Posted by gr8whyt View Post
If you have any fired brass from this chamber that has not yet been resized, measure the neck diameter and add .001. This will tell you how much expansion you are getting from the .297 neck diameter of your loaded rounds. I looked at the SAAMI dimensions for the .260 Rem chamber. It's .298. so your right near there. You might not be getting enough expansion. Good luck

-- gr8whyt
+1

Some of the guys shoot rifles that are "Tight Necked" and do OK with careful reloading.

I consider a OD of .003 larger after firing to be minimum. On low pressure loads you can
get away with .002 (.001 on each side) With big cases that typically are high pressure
I want .004 (.002 each side) for safe high pressure velocity loads.

A simple way to verify if this is the problem is to pull the bullet on a couple of the custom
loads keeping the powder charge to replace after turning the necks down a couple of
thousandths.

Fire one and see if it makes a difference.

If it does you will know where the problem is and what you have o do.

J E CUSTOM
__________________
"PRESS ON"
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  #5  
Old 02-08-2011, 12:10 PM
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Join Date: May 2009
Location: roanoke,va.
Posts: 236
Re: .260 load pressure problem

Gota 6.5, your theory seems to be correct: anytime the neck won't release a bullet there WILL be a pressure spike ! I always (well, I'm supposed to ) see if a bullet will pass through a fired case neck. It should. If not, you gotta ream. It's an easy cure and those loads, as you said, being "comfortable" should be then ok. Lots of guys posting basically the same answer. Oh, and the 260 Rem. sure is a great round,huh? I hae one in a DPMS AR10 and it really shoots tiny holes ! Loves the Lapua Scenar 139s; but the Nosler 140 Custom Comps work great as well.
Ream those necks and enjoy ! Charles
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  #6  
Old 02-08-2011, 09:23 PM
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Join Date: Aug 2007
Location: Auckland
Posts: 18
Re: .260 load pressure problem

Thanks everyone. I think we've confirmed the source of the problem. Very disappointing to have to go through this when you've paid someone to make you up some safe, accurate loads.
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  #7  
Old 02-09-2011, 02:58 PM
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Join Date: Feb 2011
Location: Halfway between Lubbock and Dallas
Posts: 5,168
Re: .260 load pressure problem

I went through a similar problem years ago with a model 70 which had originally been chambered in (if I remember right) .222 rem mag then cut and rechambered to 220 swift.

My problem turned out to be that the freebore portion of the throat was a little too short which resulted in the bullet when chambered being in contact with the riflings already producing exactly the same pressure problem and result you are seeing.

It also sounds like you didn't follow Kreiger's recommended break in cleaning regime and you may well have some built up brass in the throat of the chamber.

I'd have a long talk with my "load guy" and find out what he did to properly break the barrel in before working up the load.

I'd also have a good gun smith go over it with a fine tooth comb and mic everything out to see for sure if the chamber is correct.
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