Bullet drop compensation dials (BDC dials)

[Guest]

NO!If you shoot at long ranges in excess of 600 yds the comeups in inches falls flat on its face at a thousand yards the inch calibration of turrets are off by several moa's. I do use Leuopold scopes that I have checked for accuracy of bullet impact per click of adjustement.

 

[Bart B]

[ QUOTE ]
NO!If you shoot at long ranges in excess of 600 yds the comeups in inches falls flat on its face at a thousand yards the inch calibration of turrets are off by several moa's. I do use Leuopold scopes that I have checked for accuracy of bullet impact per click of adjustement.

[/ QUOTE ]I guess I don't understand what you're talking about.

I've never seen a scope (or aperture sights, for that matter) whose adjustment knobs (turrets) are marked in inches. I don't even know of a scope maker who says their adjustments are marked in inches except to say they move impact a given amount for a given range but at any other range, they move more or less inches (or fractions thereof) per click. So I don't understand why you think they're marked in inches.

I've done a lot of shooting at 600 through 1000 yards with scopes. Never had a problem with their adjustments marked in MOA values.

 

[7mmRHB]

Bart this is what Old Bear is getting at. If you run a program like Exbal or Sierra there are differances between the MOA and the bullet path or trajectory in inches. They do not match when you are useing a scope that tracks in 1" per min., 1/4" per click increments. Just for an example, a chart printed out on exbal for 25-06 ackly shows 14.75 MOA at 1000 yards. The trajectory or bullet path column shows 154.6" of drop. To find the true (inches) MOA so it matches the scope you take the bullet path in inches and divide by the inches per click at 1000 yrds (154.6 div. by 2.5 = 61.84 clicks) now divide that number by FOUR to get your scopes MOA which is 15.46 in this case. Now go back and look at the MOA column on the same chart and it says you need to come up 14.75 MOA to hit your target. Thats 7.5 inches diference on a flat shooting gun. If you take the 14.75 MOA figure and times it by 1.0472 it comes out at 15.446 MOA which shows that they are stll useing 1.0472 on their calculations while most scopes are in 1 inch moa. Sierras program is the same. If you want to make the proper turret corrections you better be using some math unless you are shooting with a scope that is in true 1.0472 MOA. Depending on the gun the corrections at 1k can be 3/4 to 1 1/4 Min.-----7mmRHB

 

[7mmRHB]

I always hang a grid on the target board at 100yds (32"s long)to make sure the turrets are correct and repeatable.
--7mmRHB

 

[Jon A]

[ QUOTE ]
If you live at say sea level and your bdc is calculated for this alltitude and you book a Deer hunt in another state and the elevation of your hunt is 7500 feet,the difference in correction will be considerable....How do you account for this difference with a bdc?

[/ QUOTE ]
You print a new one. Man, you guys are all over Buffalobob's case like you didn't read what he said. Of course it isn't as accurate as carrying a computer with you in the field--which you ought to for extreme ranges--and nobody has ever said that it was.

But unless you are carrying a computer with you, there's zero difference in accuracy between a BDC and a pre-made drop chart you tape to your stock. They contain exactly the same information. I wouldn't use a pre-made drop chart for 1500 yd shots from a bench with a 60lb gun either...but not everybody here is doing that.

 

[Buffalobob]

Jon A

Thanks. I don't argue with people on the forum. I just say what I have to say and go on. I have built and used BDCs. People who haven't can say whatever they want to say.

BDC is an old technology, it works.

All that said I have three scopes with target turrets.

I didn't really realize anyone was giving me a bad time. People like to bash BDCs and Weatherbys a lot. It seems to pass for entertainment here. Other days people discuss whether one should shoot game animals with Sierra Match Kings. Then if that gets boring just say "Schmidt and Bender" real loud.

Have a nice day

 

[jwp475]

[ QUOTE ]
[ QUOTE ]
If you live at say sea level and your bdc is calculated for this alltitude and you book a Deer hunt in another state and the elevation of your hunt is 7500 feet,the difference in correction will be considerable....How do you account for this difference with a bdc?

[/ QUOTE ]
You print a new one. Man, you guys are all over Buffalobob's case like you didn't read what he said. Of course it isn't as accurate as carrying a computer with you in the field--which you ought to for extreme ranges--and nobody has ever said that it was.

But unless you are carrying a computer with you, there's zero difference in accuracy between a BDC and a pre-made drop chart you tape to your stock. They contain exactly the same information. I wouldn't use a pre-made drop chart for 1500 yd shots from a bench with a 60lb gun either...but not everybody here is doing that.

[/ QUOTE ]
First off my post was not in reply to bob's.
Secound in my opion you are talking talking about a drop table for each condition taped to target turrets,that is writen in yardage, I don't think that is the same thing as a BDC.As I undrstand BDc they have a cam set for a specific load at a specific altitude and distance.If you are printing a label apparrently you are tapeing them to target turrets.

 

[Bart B]

7mmRHB, you've enlightened me. Thanks for pointing out that different folks use different strokes (MOA values, sometimes 1.0472 inch, sometimes 1.0000 inch; for each hundred yards of range). I checked my Sierra manual and sure enough, it lists a MOA equalling 1.0472 inch per hundred yards; never noticed it before.

I've sent e-mail to both Sierra Bullets and Perry-Systems asking about this. I think this has opened a can of something; candy to some, worms to others. I'll share what they say about it. And I'm going to contact all the scope makers and ask them about their MOA standard they use; more info to share. I know what those using external adjustments used; it's the internal adjustment standard I'm interested in.

Old Bear (if you're reading this), I now understand what you were talking about. Coming from his point of view, it makes sense to me. Sorry 'bout not understanding. It would be nice if all the ballistic software folks would list the scope make/model adjustment standards so we could get correct data for sight settings.

 

[7mmRHB]

Bart ,it seems that it would be a simple thing for a ballistics program to give you both options so a guy doesn't have to sit and do math all day to get his charts worked out.

The reason I discovered this discrepancy in the first place was that sierras first program didn't have a MOA function. I had to figure out the math to get MOA to match the turrets for each scope I was working with. Then when I started looking at the new programs with the MOA function built in I noticed a large discrepancy between the two. A little math solved the riddle but I still have to make my charts the hard way. LETS get this changed !!!!

Almost all U.S. made scopes are in 1" MOA. How do we go about fixing the problem with the programs.

Another problem with Sierra Infinity is that when you go to the uphill downhill function they don't have the MOA option available.(bullet path in INCHES only)

This post is buried where not too many people are reading this or there would be more response. Maybe a new post is in order in Bullets barrels and Ballistics ----7mmRHB

 

[Jon A]

[ QUOTE ]
7mmRHB, you've enlightened me. Thanks for pointing out that different folks use different strokes (MOA values, sometimes 1.0472 inch, sometimes 1.0000 inch; for each hundred yards of range).

[/ QUOTE ]
Yeah, I think it's actually more common for scopes to have MOA adjustments in lieu of SMOA. But this thread should highlight the importance of people actually measuring what their scope is regardless of what is stamped on the turrets. Apparently not many do that. Just a small amount of error can add up after 80, 100+ clicks....

It's also a reason I like Excel sheets for calculations--so easy to customize. The one I use you can enter whatever your actual click value (that you've measured) is and save yourself a lot of math.

 

[Jon A]

[ QUOTE ]
If you are printing a label apparrently you are tapeing them to target turrets.

[/ QUOTE ]
You are correct. Actually, I'm not sure what an "official" definition of BDC would be. Basically the same in operation as a Kenton, Leupold or the like, except not a $70 laser engraved aluminum piece that you're stuck with forever...or until you buy another one. To me, most of the criticism of BDC's comes from that fact--you'd need a hundred of them for every rifle and nobody is going to buy that many. But sticky labels and tape are cheap and easy to change--just like the drop chart taped to my stock.

So for that type of BDC, unless you're packing a computer into the field (which I'd guess the majority of people here aren't), there'll be no difference in accuracy. And of course, this is an entirely different thing than somebody who buys, say, an M3 scope with a permanent BDC and asks what velocity he should shoot to make it "on" or somebody who sends the load data to Kenton or Leupold and expects to use the same BDC everywhere forever...quite different.

 

[jwp475]

[ QUOTE ]
[ QUOTE ]
If you are printing a label apparrently you are tapeing them to target turrets.

[/ QUOTE ]
You are correct. Actually, I'm not sure what an "official" definition of BDC would be. Basically the same in operation as a Kenton, Leupold or the like, except not a $70 laser engraved aluminum piece that you're stuck with forever...or until you buy another one. To me, most of the criticism of BDC's comes from that fact--you'd need a hundred of them for every rifle and nobody is going to buy that many. But sticky labels and tape are cheap and easy to change--just like the drop chart taped to my stock.

So for that type of BDC, unless you're packing a computer into the field (which I'd guess the majority of people here aren't), there'll be no difference in accuracy. And of course, this is an entirely different thing than somebody who buys, say, an M3 scope with a permanent BDC and asks what velocity he should shoot to make it "on" or somebody who sends the load data to Kenton or Leupold and expects to use the same BDC everywhere forever...quite different.

[/ QUOTE ]


This is exactly what I was trying to convey. A label tapped to target turrets has exactly the same precision as a drop chart tapped to your stock.

 

[Bart B]

[ QUOTE ]
... this thread should highlight the importance of people actually measuring what their scope is regardless of what is stamped on the turrets. Apparently not many do that. Just a small amount of error can add up after 80, 100+ clicks....

[/ QUOTE ]Amidst the process of tightening the lid on this can of wiggly worms, I got some feedback from Exbal and Sierra Bullets.

Exbal e-mailed me a short paragraph explaining that one trig MOA equals 1.0472.... inches at 100 yards. Yes, that's all they said; even when I mentioned that in my e-mail to them. No mention at all of my comments about iron (aperture) sight and the 7.2-inch base separation standard for externally adjusted scopes.

Sierra Bullets, on the other hand, did better. They reasoned that maybe the two guys behind their ballistics stuff had engineering backgrounds and maybe they made a bit of a mistake using trig MOA (TMOA) instead of the shooting MOA (SMOA) I mentioned. But these two were high-end engineering rocket scientists so they were used to dealing with standard angular units and all the trig functions related to them. But Sierra also made no comments about the base spacing standard for externally adjusted scopes.

'Twas suggested this thread end and a related one be put on the Bullets, Barrels & Ballistics section, but I think the one for Scopes & Optics would be better. That's the one related to sighting equipment where this thread's info fits into. I'm trying to think of a decent title.... should get one soon....

 

[7mmRHB]

Bart,It'll be interesting to see the new post and find out how many people were aware of this problem. Maybe we'll find out there are programs out there that offer more options, such as a column for TMOA and SMOA. Michaels program might already do that so I hope he jumps in and gives us his two cents worth. I opened this can of worms ,hoping that more voices might help in getting some changes made. Might just be foolish thinking on my part!!----7mmRHB