I just started using my exbal program the other day,and I am having problems coming up with a velocity/b.c. combo that works for various ranges.I am shooting a 300wm with a 240gr smk.According to sierra,the b.c. is .711,and according to my chronograph,I am shooting 2480fps.I am sure my chronogragh is reading low,so I started playing with the velocity numbers and I can't get things to add up.I am zeroed at 200yds.,at 300 yards I dropped 8''.I then shot 492 yds,and was hitting pretty much center with 33 clicks elevation(1/4 moa).Then I had to elevate to 90 clicks at 810 yds.Based on the info. I have,is there any way I can get this mystery figured out?
Need some help with my exbal program.
- Thread starter KQguy
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ss7mm
Well-Known Member
Where you are shooting is there any canyon, cross canyon, elevation change or wind that could be affecting your shooting in any way?
Do you have access to anyone that has a quality chrono?
What scope and mounts are you using?
How did you determine the yardages?
Do you have access to anyone that has a quality chrono?
What scope and mounts are you using?
How did you determine the yardages?
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I am shooting in Ohio,very flat.I don't have access to a good chronograph,just the cheap one that I have.I am using a nightforce scope,bases,and rings.How much can published B.C.'s be off from actuall B.C. results?I am using a bushnell elite LRF also.I thought everything was good,I figured I was shooting 2660fps,with a b.c. of .711(sierra'a claim),that worked at 300 yds and 492 yds.But when I shot from 810 yds,I was about 10 clicks(1/4 moa) too low.
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Ernie
SPONSOR
Have you done a box test?
This is at the bottom of the list to check but good to do anyway.
NF's have a great reputation for tracking integrity, but you would eliminate one more possible variable.
This is at the bottom of the list to check but good to do anyway.
NF's have a great reputation for tracking integrity, but you would eliminate one more possible variable.
tjonh2001
Well-Known Member
i have had my bc change dramaticlly before. if i were you i would check tio make sure that all of the info that you are putting in the exbal is correct. double check barametric pressure and temp. you can also use altitude and temp. when you are shooting to 800+ alot of things can change. when you shot to the 800+ yard target how was the group. another this is when you change your bc to show the correct moa on the 800+ shot what did it do to the other 2 ranges. alot of the time when you change the bc to correct al longer distance you will see that it wont affect closer shots. that just my $.02.
Sendero_Man
Well-Known Member
did you enter center of bore to center of scope for the height of scope above bore ?
Yes I did,I added half the diameter of the scope(objective end),to half the diameter of the barrel(right below the the objective lens),then added the space between the bottom of the scope and the top of the barrel.I came up with 1.9".Did I do that right?I have a 20moa base also,does that have anything to do with it?
115vld
Well-Known Member
Given this information, you will have to look at your velocities at each given range. Since your bullet is slowing down once it leaves the barrel, your BC will change with each transition of velocity. So, to get your velocities at each distance, use the nightforce program or try JBM ballistics. It will tell you the velocity at each distance. I think that JBM will re-calculate the BC as the bullets transition. I have always had great success with JBM with several calibers. It is even accurate out to 1500. But that is as far as I have shot.
How far off can published BC's be off from actuall results?Sierrra has a claimed .711 on there 240MK.I am using .620 as a BC,it works all the way out to 800yds.,I just knocked off a gallon milk jug at 810 yds.The only combination I could come up with is 2650fps and .620 BC.Can my BC really be that far off from their claim?
This question was asked once already but I will ask it again. If I missed your response - my bad. Are you using a quality cronograph? I.E. Oehler?
My family owns a Oehler M33 and I find it much more reliable than my PACT Model 1-XP.
Good Luck, Don
My family owns a Oehler M33 and I find it much more reliable than my PACT Model 1-XP.
Good Luck, Don
No I don't have a good chronograph,that's the problem I am having.I am trying to manipulate the numbers to match my field tests.I am just not sure if the BC rating I came up with could really be that low.
Ernie
SPONSOR
Was their a large temp change between chronoing and shooting?
