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Mil Dots and Elevation Holds

12578 Views 14 Replies 7 Participants Last post by  m141a
I dont have all the information probably needed but in a nut shell im looking at purchasing my first Mil Dot scope to go on a nice .308 i have been eyeballing for awhile. Im planing on putting some HPBT Sierra 175Gr. rounds with a Muzzle Vel. of 2600 FPS. with a G1 Ballistic Coefficient of 0.496 through this rifle. Im trying to figure out the Elevation holds if i was to zero this rifle at 300 Yards due to my limited range access. I have been racking my brain for the past 2 weeks trying to figure all this out and im to the point where im just exhausted and frustrated on not being able to figure this out on my own. According to what i have found in my Ballistics book the PATH IN INCHES IS AS FOLLOWS : 100 yrds +2.2 , 200 yrds 0.0 , 300 yrds -9.1 , 400 yrds -26.1 , 500 yrds -52.4 , 600yrds -89.5 , 800 yrds -204.8 , 1000yrds -393.0
WIND DRIFT IN INCHES AS FOLLOWS : 100yrds 0.7 , 200yrds 3.0 , 300yrds 7.0 , 400 yrds 12.9 , 500yrds 21.0 , 600yrds 31.3 , 800yrds 60.2 , 1000yrds 111.2

If anyone can compute this for me and repost , as well as explain how you did it i would be very thankfulfor your time on this irritating issue im having. My book has an example with a .308 Fed. 168Gr. with a (600yrd Zero) as follows

yards Inches Mils
900 +158 +5.0
800 +88 +3.0
700 +36 +1.5
600 Zero Zero
500 -23 -1.25
400 -35 -2.5
300 -37 -3.0
200 -30 -4.0
100 -18 -4.5

Since the above chart is set for a 168Gr. bullet and zeroed at 600yrds , im only going to ASSUME that the round i want to use 175Gr. is heavier and that i want to zero at 300yrds there for the chart above would be incorrect. SO im asking for help on trying to figure this out cause my brain is fried , as much as i want a mil dot scope im just frustrated and about to give up with the mil dot idea. You all are my last hope on trying to "BREAK THIS DOWN BARNEY STYLE" for me. Thanks again
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The 175's behavior is pretty close to the old 173 grain FMJ in M118, but not exactly. I'm sure the military has some pretty exact tables for the 175 gr. SMK from m24's, since they use it in M118LR.

Going back to the OP, if I simply run ballistic tables in QuickTARGET, I can define each click of sight correction as having 3.375 moa. The output in clicks then gives me the necessary hold-off correction in mil dots in the scope reticule (assuming the magnification is correct for the reticule's calibration).

A couple of points: I ran this both with Sierra's published G1 BC's and velocity break points and with the G7 BC (VLD standard projectile). Since the projectile is neither a G1 form nor a G7 form and falls inbetween, I averaged results of these two to try to get a better calculation. Be aware it will not be exact unless the drag function for the 175 gr. SMK is defined exactly in your barrel.

I also ran the tables with the wind coming from both the right (first chart) and the left (second chart) for wind correction per mile per hour. That means you take those numbers and multiply by the speed of the wind in mph that you actually have.

Note that the correction is not zero for a straight on zero degree wind. This is due to precession of the bullet.

For the tables below, the muzzle velocity is 2600 fps and the barrel has a 10" right hand twist. Precession will change with twist and will reverse direction for a left hand twist. The sight zero is 300 yards.

Wind from Right:

Wind from Left:

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No. That's way too much. I set the software on autopilot and it looks like the default wind speed of 10 mph instead of 1 mph kept reasserting itself. That'll teach me to do this stuff in a hurry. I'll fix the table.

Nope. That's not what happened. As soon as I started to redo them I recalled what actually happened: I set out to do dot/mph, then realized it gave a lot of leading zeros that were hard to read, so I decided to switch to dot/10 mph and forgot to change the labels. Any way, I'll fix the labels.

Also note that they are zeroed at 300 (whatever the wind) and the tables correct off of that for the same wind speed. I think that may cause confusion? I think I'll redo them with total wind deflection instead, thus assuming the 300 yard zero is established in zero wind. Then you can go left or right of that as compensation calculators usually work.
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OK. This turned out to be quite a little adventure, uncovering no less than two bugs in QuickTARGET V. 3.6 that QuickTARGET Unlimited V. 3.6 does not share. As a result, until there is a fix, if you own these programs, do not trust QuickTARGET for windage calculations on projectiles with multiple G1 BC's, but run those in QuickTARGET Unlimited instead. It is more work to set up, but the calculations are accurate. I'll e-mail the bug data to Hartmut. The last time I found one in QuickLOAD he fixed it immediately for the next issued version.

So, here is the correct information, still averaged for the G1 and G7 models. The G7 may actually be the more accurate of the two because it is based on Litz's actual measurements from real guns of the bullet's velocity losses over a range rather than relying on Sierra's published information as the G1 BC's do. However, I stayed with the averaging because of the fit issue previously experienced. There is a maximum difference of 6.7" elevation at 1000 yards. By averaging, even if you assume one of the two is correct and the other is wrong, then the error is cut in half.

Note that one of the bugs I found is that QuickTARGET produces wind correction (in moa only) oppositely signed from QuickTARGET Unlimited's result. QTU is correct as far as mil dots in a scope are concerned. That is, if the wind blows from the right, assuming you make no click corrections, you will have to hold the reticule to the right, putting the target on the line to the left of the cross hairs. If the cross hairs define the X and Y lines of a standard Cartesian coordinate plane with 0,0 where the hairs cross, then the portion of the line held over the target to correct for wind is on the -X side of the center. Also, if you have to hold over, the vertical line below the crossing point is on the target, that is the -Y side of the line.

I also went ahead and added a decimal place so the windage corrections are truly in dots per mph. It looks uglier, but I'll leave it to you to multiply.

Based on the bugs I found, I would just ignore the first charts. They aren't way off but are off.

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