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Minimum book load w/ high pressure signs

 
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  #1  
Old 03-04-2013, 03:35 PM
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Minimum book load w/ high pressure signs

Iv'e seen this mentioned before but cannot find the thread so I need to ask the question.

Why does one get high pressure signs and a sharp recoil with a minimum powder charge load? Signs included severely flattened primers, noticably heavy bolt lift but still able to cycle by hand. I fired 2 rounds of this batch last weekend. The batch was built to allow a new shooter the ability to shoot the rifle without getting pounded by the recoil. We wound up using Remington factory ammo.

The cartridge in question is a 308 Win. assembled as follows:
  • Winchester case, new, FL sized with shoulder set back .010" Shoulder set back is out of the bag. I normally only push the shoulder .003" at the most.
  • Case length is "trim to' spec
  • CCI200 primer
  • Varget 44.0 gn. per hodgdon manual - minimum charge.
  • Sierra 155 MK set to .100 off the lands. This is the lightest .30 cal bullet I have in inventory.
The rifle is a Rem 700 BDL with the factory 24" barrel. There are approximately 670 rounds through the gun to date... never any problems with the rifle until this load. The bore and chamber are clean and unobstructed. Remington factory ammo fires fine... no pressure signs. The handloads chamber and cycle fine - no interference felt. I normally don't build light loads such as this. If anything, I'm guilty of building heavy loads that push the book limits at times.

I'm aware of problems with very light loads - under book value - but this is within the range of the published data. I don't plan on building this again and have disassembled the remaining 18 cartridges.

What I would like to understand is what happens inside the cartridge and chamber when a load like this ignites, begins to move and swell the brass and push the bullet into the rifling and down the barrel.

Could someone point me to an article that explains this or perhaps offer an explanation.

Thanks in advance

Pete

**

PS:

As for the new shooter, first time behind a gun other than a .22LR, the teen daughter of a neighbor who is not a shooter/hunter, she fired 16 rounds that afternoon. She put ALL 16 inside an 1 1/4" circle at about 130 yards. Most were inside a 1" circle. She has the best trigger squeeze I've seen in a while with perfect followthrough. And she's fearless of the recoil. And because the gun is almost as heavy as she is, I honestly thought she would shoot a couple rounds and stop. NO. She wanted to know if I had more ammo but I didn't. Her dad gave up after 3 rounds.
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  #2  
Old 03-04-2013, 04:32 PM
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Join Date: Jul 2010
Posts: 14
Re: Minimum book load w/ high pressure signs

Hello,

Back off by 2 grains.

Sounds like you have a tight chamber, and Hodgdon's starting loads (at least in 7.62x54r) tend to be hotter than other companies' max loads.

When I discovered this, I figured out 70% case fill and went with the starting load closest to that. This was from Speer data, if I recall correctly.

You might look up Sierra data. They know their bullets.

What are you getting over the chrono?

Regards,

Josh
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  #3  
Old 03-04-2013, 09:32 PM
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Join Date: Nov 2010
Location: NY State
Posts: 340
Re: Minimum book load w/ high pressure signs

Josh,

Thanks for the reply.

I don't have a Sierra book but a friend tells me that their minimum load is 41.5 or 41.4 for this bullet weight. My Hodgdons manual it is 44.0. This isn't a load I'm going to be building or shooting as this was a one time deal; an attempt to build a light load for that day.

I've checked to make sure I didn't use the wrong primer or powder and I didn't do that. The Beam scale is still set for 44.o as this was the last time I loaded anything. Those exact components were used to build some 3006 cartridges late last year with no ill effects (except for the deer). The powder proportion was different of course.

All I want to understand is what went on from the moment of ignition to the time the bullet left the muzzle. If I were to guess, I would think the bullet started to move out of the case and stopped for an instant, perhaps when it touched the rifling because pressure wasn't built up yet. Once the bullet stopped, the pressure had to go very high to get the bullet moving again. It would be that spike in pressure that mashed the primer, stretched the case, and then kicked like a mule.

But if there is a different scenario, I want to know and understand what went on in there.


Thanks again

Pete
__________________
"It is the duty of the Patriot to protect his Country from its government." ~ Thomas Paine ~

A 'do nothing' Congress is not always a bad thing. If they pass enough laws, it should be entirely possible to sterilize one's complete life experience.

"The 'tolerant' left has come full circle... and has now become what they so despised."
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  #4  
Old 03-04-2013, 10:16 PM
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Posts: 193
Re: Minimum book load w/ high pressure signs

Sierra load manual for 308win 150gr and 155gr palma...

Varget min 41.4gr/ 2600fps 43.1/ 2700fps 44.8/2800fps is max

If you sized your case too small you may have caused a pressure problem even .8 off max load. How did you come up with the .100 off the lands? From the newly sized case or an old one of diff dimension? I question this because you stated you set the back .010 out of the bag when you normall only go .003 at the most.
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  #5  
Old 03-05-2013, 08:30 AM
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Join Date: Nov 2010
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Re: Minimum book load w/ high pressure signs

Quote:
Originally Posted by Red hunter View Post
Sierra load manual for 308win 150gr and 155gr palma...

Varget min 41.4gr/ 2600fps 43.1/ 2700fps 44.8/2800fps is max

If you sized your case too small you may have caused a pressure problem even .8 off max load. How did you come up with the .100 off the lands? From the newly sized case or an old one of diff dimension? I question this because you stated you set the back .010 out of the bag when you normall only go .003 at the most.
Red,

Here's how I arrive at my numbers. I use a Stony Point OAL gauge with the particular bullet used to locate the lands. The bullets are then seated .100" back from that measured point. Hornady makes the tool now. I've been using it to locate the seating depth for quite some time now, especially with VLD bullets. For me, this has become a matter of practice now.

One may ask why .100"? In most of my other rifles, in various calibers, my accurate loads are not set close to the lands but back; sometimes quite a bit. That includes the VLD's.

As for the cases, they are brand new, out of the bag. They were sized as a matter of practice. The sizing die is set to the shoulder height of a recently fired case, minus .003". I check this at each loading session but this has never needed to be readjusted since the initial setting. After resizing the new brass those shoulders were approx.009" to .010" from that length just mentioned. Out of curiosity last night, I measured a few of the untouched cases in the bag and they are also about .010".

What I'm finding interesting is the differences between loading data. Minimum in one book is near max in another.


Pete
__________________
"It is the duty of the Patriot to protect his Country from its government." ~ Thomas Paine ~

A 'do nothing' Congress is not always a bad thing. If they pass enough laws, it should be entirely possible to sterilize one's complete life experience.

"The 'tolerant' left has come full circle... and has now become what they so despised."
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  #6  
Old 03-05-2013, 04:08 PM
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Join Date: Jan 2012
Location: Bean Patch
Posts: 193
Re: Minimum book load w/ high pressure signs

My uneducated guess is the bullet design. Mostly the base and maybe some of the internal design and possibly jacket thickness. Maybe someone else has the info. I am lucky enough to have friends with the books I don't.
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  #7  
Old 03-05-2013, 04:58 PM
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Join Date: Jul 2010
Posts: 14
Re: Minimum book load w/ high pressure signs

Quote:
Originally Posted by Red hunter View Post
My uneducated guess is the bullet design. Mostly the base and maybe some of the internal design and possibly jacket thickness. Maybe someone else has the info. I am lucky enough to have friends with the books I don't.
Jacket thickness, and bearing surface.

Back when the military folks of several countries were playing with their first smokeless powder rifles, they had long-bearing surface, round nose jacketed bullets.

Lots of people got blown up by pressure spikes. This is one reason the bore on the Gewehr 88 was increased from 0.318" initially to 0.321" and eventually 0.323" to help reduce pressure spikes due to the bearing surfaces of those original, 221 grain (IIRC) bullets.

When low bearing surface 150 grain S bullets were introduced with the 7.92x57js (is) round, they were able to keep full-bore bullets.

This is how the modern 8mm round became more than 8mm!

Regards,

Josh
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