Scratching my head on this one????

[SOUTHTEXASBOY]

Ok guys I need your help, I don't know what's going on, I have a 28" 8.5 twist barrel, 7 rem mag shooting 180 Berger hybrids. Norma brass, F215m primers, 70.5 grains H1000, 2.700 base to ogive seating 10 thousands off the lands. I also attach my ultra 9 suppressor when shooting. I sighted it in at 100 yards. Shoots in the .1-.2 all day if I do my part. 75 degree day, 50% humidity, 30.02 pressure. Using a lab radar I got these muzzle velocities, 3064, 3063, 3067, 3066, 3069. So I have a 3065.8 average. ES of 6 and a SD of 2.3. I shot with a Magnetospeed a different day and the muzzle velocity was almost identical to what the lab radar read. I plug all of this into my ballistic calculator and sig kilo 2400 along with sight height of 2.5. At 300 yards it calls for 2.4 MOA up, I dial 2.5 moa and shoot 3 shots, group measure less that .5 inch but is 1.5 inches low. So I dial up to 3.0 moa and shoot one and it's dead on. Back up to 600 yards dial to 9.0 moa shoot a 5 shot group and it's about 2 inches low so I probable needed 9.25 moa. Back up to 997 yards. Calculator says to dial 20.8 moa so I dial 20.75 and shoot. I hit about 20 inches low. Dial up to 22.75 and hit dead on. Now if I true up my data and play with my muzzle velocity it shows my bullet is moving around 2950 fps, has anyone ever seen their muzzle velocity be off over 100 feet per second. What should I do next? I don't know what to do at this point? Is there anyway my muzzle velocity is off by that much. The scope on the gun is a ATACR 5-25 Moart.

 

[Barrelnut]

Happens to me all the time. Maybe not 100' per second but getting up there. I usually change muzzle velocity so the App matches the physical drops. But you can change the BC instead. Bottom line is the BC for the bullet, in the App, may be causing the issue. But your velocity back up to 3065, start adjusting the BC and see how it works out.

 

[Alaska2006]

Yup what barrel nut said. Have to get your true drop then adj MV until your round goes transonic then you use the DC in your AB program.

 

[Alaska2006]

Go to " rifles, bullets & ballistic forum" on this site, Doc posted a great how do document on this topic and all things AB/Kestrel related about a month ago.

 

[SOUTHTEXASBOY]

Have you had that happen shooting Berger bullets? I didn't think they would be that much off

 

[Barrelnut]

Have you had that happen shooting Berger bullets? I didn't think they would be that much off

Yes, I shoot mostly Bergers. You don't mention your App name and I don't use the Sig. But, If you use Shooter or Applied Ballistics App, use the bullet profiles verified by Brian Litz. I have the best luck with those.

 

[SOUTHTEXASBOY]

I have been using bullet flight app and ballistic AE

 

[SOUTHTEXASBOY]

When using the kilo 2400, i used Brian's custom curve.

 

[Barrelnut]

In Shooter, you cannot adjust the Brian Litz profiles. I'm assuming that is not possible with the Kilo 2400 either. So I adjust the velocity. But it seems I almost always have to adjust some.

I think the actual BC between lots of bullets, may be a little more than we think.

 

[SOUTHTEXASBOY]

The bad thing is you can not run the numbers and mess with the velocity and see what the drops would be on the kilo 2400 without sitting out and ranging at different yardage.

 

[Rich Coyle]

tag

 

[J E Custom]

Most bullets are advertised with the highest BC possible and I have found that adjusting the BC until your drop chart is accurate is the simple way to solve this issue.

I have rarely found the BCs to be spot on, so I simply shoot my favorite load at 100 yards to 600 yards and measure drop and then go to the tables and adjust the Bullet BC to get the trajectory that matches, then use the new BC to do the calculations at longer distances. This can be verified if you have a good range finder and plenty of room/distance.
I call this BC the actual bullet BC.

Crude, but it works.

J E CUSTOM

 

[Sevy]

I had the same problem with my 300 win. It blew my mind for a bit like something was wrong with the gun. But I kept shooting and keeping records of my drops and adjusted my mv in my app until I started hitting consistently at distance. Everything seems to be fine once got it figured out.

 

[Rhovee]

Do you have the BC on the gun profile set to Custom Drag? I haven't got a chance to test mine past 425. But at that distance the custom drag seemed spot on for both my rifles.