Is three out of four bad?

bassassassin007

March 03, 2007 15:08

So I am at the range today working on new recipes. The plan I followed was to shoot one 4 shot group every 1/2 hour to allow my barrel to cool. (I would have gone for 5 shot groups but I felt the barrel was getting a little too hot to be considered cold).

Regaredless, 8 times out of ten I ended up having 3 shots in each group on top of one another, and one of the four was astray. It wasn't very far, but far enough that my .2 and .3 groups became .7 to 1.0 groups.

Now it wasn't the fourth shot in each group, just one of the four.

The brass is the same, it is trimmed to the same length (within 4/1000, the C.O.A.L. is the same (again with in 4/1000), the powder charges were measured to be the same within .1 grains.

What is the likely cause? I am ok with it being operator error as long as I knew what the error was.

Thanks in advance, Dave

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  HandgunHTR52

  March 03, 2007 16:55

  The problem could very well be that the bullet is not concentric to the case mouth.
  There is a pretty good short article about this in the lastest Guns & Ammo mag.
  Also, .1 grains is a pretty big variable if it is a small to medium powder charge. What caliber are you reloading?

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  bassassassin007

  March 04, 2007 02:44

  The caliber was 7mm Rem Mag. Loads were from 46 to 54 grains.
  
  Thanks

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  nononsense

  March 04, 2007 07:23

  bassassassin007,
  
  It can involve several variables that you can check individually to eliminate what it isn't.
  
  - Depending on which powder and bullet you've chosen, your top load of 54.0 gr. of 'x' powder leaves about 20% of your case unfilled so you could be having problems with powder position. This leads to inconsistencies in ignition and burning.
  
  - It can be the batch of bullets in that they may contain very slight inconsistencies in the manufacturing; tiny bubbles in the core, slight air spaces between the core and the jacket, inconsistencies in the jacket thickness, poor skiving, etc.
  
  - The individual cases have varying wall thicknesses and therefore varying capacities. I've found more variations in our belted magnum cases than those that are beltless.
  
  - Slight variations in the barrel harmonics brought on by the factors above. Heat, even a slight increase, can cause the barrel to move slightly because the internal stresses are being relieved.
  
  - Stress induced from the bedding or lack thereof and being accentuated by the heat or harmonics or both.
  
  - Thumb pressure (position) change on the stock.
  
  - Variation in your cheek weld and eye position.
  
  - Potential lack of concentration and follow through on the shot release.
  
  - Mechanical inconsistancies in the trigger; contact surfaces, spring set and re-set, dirt, etc.
  
  When you start measuring groups in the .2's and .3's, the smallest of flaws can bring about flyers.
  
  Best.

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