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  #1  
Old 07-21-2021, 07:26 PM
Grump Grump is offline
 
Join Date: Feb 2018
Posts: 3
Default How Much Will Case Mouth Contact Affect Accuracy?

My ammo testing with two powders a few years ago was complicated by mysterious failures to eject. Sometimes the fired case would get re-chambered after the bolt reset the hammer to that rooster position.

Turns out my once-fired brass was longer than 2.015". Some 2.025" and even longer. Later learned that many of these would be impossible to manually extract from the case mouth jamming at the front of the chamber.

So I'm wondering if anyone has seen how much that jamming could have been causing the erratic accuracy I was getting. A nice 1.1-inch 100 yard group would be followed by a 2.1. Some sessions went great except for "that one group" of 2.25" or even 2.75". With a well-mounted scope. Same scope gives me groups in the .4s to .6s on a Creedmoor bolt gun.

Have some well-prepared brass ready to load up and spend some of my precious primers on for follow-up testing. So I might be the one who puts numbers on the question in a few weeks.

Thoughts?
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  #2  
Old 07-22-2021, 06:00 AM
ceresco ceresco is offline
 
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Well: generally, accuracy is dependent on consistency and uniformity. A common problem with .308 cases is excessive stretch. I would suggest minimal shoulder setback during FL resizing and carefull monitoring of case length. There is a fudge factor on case length so slightly over (.004") is generally OK, but .010 is too much and any case that jams into the neck is obviously too long--for safety or accuracy. All the potential problems are enhanced if you are using max loads..... Good Shooting. ...
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  #3  
Old 07-22-2021, 06:54 AM
milprileb milprileb is offline
 
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Location: Saigon, RVN
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Its called "neck tension" and it does play havoc on accuracy if your reloading procedures do not produce consistent accuracy. The M1A / M14 does stretch cases so you simply must properly full length size, trim/chamfer / debur all cases. Do that and neck tension should be consistent and accuracy results consistent as well. To attain that neck tension more effectively, I anneal all cases before I proceed with full length resizing.

With diligence, your handloads properly prepared will serve you amazing accuracy.
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Old 07-22-2021, 07:09 AM
col b col b is offline
 
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Hold my beer and watch this! Can't be a serious first post.
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  #5  
Old 07-22-2021, 10:40 AM
milprileb milprileb is offline
 
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Guy has 1 post on CMP. Lets cut some slack, we've all started somewhere and had a learning curve.
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  #6  
Old 07-23-2021, 06:29 AM
Grump Grump is offline
 
Join Date: Feb 2018
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Note the difference between "Will it. . . ?" and "How much did. . .?" I'll stop now.
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  #7  
Old 07-23-2021, 08:05 AM
NMC_EXP NMC_EXP is offline
 
Join Date: Jul 2010
Location: New Mexico
Posts: 297
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Quote:
Originally Posted by Grump View Post
Note the difference between "Will it. . . ?" and "How much did. . .?" I'll stop now.
In general, jamming a bullet into the rifling vs jumping the bullet (if I understand your post) should result in chamber pressure differences which should result in differences on target.

As mentioned the 1st requirement for precision/accuracy is reloading technique which produces a uniform finished product. Then experiments with reloading recipes will provide valid data.

I'm with milprileb: for a gas gun I full length resize and trim after each firing. This allows me to use the ammo in any rifle. The other benefit is reduction of risk of slam fire.

I highly recommend getting a cartridge case headspace gage. Use that gage to verify your resizing die is adjusted to bring the case back to nominal. After resize, trim OAL back to nominal and deburr ID and OD of the mouth.

That way you are starting off with brass dimensionally the same as new factory stuff.

The M14/M1A is hard on brass. I expect only 4 or 5 firings before signs of case head separation show up.
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“After all is said and done, successful rifle shooting on the range is nothing more than first finding a rifle and lot of ammunition which will do precisely the same thing shot after shot, and then developing the same skill in the rifleman.” ~ Capt. E. C. Crossman (Book of the Springfield)
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  #8  
Old 07-23-2021, 10:20 AM
ZvenoMan ZvenoMan is online now
 
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I'm no reloading expert.
But I do know that if the brass itself is too long, such that it's hanging up during ejection, nothing you do regarding reloading and accuracy will matter until the brass is trimmed to the proper range. Every other discussion is simply wasted breath!

JH
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  #9  
Old 07-23-2021, 11:34 AM
Grump Grump is offline
 
Join Date: Feb 2018
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I had long reloaded 1x-fired brass in the '06 and trimmed them after that second firing. No can do with this .308 stuff. Military (LC, WCC), commercial FC, all were found to be quite long. Don't do that any more and all 1x-fired is trimmed from the get-go. And annealed since I've had some slack time. My own testing with the same now-trimmed brass will tell. Will include some 300-yard charge weight vs. velocity vs. point of impact testing as well.
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