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Old February 3, 2013, 06:12 PM   #9
Join Date: March 4, 2005
Location: Ohio
Posts: 13,727

If you see Tank's post #3, you will see he already changed scopes out.


Switching between primers in .223 causes different pressures and muzzle velocities. Charles Petty did a comparison in a 2006 Handloader issue with one fixed powder charge and bullet and got up to almost 5% velocity difference from different primers under the same charge, indicating the warmest primer he used was making higher pressure. Obviously the barrel time is shorter when the pressure and velocity are higher. This can move a round off a sweet spot, so you may need to adjust the powder charge for each primer to find the best accuracy load with it.

Changing the powder charge half a grain in that small case can cause you to sail right past the middle of a sweet spot range. That's roughly a 2% change, which is fine for searching for pressure signs, but is too coarse for finding the ends of a sweet spot range with some powders. 0.7% to 1.0% is better for that, so try 0.2 or 0.3 grain steps. Take a look at Dan Newberry's round robin procedure as one way to help identify the middle of an accuracy sweet spot with those small increments.

You didn't say what powder you are using, what bullet you are using, or what your barrel twist rate is. These affect best choices of bullet and powder. It's perfectly possible to have a marginally stable bullet or an excessively stabilized bullet that is happy under one set of atmospheric conditions, but not another.

To have each group the same size within 10% of one another with 95% confidence (meaning it will be true for 19 groups out of 20) takes about 25 shots per group to achieve. With 10 shot groups, 19 out of 20 will vary about 22% randomly, so if you have a difference that small, it doesn't mean your load has lost its touch. If the difference is much larger, and consistently so, then something else has changed.

Seating your primers properly can have a big influence on ignition consistency. That not only affects muzzle velocity, but it can introduce ignition delays that are too small for the shooter to notice, but that are as long as the rifle lock time in some instances. That allows small mechanical firing disturbances to get to the muzzle ahead of the bullet and open groups up. To avoid this, you want to set the bridge on primers properly. This generally means seating fairly hard. You want the primer to go in at least two additional thousandths (small rifle; three thousandths for large rifle) and as much as six more thousandths after the anvil feet find the bottom of the primer pocket.

Forster solved the above fairly neatly on the Co-ax press by creating a priming mechanism that forces the primer 0.005" below flush with the case head. You can also set up the Sinclair hand priming tool to do this. The K&M Primer Gage tool will let you measure exactly how far you seat below anvil contact on an individual case and primer basis, but it is slow going and intended for brenchrest shooters with lots of time to dote on every detail of every round. Though not practical for AR shooting volume, it can help diagnose an ammo issue by process of elimination.

With any priming tool, you can see if it has pushed deeply enough by taking some 20 lb bond printer paper and cutting a thin strip about a quarter to a third the width of the primer. Then bridge the primer pocket with the flat blade of a screwdriver and see that the strip of paper slips between it and the primer. If it gets trapped, you need to push in a little harder.

I can think of other factors, but further diagnosis will get you into more tools. If your chamber is sensitive to bullet tilt, as some are, you may need a runout gage to check how straight the ammo is. If your chamber is sensitive to bullet seating depth, you need to get that dialed in. If you have a velocity consistency issue do to other factors, you'll need a chronograph for diagnosis. Of particular interest would be whether or not the average velocity was different on days when a load shot well or didn't shoot well. If not, you need to figure out why not?

After all that's sorted out, you'll be able to tell if match primers help or hurt your cause.
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