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Velocity deviation question for 700yd gun

 
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  #1  
Old 08-10-2010, 03:38 PM
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Velocity deviation question for 700yd gun

I got a very accurate load worked up for my 270WSM, 140gr accubonds at 3020fps average. I am going to send off my Leupold to get a couple CDS turrets made. My max deviation for 5 shot groups is usually 30-35fps. Is this acceptable for a 700yd rifle or should I keep working on loads. 5 shot groups with this load are running .4" in perfect conditions, and if it's over .7" I screwed up.
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Old 08-10-2010, 04:43 PM
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Re: Velocity deviation question for 700yd gun

Plugged your data into a ballistic program I have here in my office....

140grain Accubond
BC = .496
Velocity = 3020

With 100 yard zero, calculator said +15moa adjustment for 700 yards.

Same data, but adjust velocity to 3055 and it's +14.5moa for 700 yards.

That .5moa equates to ~3.5" at 700 yards. I would put that right on the edge of acceptable. Personally, I would look reduce your extreme spread (ES) down to 15 or better.

I recently had a load for my 280 that shot accurately, but the ES was terrible. Changed primers and worked the load up again and the ES came down to 12. So maybe all you need to do is change your primer.
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Old 08-11-2010, 09:14 AM
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Re: Velocity deviation question for 700yd gun

trebark statement that your ES results in marginally acceptable variations at 700 yards.

30xx FPS MV is a bit light for the 140s in a WSM. I'm bettin' you'll find another accuracy 'node' as you increase powder more towards a higher load density.

trebarks primer change consideration is a good one also.

For shots beyond 800 yards an ES of 15 would be an upper limit for coyote size targets.
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Old 08-11-2010, 10:08 AM
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Re: Velocity deviation question for 700yd gun

Boy that is a tough call----for one of my competition rifles no way but if the load shoots dots at 300 then it is a tough call to make for a sporter. Right on the edge---40 fps is my aboslute cut off for sporters and that is if they shoot DOTS!!

Also when you can shoot a couple of 5 shot groups (I use 10) that are the same then you will know for sure. Also remember what the temp changes will do to your velocity before you have custom adjustment disks made.
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Old 08-11-2010, 10:30 AM
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Re: Velocity deviation question for 700yd gun

Another thing you may try is to adjust your load up and down .1-.2 grains to try to reduce your ES. Generally with any good load, there is a accuracy sweet spot of about .5 grains. Some call it the accuracy node. You want to be right in the middle of that sweet spot to give it reliable consistency to cushion the effects of temperature induced pressure variations.

I have found that .1-.2 grains can really imporove the consistency of a load. If it doesn't work, you can always return to your proven load.
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  #6  
Old 08-11-2010, 03:43 PM
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Re: Velocity deviation question for 700yd gun

It rained us out of the hayfield, so I tried H4350 extreme today, I've had good luck getting low deviation with it in my 270 win. I improved my groups slightly and increased my speed slightly to an average of 3046fps over 20 shots. I had a couple 4 shot 1 hole groups, and only 1 went over 1/2". My rifle is an A-bolt with the 23" barrel which probably accounts for the slightly slower MV. My deviation remained similar, 32fps. I am using CCI magnum primers. I am down to 12 accubonds, so I loaded them to shoot critters with and will get to town in the next few days to buy more. I am very happy with the accuracy in a rifle that weighs 7.25lbs with scope. I'm using 4 shot groups because of the light barrel and 100 degree heat. It's going to be in the low 80's next week so I can see if my velocity changes. I'm hoping to avoid much velocity change with the extreme powder. I'm going to use this rifle on a muley hunt at 7-9000ft in October so I think I will get 2 turrets built for my elevation (1200ft) and for the area I hunt most years in the mountains.

With 32ft deviation over 20 shots should I call it good on load development?
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  #7  
Old 08-11-2010, 04:13 PM
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Re: Velocity deviation question for 700yd gun

Quote:
Originally Posted by mcseal2 View Post
With 32ft deviation over 20 shots should I call it good on load development?
No...see the previous calculation and royinidaho's comment that your ES needs to be 15 or better if you intend to shoot 700+ yards.
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