Zeroing scope with BDC theory, feedback requested

[Fathead302]

I have a scope with a BDC reticle (Burris Tac30) that has holdover dots out to 600 yards. These holdovers are calibrated by the manufacturer for using certain projectiles (62gr) at a certain speed (3025fps), and zeroing the scope at 100 yards using the 100 yard center dot. I am loading my own 69gr SMK HPBT ammunition. When I run the numbers for my loads through the Hornady Ballistics Calculator, the "Come Up in MOA" differ from the scopes holdovers as follows:

Range(yds) Scope Hold Over(MOA) Come Up in MOA from calculator

100 0 0

200 1.1 2.0

300 3.3 5.0

400 6.1 8.8

500 9.6 13.3

As I have seen from the above table, the farther out in distance, the more the numbers differ. If I were to sight in the rifle and scope at the manufacturers recommended distance of 100 yards, the holdovers will become increasingly further off from true POA/POI the further out the targets become.

Here is where the theory comes in, and my request for feedback from the shooters on this sight who have undoubtedly more experience at shooting longer ranges than I do. Would the following be correct assumptions if I were to zero the scope at 300 yards, using the 300 yard holdover dot in the scope?

The holdovers for 100yards-200 yards would be off, but only marginally so?

The zero for 300 yards would be dead on.

And the 400 yard POI would be much closer to the holdover dot for 400 yards than it would have been with a 100 yard zero?

Or

Am I just thinking too much and I should learn that the 300yard holdover dot will actually be closer to a theoretical 250yards?

Thanks everyone!

Edited August 13, 2011 by Fathead302

[bigbear_98]

I took the MOA correction from each mark on the reticle in my XTR and compared it to the calculator. I then used the yardages that the calculator had for each correction instead of the factory set points. I came up with numbers like 329 and 456 but I'm not having to guess where the mark will hit. It was pretty true.

Original reticle

Edited August 13, 2011 by bigbear_98

[outerlimits]

Uae jbm ballistic calxulator, plug in yerdata and determine where to adjust yer zero to as closely match the reticle ballistics.

[MarkCO]

You also have to realize that the manufacturer data is for STP as well. The data varies some with humidity, temperature and barometer, so it will never be "perfect" unitl you hold all the parameters constant. For some powders, going from the 59 degrees to 100 degrees can result in a velocity increase of of 60 fps or more. Go up in altitude and dry out the air at the same time...your "perfect" BDC reticle is no longer...

Also, one of the far targets at RM3G shot like true distance (585 yards) some of the time, but I saw experienced shooters holding a 300 yard hold to get the hit just after a thunderstorm rolled in.

I use the MOA of the dots and run my data, or look at my data sheets, for each stage. During RM3G, there were differences of almost 2 MOA at 500 yards during the match. And yes I got my long range hits, some even in a downpour.

[Fathead302]

MarkCO, I don't know if you intended to, but you have blown my novice rifleman mind! This site is so informative, but sometimes I feel like I am back in my college physics class learning Quantum Mechanics!

If it wouldn't be too much of a trade secret to share, could you expand on some of what you said in your previous post?

When discussing RM3G and the different holds shooters were using, do you mean that at one time during the match all of the data you have that says where to hold for 585yards actually worked on a 585 yard target, but after conditions changed you would have had to hold on the 585 yard target more like it was at 300 yards? Would this information be discovered on the clock when moving your hold over and walking the rounds on target?

When you say "run your data" and "look at your data sheets" do you mean from a ballistics calculator as was posted earlier, or real world range data, or both? I ran the JBM Ballistics Calculator (thanks outerlimits)for my reticle and came up with this:

dot moa actual yardage

100 0 100

200 1.1 160

300 3.3 250

400 6.1 330

500 9.6 420

600 13.6 500

So theoretically if a target had been found to have an actual range of a little over 300 yards, I would actually be using the "400 yard" reticle? Is this what you mean by "using the MOA of your dots."

Thanks

Edited August 13, 2011 by Fathead302

[Jesse Tischauser]

Build your load then go shoot the rifle using the reticle at all distances out to 600. Then measure where the bullets are hitting versus POA. If you can add more or less velocity to make the reticle work for your load great. If not just know what distance each holdover mark in your reticle equates to. i.e. The 3rd hash is 422 yards not 400 but a center hold will be good for 410-435 on a 10" MGM flasher.

Edited August 13, 2011 by Jesse Tischauser

[MarkCO]

MarkCO, I don't know if you intended to, but you have blown my novice rifleman mind! (SNIP)

When discussing RM3G and the different holds shooters were using, do you mean that at one time during the match all of the data you have that says where to hold for 585yards actually worked on a 585 yard target, but after conditions changed you would have had to hold on the 585 yard target more like it was at 300 yards? Would this information be discovered on the clock when moving your hold over and walking the rounds on target?

When you say "run your data" and "look at your data sheets" do you mean from a ballistics calculator as was posted earlier, or real world range data, or both? I ran the JBM Ballistics Calculator (thanks outerlimits)for my reticle and came up with this:

dot moa actual yardage

100 0 100

200 1.1 160

300 3.3 250

400 6.1 330

500 9.6 420

600 13.6 500

So theoretically if a target had been found to have an actual range of a little over 300 yards, I would actually be using the "400 yard" reticle? Is this what you mean by "using the MOA of your dots."

Thanks

Sorry, did not mean to.

Yes, when I shot, my holds were dead on. My chart and real time data (I use a ballistics calculator on my phone that gets real data for the location) matched and I got first round hits. Yes, some competitors found out on the clock that their hold was not working so spent some time working it out and it was shooting more like it was at 300 yards than 585. Granted it was a large target, and one competitor who I know to be a skilled rifleman claimed he used a 200 yard hold on the head and got a hit. Seems there is a stage like that at RM3G every year. If you read about flying planes over dark timber and getting the thermals, it will give the best explanation as to why you can end up with lift shooting over some types of terrain, vegetation and water.

Yes, don't think of those dots as "yardages" think of them as defining a certain MOA. "Run your data" does mean using a ballicitcs calculator of some sort. BUT, like Jesse said, that is really only the start. In known atmospheric conditions, shoot your gun and your load at a variety of distances and compare to the data from the calculator. I shoot my loads out to 500 yards, on targets, to verify holds.

If you consider those dots to be merely a MOA indication, it should simplify your memorization and process.

[Fathead302]

Great info MarkCO, thanks for sharing.

What is the ballistics calculator that you are using on your phone?

[MarkCO]

Great info MarkCO, thanks for sharing.

What is the ballistics calculator that you are using on your phone?

I have Android, I use "Shooter"...very good.

[MarkCO]

In the FWIW category, most matches, you don't need to get too carried away. If you use a fast 55 grainer, you should be good to go for the VAST majority of matches with a little hold over here and there. Some matches, like Blue Ridge and RM3G stretch it out a bit so you do need to know your data from 300 to 600, be prepared for wind and be able to see what is going on and make adjustments if you happen to be one of the unlucky few who get caught in weird conditions.

I was shooting stage 1 of RM3G when one of the torrential downpours came, temp dropped 25 degrees and it was literally dumping. Never found the 370 yard target, but I could barely see the 425 yard targets and I held 7 MOA on the top and kept walking it up til I got a hit at 10 MOA.

[P.E. Kelley]

Build your load then go shoot the rifle using the reticle at all distances out to 600. Then measure where the bullets are hitting versus POA. If you can add more or less velocity to make the reticle work for your load great. If not just know what distance each holdover mark in your reticle equates to. i.e. The 3rd hash is 422 yards not 400 but a center hold will be good for 410-435 on a 10" MGM flasher.

Simple and effective!

I pretty much follow Jesse's thinking too.

BDC ABC BFD

Zero the rifle on one of the dots line or hash marks that YOU like and can reference.

Shoot to see where the other lines/marks fall and adjust load or re-zero to suit you.

In a perfect world having each mark line up with a even distance 100,200,300,400 and so on

is not all that helpful when our targets never quite follow that same pattern.

I like to have one of my lines/dots (usually the first one down from center) on the money at 300 and then check the rest to see where they fall in relation to my known 300 mark.

It works for me (when I am)

Good luck

Patrick

[Fathead302]

I have an iphone so no luck on "shooter", but I was able to find "Ballistic" which is based off the JBM ballistics engine that also uses current weather conditions for corrections. I will give that one a shot and see how it does.

Most of the matches I attend are local and utilize a 100 yard range maximum for rifles, but I will be attending the Ozarks 3 Gun Championship in September, and my poor performance from last years longer range stages still causes me to wake up in the middle of the night in a cold sweat!

I only have access to a 300 yard range to do testing on, so the info you have posted will help to get me in the ballpark when it comes to game day.

Thanks

Edited August 14, 2011 by Fathead302

[MarkCO]

Look at Post #10 in this thread: http://www.brianenos.com/forums/index.php?showtopic=133336&st=0&p=1501696entry1501696

Yep, it is for the new MTAC, but the Tac30 and the MTAC have the same reticle. I keep this in my range bag and under my pillow

[Jesse Tischauser]

300 is the max useful distance for most matches. I just found out that Rocky mountain requires knowing a good hold for 350-450 as well. You also have to take into account varying targets sizes. The norm target is the MGM flasher. It uses a 10" circle with a 5"x5" square head on top. So you have essentially 15" of vertical target presentation. At RM3G we shot mini IPSC shaped steel, R&R flashers and Metal man flashers all of which are different shapes and sizes. I was missing and ringing that SOBing splash guard on the R&R flasher at 365 and was doubled plugged and couldn't hear my misses hit the steal. Typically on an MGM flasher those would have been low hits.

Dial in that 100-200 main zero then get a solid 300 yard zero and know where the rest hit in all possible target size/shapes. At least that's what I'm gonna do from now on.

[Fathead302]

300 is the max useful distance for most matches. I just found out that Rocky mountain requires knowing a good hold for 350-450 as well. You also have to take into account varying targets sizes. The norm target is the MGM flasher. It uses a 10" circle with a 5"x5" square head on top. So you have essentially 15" of vertical target presentation. At RM3G we shot mini IPSC shaped steel, R&R flashers and Metal man flashers all of which are different shapes and sizes. I was missing and ringing that SOBing splash guard on the R&R flasher at 365 and was doubled plugged and couldn't hear my misses hit the steal. Typically on an MGM flasher those would have been low hits.

Dial in that 100-200 main zero then get a solid 300 yard zero and know where the rest hit in all possible target size/shapes. At least that's what I'm gonna do from now on.

I don't know if I want to attend a match that utilizes "Metal man flashers"