260 Guys, Help?

L

lencomatt

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Minuteman
Jul 19, 2009
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COS, CO
#1
I took my 260 out today and was planning on just breaking in the barrel so I didn't really go crazy with the loads. No trickling the powder weights, whatever the measure dropped was good. It was around 41.1g of H4350 with Nosler custom comp 140's at 2.83", it shot just a hair under .5" at 100, I'm happy with that. I didn't have my chrono with me so I don't know what speed it was. I walked it onto target at 1000 yards and it took 8 mils from the 100 yard zero. Now if I do a trajectory validation on Shooter or Strelok they come up with about 2900fps, I have an Obermyer 5R tight groove which is possibly faster than a normal barrel but 2900 out of a 25.5" barrel seems to fast, I was expecting 2650-2700. I was in the mountains with an altitude of 7800' and a density altitude of 10,500'. Is 2900 really possible or is the altitude making it look better? I'm pretty sure I entered all the info right in Shooter, which I usually trust. Regardless I'm pretty damn happy with my M2013 AX :)
 
#3
I have the same set up times 2 (barrel/caliber), I would guess you are right around 2790-2820 with that load. I think that the elevation is helping (however I have never shot at that alt). At sea level (43.5g H4350 24" krieger barrel) runs right at 2830 which is 8.5 mils to 1000, my other rifle is running 2870 with 44.1g of H4350 (Obermeyer 5r tight bore 25") which makes 8.2mils at 1000. My dads rifle is a Obermeyer tight bore as well and runs 43.8g of H4350 at 2910 on a 26" barrel 8.0mils at 1000yards.

There are a lot of smart people here and I am sure someone will chime in with some more accurate knowledge.
 
#4
Shooting at 100 ft ASL versus my shooting at 3600 ft ASL usually jumps me up about 125 fps. I definitely think your altitude is helping with the speed of your rounds.
 
#8
lencomatt said:
I took my 260 out today and was planning on just breaking in the barrel so I didn't really go crazy with the loads. No trickling the powder weights, whatever the measure dropped was good. It was around 41.1g of H4350 with Nosler custom comp 140's at 2.83", it shot just a hair under .5" at 100, I'm happy with that. I didn't have my chrono with me so I don't know what speed it was. I walked it onto target at 1000 yards and it took 8 mils from the 100 yard zero. Now if I do a trajectory validation on Shooter or Strelok they come up with about 2900fps, I have an Obermyer 5R tight groove which is possibly faster than a normal barrel but 2900 out of a 25.5" barrel seems to fast, I was expecting 2650-2700. I was in the mountains with an altitude of 7800' and a density altitude of 10,500'. Is 2900 really possible or is the altitude making it look better? I'm pretty sure I entered all the info right in Shooter, which I usually trust. Regardless I'm pretty damn happy with my M2013 AX :)
Click to expand...

I think you entered your data incorrectly. You are probably in the 2725-2750 range.
 
#9
Roddd said:
Shooting at 100 ft ASL versus my shooting at 3600 ft ASL usually jumps me up about 125 fps. I definitely think your altitude is helping with the speed of your rounds.
Click to expand...

Are you saying that a change in altitude, with all other possible factors remaining constant, has a corresponding change in muzzle velocity? HIGHLY unlikely.
 
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#10
You're probably at around 2750fps, but the high altitude is giving you a lower Density Altitude which lowers the resistance of the air on the bullet accounting for the flat trajectory. Altitude doesn't change MV, but the reduced air density improves the bullet ballistics.
 
#11
19Scout77 said:
Are you saying that a change in altitude, with all other possible factors remaining constant, has a corresponding change in muzzle velocity? HIGHLY unlikely.
Click to expand...

I thought the same and ran some rough numbers. Changing the altitude that much changes your DA by about 4000'. That corresponds to about a .6 mil change in elevation at 1000 yards, which coincidentally would be roughly 125 fps.
 
#12
How does elevation possibly change muzzle velocity? Will change your drop because of thinner less dense air but I don't see how muzzle velocity can possibly change with altitude.

I don't mean to get your thread off track
 
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#13
.284 said:
How does elevation possibly change muzzle velocity? Will change your drop because of thinner less dense air but I don't see how muzzle velocity can possibly change with altitude.

I don't mean to get your thread off track
Click to expand...

Well, I don't know how much it affects mv, but before every shot is made there is a tube of atmospheric pressure dead air sitting in your barrel that the bullet needs to posh out of the way before it can leave the barrel. Air has less mass at higher altitudes so the bullet doesn't have to do as much work to get that air out of its way. It does result in higher mv, but in the order of feet per second, not tens or hundreds of feet per second.

PCR/XLR/TAC338 http://i813.photobucket.com/albums/zz53/bodywerks/IMG_20130816_111453_255_zps1b498f0d.jpg http://i813.photobucket.com/albums/zz53/bodywerks/IMG_20130816_111325_951_zps290ebdd0.jpg
 
#14
It cant be different muzzle velocitys between altitude and if it is its minamal. There is no way your getting 2900 fps with only 41.1 grains of h4350 IMO ..... I have my velocity at 2800 and bc set at .6 (berger 140 vld)in shooter and I verified dope the other day at 8500 da and at 1000 yards with a drop of 7.5 mills. shooter tells me at a da of 10500 I need 7.3 . I think you need to probably raise your bc and lower your velocity IMO. But try setting your targets at like 400 600 800 and 1000 and then test your shooter app you will probably find the arch won't match. I'm shooting 42.4grains with a 5r bartlein barrel at 24.5inches and am right under 2800 fps
 
#15
I had some Berger 140's loaded with 42.5g's of H4350 and they went 7.2 mils to 1000. They weren't loaded on the lands though and they weren't very accurate. For them to be on the lands in my gun they won't fit the mag so I won't be playing with them right now. A little more powder and a better BC on the Bergers so obviously less drop. From asking around it seems like most people use around 43g or so and 2800fps is about max, I'm going to have to chrono some this Sat and hope for the best for sundays match.
 
#16
What did you enter as a BC for that nosler bullet? Run your program again and try .56 or .57. I think nosler a listed BC is a bit low on that one. They usually error the other way but....

Bb
 
#18
I was using Shooters BC for the Nosler, they have it listed as .552, or close. I went back and looked at the programs, neither have an altitude factor for the trajectory validation. There's the issue, at a DA of 10, 500' there isn't much air pressure pushing the bullet back down so the programs assume the bullet is going faster. If a chase the speed in the table it comes in around 2725, which seems more right to me.
 
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