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Standard deviation in .223 Rem

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Unread 07-11-2009, 10:39 AM
dwm dwm is offline
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Join Date: Feb 2003
Location: Texas
Posts: 738
Re: Standard deviation in .223 Rem

+1 try anealling your cases

Someone once suggested to me that partly cloudy skies can impact the output of the chrony, was the sky clear that day? Cloudy, then clear, then cloudy, etc effects velocity readings or something like that. I have no way to validate that claim though.

They also suggested that if the chrony readings are taken over a long period of time the change in sun angle would effect the chrony readings. Measured velocity in the morning may be different than measured velocity in the afternoon due to a change in sun angle. (using the same chrony setup/location) Again, I have no way to validate that claim either.

Some things to consider though.
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Unread 07-11-2009, 11:24 AM
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Join Date: Jul 2004
Location: Texas
Posts: 7,233
Re: Standard deviation in .223 Rem

Originally Posted by remingtonman_25_06 View Post
My .222 rem gives me at most 15fps, and is usually within 10fps with its couple better loads. I am about .010"-.015" off the lands using CCI400 primers. Use IMR-4198 w/40g bullets and IMR-4895 w/55g bullets. Neck sized in a RCBS die. I dont know, I always thought smaller cases were easier to get lower deviations??? Never had a problem with my triple deuce getting low spreads.
Those are very good SDs !

It is the smaller bullets that are harder to get low SDs because of the weights of the bullets
compared to the velocity.

The smaller the bullet the more it is effected buy any difference in primers, neck tension, seating
depth, temperature and even the accuracy of the bullet weight . For example= If 500gr bullets
vary by 1/10 of a grain the difference will be 2/100 of 1 percent. but if 50gr bullets vary
by 1/10 of a grain then the percentage is 2/10 of 1 %.

It is not hard to get big bore bullets in the 400 to 600gr range down to single digits because slight differences have less effect on them.

The smaller cases do have an advantage in combustion Pressures because of faster powders
and smaller volume's .

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