How to true the BC

[Ucsdryder]

I imagine this has been discussed before but a quick search didn't come up with anything recent!

I am using a ZEISS rf10x42 that calculates elevation and atmospheric conditions, into the ballistic calculator. I am always low on my shots past about 600 yards, give or take. A couple weeks back I was shooting at 1250 and was hitting a good 16" low. My muzzle velocity is on per my magneto v3 so I'm thinking the 180 eldm doesn't have quite the BC advertised. I guess the easy way would be to shoot at 900 yards and fidget with the bc calculator until it gives me the same mil correction. Is there a better way? I don't have access to another chrono and the magneto doesn't work downrange.

 

[Recon$$]

Can you true the rifle with that system?

 

[Ucsdryder]

Can you true the rifle with that system?

I don't know what you mean? That's what im trying to do. Assuming the ZEISS are giving me correct atmospheric conditions and the v3 is giving a correct MV, the only thing left is the bullets BC. I'm trying to figure out the best way to confirm that. If I manually drop the BC then of course the mil correction increases, my hold over increases, and I don't hit low. The alternative is to play with the MV but since I know that's accurate it seems like the BC would be the number to change.

 

[TheSouthpaw]

What value are you using for BC of the 180 ELDM? My library shows .703/.360 G1/G7.

 

[Recon$$]

Trueing the rifle. Means you shoot the rifle around where the bullet starts going transonic. So let's use a 20" .308 at 2700 fps. That is around 750 yards with a 175 smk. So instead of confirming your dope at 200, 300, etc... you simple shoot out to 750 and record the dope. Say you are off by .2 .3 mil. You go into the program and say at 750 the rifle actually shoots at 4.5 mil instead of 4.2. Then the program adjusts the rifles dope out to 750 to be exactly correct. You can do this past transonic etc...

 

[Ucsdryder]

What value are you using for BC of the 180 ELDM? My library shows .703/.360 G1/G7.

Depends on what program you're using! I've seen .796, .777, .714. I don't know what the program uses, if I remember correctly it was around .750. When I played around with the bc to get to 11.0 mil at 1240 yards I think I had to get down to around .690 but I'd have to double check.

 

[Ckleeves]

What's your twist? IME Hornady's published BC's are pretty accurate with their stepped BC's for velocity and twist rate.

 

[Ucsdryder]

1-8 out of a 20" 7saum. I am only starting out at 2770 which is why I think I might be a little low on the bc.

 

[338 dude]

I imagine this has been discussed before but a quick search didn't come up with anything recent!

I am using a ZEISS rf10x42 that calculates elevation and atmospheric conditions, into the ballistic calculator. I am always low on my shots past about 600 yards, give or take. A couple weeks back I was shooting at 1250 and was hitting a good 16" low. My muzzle velocity is on per my magneto v3 so I'm thinking the 180 eldm doesn't have quite the BC advertised. I guess the easy way would be to shoot at 900 yards and fidget with the bc calculator until it gives me the same mil correction. Is there a better way? I don't have access to another chrono and the magneto doesn't work downrange.

 

[Ucsdryder]

View attachment 369356

That's good stuff! If I'm starting at 2770 it isn't long before my bc is down closer to .700…or do I work off G7? Back to my original question, how do I true up my bc to match my actual rifle.

 

[257 STW]

Not sure if I`m any help, but I always use the G7 BC and make minor velocity changes. I shoot Bergers and their BCs seem to be pretty close. Seems to work for me. My .02

 

[Ckleeves]

I would work everything in G7, whenever I was forced to use G1 with the first gen Leica RF's I always had trouble truing. I would probably start with .357?

Make sure your zero is dead on and I mean dead on at 100 yards. If your impacts are perfect out to 500 then chances are your input velocity is correct so I would try not to cheat it much.

Try to find a big piece of steel you can see your impacts on then paint a horizontal line and shoot at distance (800+) and use your reticle to measure how low you are then start cheating your input to match what your seeing on the target. Then try other distances closer and further to double check.

It also might be worth doing a quick check on your scopes tracking to make sure it's not tracking a little on the weak side.

 

[338 dude]

That's good stuff! If I'm starting at 2770 it isn't long before my bc is down closer to .700…or do I work off G7? Back to my original question, how do I true up my bc to match my actual rifle.

A G-7 more than likely represents your bullet better, a G 1 would be more of a flat base bullet where a G7 is a more aerodynamic rifle bullet with a boat tail

 

[338 dude]

That's good stuff! If I'm starting at 2770 it isn't long before my bc is down closer to .700…or do I work off G7? Back to my original question, how do I true up my bc to match my actual rifle.

I run my own custom curve from stepped velocities exactly as printed in this book simply adjust your velocity and everything should match up. The ballistics app Shooter allows you to set up and run multiple Bc's others will not.