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Old September 12, 2010, 12:05 PM   #1
Mattp27
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Reloading question 5.56/.223

I have been reloading a couple years. Maybe someone can help explain this issue I have. Because I do not understand.

I have been shooting 55gr Nosler BT out of my AR-15 getting around sub MOA at 100 yards. Recently I have been trying to shoot out farther 300-600 yards and the 55gr bullet seems to be doing good on calm days. Around 1 MOA at 600 yards I am fairly pleased with that.

I recently bought some 69gr Nosler HPBT to help out with the wind. My barrel is a free floated 16" 1-9 twist so all the reading I've done shows that's the max weight I should go.

After loading some of the 69gr loads I took my Crony out and verified I am shooting about 2810fps.

My 55gr loads are around 3050fps.

Now what I am confused about is why are the slower 69gr bullets shooting about 3" high at 100 yards? Shouldn't they be shooting lower then the 55gr at 3050fps?

Here is a picture from today's shoot. The high ones are the 69gr .305BC @ 2810fps the ones next to the bullseye are 55gr .267BC @ 3050fps



Here is a picture with the 55gr only at 100 yards.



Any explination would be appreciated.
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Old September 12, 2010, 12:15 PM   #2
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Heavier bullets almost always shoot higher. They take longer to get out of the barrel, so the barrel has time to recoil up to a higher angle of departure before the bullet clears the muzzle.

The above is why vertical POI also changes when you change your hold on the rifle. Try sometime shooting prone with the tight military sling pulling the butt hard into your shoulder. Try again without the sling and butt pressure being light, and measuring the resulting average vertical POI.

With longer, whippier rifle barrels, you sometimes see the reverse happen, and a heavier slug impacts in the same place or lower than a lighter one. This is due to the slower, heavier bullets getting caught in the downward phase of a barrel harmonic whipping motion. You can see this influence shooting an Audette ladder. But if you adjust the load so the bullet exits in an upswing, the reverse happens.
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Old September 12, 2010, 02:34 PM   #3
Mattp27
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Ok that is crazy.

So if I decide to use the heavier bullet combo would it be safe to just zero it in at 100? It seems fairly consistent.

My next question is at farther ranges how bad does it affect it?

And if I would not have been reloading my own rounds I would have never known/thought of this happening. Good to know.

Thanks
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Old September 12, 2010, 02:37 PM   #4
fishluv
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again, unclenick teaches me something

thanks!:
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Old September 12, 2010, 02:38 PM   #5
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now i have to learn how to adjust loads with the harmonic whip!!!!BLARRRRGH
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Old September 13, 2010, 10:54 AM   #6
demigod
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Quote:
now i have to learn how to adjust loads with the harmonic whip!!!!BLARRRRGH
Anyone who develops an accurate load for a specific rifle is doing this whether he realizes it or not.
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Old September 13, 2010, 05:45 PM   #7
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Correct. The procedure I like to follow is not the Auddette Ladder these days (if you want to see how much trouble you may have to go to with one, read this old thread here). I now use Dan Newberry's method and recommend that as a systematic way to find sweet spot loads.
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Old September 14, 2010, 11:32 AM   #8
DANNY-L
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I want to try that load for my mini,what primers did you use,and what was your oal.thanks
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Old September 14, 2010, 11:36 AM   #9
Mattp27
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Quote:
DANNY-L I want to try that load for my mini,what primers did you use,and what was your oal.thanks
Which load? The 55gr or the 69 gr?
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Old September 14, 2010, 05:40 PM   #10
DANNY-L
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sorry,55gr. thanks
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Old September 14, 2010, 07:00 PM   #11
Mattp27
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Nosler Ballistic Tip Varmint Bullets 55grain

AA2230 25.0gr
(This is the upper imit so be careful. I am actully backing down because my primer pockets are loosing up.)

Wolf Small Rifle Magnum Rifle Primers

2.25" OAL

3049.8 fps 16" Bushmaster Carbine Barrel
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Old September 14, 2010, 08:07 PM   #12
deepcore
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Thanks for the link Uncle Nick.
This thread peaked my curiousity about barrel vibrations
of shorter barrels (like the one I have).
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Old September 14, 2010, 08:26 PM   #13
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thanks for the data. By the way thats pretty good shootin.

Last edited by DANNY-L; September 14, 2010 at 08:36 PM.
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Old September 14, 2010, 09:06 PM   #14
Mattp27
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Thanks. I am not happy with those groups though. I need to change the barrel out if I want to get the 1/2 moa groups. Right now I'm averaging 1 moa or so.
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