Need Help, tune mv or bc

[Excavator]

I would have to change somewhere around, off the top of my head 75fps to compensate for the 479 yard group I shot this evening. To me that seems like a lot. My thought on bc was because I am starting 90fps slower then hornady advertises this round.

 

[Bstick]

I personally would purchase ABM and use their CDM, that is to much to adjust MV

 

[Clucknmoan]

I don't think changing MV only is going to solve the issue. But it's where to start. Changing the BC to get the drops @ 500 is going to create a lot more problems.

Validate zero, adjust MV at a mid range, adjust BC to fine tune at distance.

 

[Bstick]

ok since you want to change known factors, what is an acceptable limit to change the known factor? 3%? 10%?

 

[Clucknmoan]

I would have to change somewhere around, off the top of my head 75fps to compensate for the 479 yard group I shot this evening. To me that seems like a lot. My thought on bc was because I am starting 90fps slower then hornady advertises this round.

Check to see if your scope needs a correction factor by performing a tall target as suggested above. I'm not saying MV is your only issue, or even an issue. I'd think your zero or correction factor have something to do with it.

 

[Clucknmoan]

ok since you want to change known factors, what is an acceptable limit to change the known factor? 3%? 10%?

I was also using that percentage as an example, just as you did with your velocities. Just to show that MV has way more of an affect than BC at mid range, which is where he tested. I'm not going to turn his post into a ****ing match.

Good luck with your hunt the weekend excavator!

 

[Excavator]

Thx again guys!

 

[Bstick]

I was also using that percentage as an example, just as you did with your velocities. Just to show that MV has way more of an affect than BC at mid range, which is where he tested. I'm not going to turn his post into a ------- match.

Good luck with your hunt the weekend excavator!

not a ****ing match but a question.

Why would you change something that you know is right?

 

[donwill]

I would have to change somewhere around, off the top of my head 75fps to compensate for the 479 yard group I shot this evening. To me that seems like a lot. My thought on bc was because I am starting 90fps slower then hornady advertises this round.

Keep in mind that you are using a 20" barrel. what lenght of barrel was used by hornady? you will have lower MV witha shorter barrel (about 50 feet/sec per inch 0f lenght) so that would place you really close to their spec with a 22 inch barrel. I would adjust the bc not the mv as adjusting mv will make all your shots past the zer0 range drop off the chart as the velocity declines with distance.

 

[Excavator]

That was my thought. I bet they were testing with a 24" at 2710fps, when they recorded the posted BC.

 

[skipglo]

so, I just had a 6.5 creed with a 20" tube put together for my little girl. We shot a hand load 130 accubond at 2675. And eld-m 140 at 2620. Mv were through a lab radar. So here is the question. Once I stretch the range out, with both loads. Both shoot low. My thought is to tweek bc to match drop instead of mv. My reason is because I have a shorter tube and am starting slower, then they were probably shot at the factor when bc was recorded. Gun was built by a very reputable smith that has built 10-12 setups for my brother and I. First time I have had this problem. What are y'alls thoughts? Thanks in advance. We have a hunt this weekend, and trying to fix drop tomorrow.

I think it would be easier to adjust the mv to meet the drop. The BC is manufactured into the bullet!

 

[jkwright]

Shoot a several known distances and measure the drop. Then you can play with the bc and velocity untill you find a combination that works for your known distances. Like fitting a curve

 

[Excavator]

But my understanding is BC is related to MV. So if I am starting 90fps slower then hornady was when they recorded BC, could I be starting with a lower BC right out of the barrel. An is the BC decreasing faster do to the slower initial MV difference?

 

[B-P-UU]

I would start by changing to a solid 100 yd zero. Don't worry about your barrel length vs. whats on the box or your velocity vs. what's on the box. Your goal is to get your data to work. As others have stated, its generally best to start by tweaking your MV whether you agree with it or not. It has a far better effect inside of 600 yds. With that said, 75 fps is quite a bit and I think there is something else going on. I have little experience with swaro.. are they IPHY or MOA adjustments?

The Applied Ballistics guys recommend truing MV @ 1.2 MACH. So determine the distance at which your rounds hits 1.2 Mach (1125 fps x 1.2 = 1,350 fps and shoot at that range). tweak velocity to get it your come-ups to match what you see on target. For me, that's around 1,100 yds for my dasher. I've had pretty good results using this method.

Others prefer to adjust velocity in the 'linear' portion of the drop curve (say, inside of 5-600 yds) and BC for the non-linear (700+). I would not recommend solely adjusting BC at the distances you are shooting.