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Old July 30, 2012, 09:29 AM   #1
tpcollins
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How do you verify a BDC reticle?

I was thinking of getting a BDC type scope for my Ultra Slug with just 2 or 3 holdover lines/dots. I'd like to sight it in with the center duplex hitting at 100 yards. I was thinking of then shooting at the same 100 yards using the holdover lines so they should incrementally each hit higher on the target. I could input the bullet weight and velocity into a ballistics chart and keep adjusting the "zero yardage range" until the inches high at 100 yards lines up.

I'm not interested in finding a load to match the BDC, I just want to know what the yardage is for each line with the ammo I choose. Will this work or is there a better/different way? Thanks.
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Old July 30, 2012, 10:20 AM   #2
emcon5
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Sure, but be sure to reality check what the chart/program tells you. Charts don't always match real life.
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Old July 30, 2012, 10:53 AM   #3
Double Naught Spy
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There is no better way that actual verification via shooting.

Ballistics charts are nice. So do you know the exact velocity the round is leaving your gun? That would actually involve chronoing your ammo, so why not target check your POA and POIs?

I find that once you know your drop information for 2 given distances, then the ballistics charts are the most beneficial for use at other distances once you have at least the two points of reference. However for my shooting, the 2 points of refence usually cover most of the range I need.
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Old July 30, 2012, 11:12 AM   #4
wogpotter
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There is only one way.

Use the load you're going to use most & shoot at the indicated ranges, noting discrepencies (if any) from the charts. Charts are computer models only & don't exactly match real world conditions.

I have the Nikon BDC reticule & it is pretty good out to 300yds, but after that it diverges quite a bit.

Shoting at a fixed distance & calculating back the trajectory just gives you a different view of the computer model, again, not real world.
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Old July 30, 2012, 11:40 AM   #5
tpcollins
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Once I have an idea what the trajectory should do, I would verify those distances to be sure. I do have a chrono and a decent ballistics program. But the Lightfield slugs I use cost $15 for a box of 5 - I'd like to be able to find out what the trajectory "should be and is" at a minimum expen$e . . .
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Old July 30, 2012, 03:23 PM   #6
Double Naught Spy
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Quote:
Shoting at a fixed distance & calculating back the trajectory just gives you a different view of the computer model, again, not real world.
Yes and no. Unlike just using the computer model, having knowns removes some of the problems of solely using the computer program. Turns out, claims of manufacturers are not always 100% and so when you plug in their data, the results from the program can be of significantly from reality.

So you shoot a couple of known distances. If the results of the program along the trajectory do not match the reality of the two results actually obtained, tpcollins can tweak the parameters of the program to match the two known impacts and then the trajectory provided based on the two actual knowns should be very close to what will be the real world result, which sounds like it should be more than sufficiently accurate for the caliber being used within the short shot ranges. That information can be correlated with the reticle which can be field checked for its accuracy by noting distances between the lines at a fixed distance or distances.

I am all for shooting a target every 10 or 20 yards which I have done for 5.56 out to 200 yards. It is enlightening, time consuming, and in the end, the refinement wasn't amazing. The quarter and half inch differences really didn't turn out to be of any significance.

So it isn't absolutely real world every shot. You are correct. However, with two knowns, you can work out a realistic trajectory with the ballistics calculator that will likely be more accurate than the shotgun ammo, depending on the ammo and the rifle from which it is fired. The method works best with the known distances being longer, so for tpcollins, he might want to do 100 and 150 yards, compare with the trajectory program and everything from 150 and back should be very good and from 150-200 should be functional as well.

tpcollins, I have shot with 4 or 5 BDC scopes. Two I have shot with the correct ammo and rifles (resulting in the correct velocity and related variables). In the cases where everything matched, the BDC was reasonable, but far from perfect and certainly the scopes didn't change with the shooting conditions. The hotter the day got, the further off the scope was. That will hapen with any fixed BDC or normal reticle. The POI will change with changing conditions.

You change the ammo, gun, mounting height of the scope and things can be off significantly. The lines are just that, lines, when you don't have everything that is right (original parameters for the origical setup of the BDC), but that does not mean they aren't just as useful. You just have to recall that the 100 yard distance in the reticle is really accurate for 88, 200 is for 190, etc. (whatever your results are).

Here is a BDC reticle from a Trijicon ACOG with my POI at various distances using this with a Remington 788 firing Lithuanian .308 surplus ammo. I did verify every dot for the distance indicated. 200 yards was the actual maximum intended used distance and chances were shots were going to be inside of 100 yards. So my setup had everything happening withn 5 MOA on my scope, but I could put the top of the reticle on the same place of the animal (hog) out to 150 yards and get the desired result. The point here is that depending on the use, the computer statistics, if correlated with a couple of knowns, aren't apt to be off enough distance to matter for the shotgun situation. His POI spread will undoubtedly be somewhat larger, but to get the two knowns to match with the trajectory program, the trajectory track is going to have to be awfully close between reality and what is calculated.

What is interesting is that the donut (in red) is supposed to be 4 MOA. Verification of your BDC on the target with a ruler can be enlightening. It is closer to 5. The scope is actually for 62gr. M855 ammo fired out of a shorter barrel. However, on the previous gun using a 24" barrel, I was shooting Privi Partisan 69 gr. ammo and the POI dots shown here were all very close to 200 yards. I tweaked my cheatsheet image slightly for the new gun and ammo, but not much.

All this was even more confusing when realizing that the measurements of the BDC in the ACOG were problematic. The manual refers to distances in both yards and meters, introducing a possible error of about 9.4%. You can read Trijicon descriptions today about distances in meters, but give fields of view at ranges such as 100 yards. Adjustments are X clicks per inch at 100 yards, but the reticle is apparently in meters. How is that for wonderful?
http://www.uscav.com/pdfs/acog.pdf

I shot a Springfield scope, an older model, that had its reticle lines at every 2 MOA as I recall and they were actually closer to 2.7 MOA when put to the ruler at the target.

So if you don't want to shoot a lot because you have really expensive ammo, I would shoot 3 shots at 2 known distances and record the difference between the POA and the average POI of the group at each distance. Ruler verify that your reticle is correct as per claims or any measurement about which you will based making a shot. Plug in the chrono data and the BC for the ammo into your ballistic calculator along with all the other relevant data necessary and see if the calculator predicted accurately the impacts at the 2 known distances. If not, work backwards from the distances to determine your trajectory. As your BDC likely is not set up for your setup, you can make a numeric card cheatsheet to go with your setup or do like I do and make a picture version. It requires less cognition in the field to figure out where to aim for your actual sight picture, but the numeric method is the traditional method used by most hunters.
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Old July 30, 2012, 04:11 PM   #7
tobnpr
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I can't see a BDC reticle being useful unless it's calibrated for a specific commercial round (like M855), that you shoot.

Kenton makes custom engraved BDC turrets for many scope manuf's/models, as do others. If I were going that route, I'd chrono my ammo, and get custom turrets made to match my ammo, exactly-rather than half-assing it with a reticle that's not dead-on accurate.

Seems to me that having to make allowances/hold-overs, completely defeats the purpose of a BDC scope, whether it's the reticle, or the turrets.
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Old July 30, 2012, 05:26 PM   #8
tpcollins
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Thanks for the good comments but here's what I'm going to try. The ammo manf says the bullet/sabot I intend to use has an 1800 fps at the muzzle. I'll check it with my chronos to use that data. I'll sight the crosshairs POA to hit POI dead center of the bull at 100 yards. At the same yardage I'll use the first BDC line under the crosshair and shoot at the same 100 yard target. If the POI is 3" higher for example, I can play with the "Zero Range" on the ballistics calculator until I find a trajectory that is 3" high at 100 yards - which is 135 yards. So that sets the first line under the plex. Seems like I could do the same thing fo the next BDC line, see how high it hits at 100 yards also, and recalculate the "Zero Range" until I found what that yardage is.

Seems to me that would be a quicker way to find out what the distances are for the ammo I intend to use.
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Old July 30, 2012, 05:36 PM   #9
emcon5
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Honestly, for the calculations, you shouldn't need to fire a shot after the initial sight in.

Get a red sharpie, and a ruler, and mark 1" lines above and below your point of aim. Look at the target and determine the spacing in MOA, then run your numbers.

Once you have figured out where the math says they will hit, go shoot at that range and see if it matches reality.
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Old July 30, 2012, 05:58 PM   #10
tpcollins
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Thanks emcon5, that makes really good sense. I'm just a bit confused now whether the recticle's lines are going to change with magnification based on whether it's a first focal plane or second focal plane scope.
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Old July 30, 2012, 06:36 PM   #11
warbirdlover
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My Nikon with the BDC has you sight in at 200 using 9 power (3-9X). Then the others are on at 300, 400 etc. And it works.

You just have to remember to crank to 9 power when using the BDC.
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Old July 30, 2012, 06:38 PM   #12
Double Naught Spy
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That is interesting. I have only used fixed magnification scopes with BDCs. That would definitely get me screwed up, LOL.
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Old July 30, 2012, 06:41 PM   #13
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Nikons spot on software is pretty awsome. You can plug in every aspect of your weapon and I will say they are dang close. Close enough that I could buy a rifle and scope zero it at 100 do my homework on the spot on software and hit a 12" area at 500. Not perfect but its dang close for some basic computing
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Old July 30, 2012, 07:27 PM   #14
tpcollins
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Quote:
Nikons spot on software is pretty awsome. You can plug in every aspect of your weapon and I will say they are dang close. Close enough that I could buy a rifle and scope zero it at 100 do my homework on the spot on software and hit a 12" area at 500. Not perfect but its dang close for some basic computing
Thanks buckslayer, actually I just downloaded that Nikon Spot-On chart the other day. I went back to look at it again and things are starting to come together. Thanks for the reminder - duh.
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Old July 30, 2012, 08:00 PM   #15
Pat T
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Quote:
I'm just a bit confused now whether the recticle's lines are going to change with magnification based on whether it's a first focal plane or second focal plane scope.
TP-

Reticle remains in same position relative to lenses, image grows larger or smaller relative to reticle depending on power setting.

My particular load in my .30-06 works well at 8x in the summer, and equivalent yardages are at 7x in winter. This is due to velocity loss because of the colder temps. I've shot mine out to 600 yards with a 200 yard zero and the BDC and Spot On did a good job.
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Old July 31, 2012, 07:26 AM   #16
wogpotter
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Sorry, Pat that's with an American-style second focal plane scope only. With a European style first focal plane scope the reticule size varies with magnification. Thats one of the major differences between the two if you're deciding on a model .

Both have advantages, the second focal plane has a fixed reticule post thickness as seen, but the first focal plane has a fixed relationship between the reticule gradations & the target no matter what magnification is used. That's why the "Shepherd" scopes have the one shot zero ability they have 2 reticules, one in the first & one in the second focal plane.
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