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How did I get an extra 150fps? 300Wby

 
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  #1  
Old 10-18-2013, 09:38 AM
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Location: The Republic of Texas
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How did I get an extra 150fps? 300Wby

This may be a stupid question, but on the surface it's perplexing for a simpleton like me.
Here's the data, an ideas what may be causing this?

300 Weatherby Accumark, same gun, same brass/primers, same powder load, same OAL:

168 gr Berger Classic Hunter (BC .496) = 3125fps

168 gr Nosler BT (BC .490) = 3278fps

I have a theory but tell me what you think.
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  #2  
Old 10-18-2013, 09:48 AM
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Re: How did I get an extra 150fps? 300Wby

Bearing surface or seating depth of ogive to the lands? Is it just one test or multiple tests with similar results? Crazy amount of speed increase.
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Old 10-18-2013, 09:57 AM
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Re: How did I get an extra 150fps? 300Wby

It's a Weatherby, so not sure about distance to lands (freebore). I set the OAL as long as possible. This was the first test of the loads--5 rounds each and I averaged the velocities. The faster group was shot after the slower group but I let the gun cool several minutes.

I'm guessing it's probably bearing surface but haven't looked closely at bullet dimensions yet.

For what it's worth, the Berger group was .666, the Nosler group 1.00.
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Old 10-18-2013, 11:22 AM
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Re: How did I get an extra 150fps? 300Wby

I think you have an increase in pressure on the nosler group due to the design of the bullet putting the ogive closer the lands. Bearing surface will have some affect but for example on my 6.5 .100 from the lands with the same powder vs .000 to the lands produced 130 fps difference, with the same bullet. The pressure was off the chart though and looked like was about to blow holes in my primers.
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Old 10-18-2013, 12:09 PM
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Re: How did I get an extra 150fps? 300Wby

Lengths:
Berger 1.254"
Nosler 1.304"

I think I might be getting more case pressure because of the length and shape of the Nosler (seated deeper because it's longer). The Berger looks like it has less bearing surface, so I don't think that's the culprit.

Any suggestions for seating depths on a Weatherby Accumark with a freebore barrel??
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  #6  
Old 10-18-2013, 12:31 PM
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Re: How did I get an extra 150fps? 300Wby

sorry no. I always measure the ogive seating depth via a Hornady OAL guage or the old method of inserting a bullet in a case and chambering it. I always record the data for each bullet design and set my dies to achieve the OAL to the OGIVE that produces the best accuracey etc. I adjust powder accordingly as well.

Someone on here should be able to help. I am no expert so my help is limited.
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  #7  
Old 10-18-2013, 02:11 PM
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Re: How did I get an extra 150fps? 300Wby

If your barrel's bore and groove diameters were smaller than what was used to get the load data and you held the rifle differently, too, that extra muzzle velocity is normal. The use of different primers and powder lots as well as firing pin impact force, case neck tension than what the load reference used compounds the issue.

It's not abnormal to shoot a given load's bullet 200 fps faster or slower than what some published load says. All sorts of variables that are not listed for comparisons.
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