Brunton 3x9x40 BDC reticle question

[pandjbox]

I recently bought a Brunton 3x9x40 Echo rifle scope with the BDC reticle. So far I like the scope however I think the BDC MOA values for the three bars implied by the scope literature are way wrong. After calling Brunton (no help partly because they are out of the business now) and quite a bit of head scratching, I put a yard stick out at 100 yds and measured the distance from the cross hairs to each bar. I measured approximately Bar 1 - 3in, Bar 2 - 11in and Bar 3 - 23in which about double what the literature implies. Does any one out there have better values or a better way to measure the values? I know in the end I'll do the shooting but I'm hoping good values and a good ballistic calculator will get me started. TKS much in advance.

 

[7magcreedmoor]

To accurately measure your reticle subtensions, you need to measure the distance as exactly as you can. If you are at 100 yards precisely, put up a target with markings on it that are spaced 1.047 inches center to center (1 minute of angle at 100 yards). If the distance is other than 100 yards, you must adjust the size of the test target accordingly. The scope (on a rifle or off) must be rested as solidly as you can make it. Turn the scope magnification to maximum (most bdc units are specced at max mag) and check your reticle lines against the target markings. It is a good idea to have half and quarter MOA intervals as well. I would then play with the mag settings and find (and mark on the mag ring) where the intervals are doubled, tripled etc. I am not familiar with the Brunton you have, but it is probably a second focal plane reticle, which means the apparent size of your reticle markings will change against the target when you shift magnification. If it does not, then you have a first plane unit. I have measured my various reticles in this fashion as I acquired each one, and also performed the "box-test" to verify the accuracy of the turret adjustments on each scope as well. If the scope reticle isn't what the literature says, that isn't a problem once you have measured it and know what it actually is. Same thing applies to turret tracking. If you have parallax adjustment, the same thing applies to those markings also.

 

[bruce_ventura]

[quote="pandjbox;1019989"...After calling Brunton (no help partly because they are out of the business now)...[/QUOTE]
Do you know if the entire company is out of business, or did they just terminate the optics products?

[QUOTE="pandjbox;1019989"...I put a yard stick out at 100 yds and measured the distance from the cross hairs to each bar. I measured approximately Bar 1 - 3in, Bar 2 - 11in and Bar 3 - 23in which about double what the literature implies...[/quote]
Yes, those holdoffs are too big by about a factor of two. That could happen if the magnification was set to 4-5X instead of 9X. Or, it could be a poor reticle design.

 

[pandjbox]

TKS much for your response. I now have confidence in the method, I just need to work on accuracy.

 

[pandjbox]

I did do the measurements at 9X. For the 270 I have the scope on, the holdoffs give me roughly on target at 400, 700, and 1000 yds. Is that out of line with other brand BDC scopes? TKS

 

[7magcreedmoor]

I did do the measurements at 9X. For the 270 I have the scope on, the holdoffs give me roughly on target at 400, 700, and 1000 yds. Is that out of line with other brand BDC scopes? TKS

Those are some very widely spaced intervals. Are those drops you mention from a 200 yard zero? You could opt for a 100 yard zero and then work out what mag setting gives you the drop for 300, 400, etc., but it will be rather vague and I would never bet an elk on it. If you have a "normal" range hunting rifle you could just zero that scope based on "maximum point-blank range" and let it be what it is. For LR hunting, I think I'd want to move on to a different optic.

 

[bruce_ventura]

Most BDC reticles are designed to have the crosshair at 100 yds, the first line below it at 200, the second line below it at 300, etc. personally, I would not use a BDC reticle beyond 500 yds.

 

[pandjbox]

The 270 I have the scope on and my 300wm are sighted in for 250yds. BDC bar ranges were based on a 130gr bullet @ 3100f/s. I often shot at the range out to 500yds and feel comfortable hunting at that range depending on the rifle, game and situation (sturdy rest, low wind, time). I must admit that I bought the scope primarily for the cost, power, and Brunton name and the BDC presented and interesting potential which I'm just beginning to explore. Without a range finder and time on the range at the longer ranges, I probably won't use BDC bars 2 and 3 for any thing but the range. I appreciate your comments.

 

[sscoyote]

Something definitely sounds a bit fishy with those subtensions. I've seen that optic before and the stadia didn't appear to have that wide of a spacing. While you're at it measure and apply the plex "windage" stadia for windage apps. too.

 

[pandjbox]

Is the "plex "windage" stadia" the horizontal lines associated with each bar? And I assume I'd measure them in basically the same way. And if so, how do I determine what wind speed/angle to bullet path that equates to? By the way, I had a chance to look at a Brunton 3x9x40 Eterna rifle scope which is similar to the Echo I have but a higher end scope. The Eterna literature shows Bars 1, 2, and 3 at an MOA of 3.5, 11.5. and 23.5 (and horizontal MOA values 2, 4 and 8). A scopes side by side look at the same object shows the Eterna MOAs are clearly slightly bigger than the Echo. So to me, it's another data point to say my measurement of the Echo MOAs are in the ball park.

 

[sscoyote]

I saw that Barsness did a review of the Eterna and his subtensions had to have been far different, since the 3rd line down was basically a 400-yd. zero from 200 at x-hair axis with a 30-06, which would be far less than 23 MOAish.

I didn't realize that reticle was a tree-style BDC reticle (it is different than the older Bruntons i saw). When i use a reticle like the Burris Ballistic Plex i apply windage with the plex post tips, for a windage system, usually at 10 mph full value calcs.

 

[pandjbox]

Thanks much for pointing out the article by Barsness on the Eterna. It does, however, leave me with more that I can't explain. Let me summarize what I think I know. 1) The Eterna literature shows bar values of 3.5, 11.5, and 23.5 MOA

(a jpeg picture would be here if I knew how to create it as a URL. I would be glad to send it as an email attachment if you would like to provide an email address)




2) The Bar values are consistent with the values I measure for my Echo scope and a side by side comparison with the Eterna and Echo looking at the same object. 3) My measurement of the first Echo Bar value is consistent with a side by side comparison with my Leupold 3x9x40 VII with a spec value for the distance from the top of the broad part of the lower post to the crosshair of a 3.1 MOA. Based on range experience (the Leupold is on a 300wm) out to 500yds and an elk kill at 525yds, I believe Leopold spec value.



From the Echo literature



For example: A 30-06, shooting a 168 grain Sierra Match King with a muzzle velocity of 2700 fps would have the following ballistic drop.
Vertical Sight Reference Distance Estimated Drop
Sight-in (Main Cross-Hair) 200 Yards 0 "
Bar Number 1 300 Yards -3.5"
Bar Number 2 400 Yards -24"
Bar Number 3 500 Yards -48"

This is assuming the rifle scope was "Zeroed" at 200 yards.

This implies (I think) MOA values of approx. 1.2, 6, and 9. When I run the above bullet parameters through the online JBM ballistics calculator, drops at 300, 400 and 500yds are 8.6, 25, and 50.9in. So I'd give Bar 2 and 3 a "consistent" grade but not Bar 1. When I run the above bullet parameters through the Brunton calculator which gives the distance when the bullet crosses each Bar but not the value of the drop below LOS, it gives distance values of 247, 307, and 372 yds for Bars 1, 2, and 3. This is very consistent with the Barsness article but very inconsistent with the Brunton Echo literature. And when I look at the MOA values for the 247, 307 and 372yds from the JBM calculator I get approx. an MOA of 1.3, 2.7. and 6 which is inconsistent with the Echo literature and very much with my measurements.



So the conclusions I'm coming to are that the Brunton calculator has the wrong MOA values in it for the bars (which a Brunton representative essentially admitted) and at some point the BDC MOA design values changed. I'd be very interested in hearing your assessment. Sorry for the almost OCD response but as a physicist, engineer and avid hunter I am somewhat perplexed.

 

[sscoyote]

P--I looked at a website that sells that optic and the diagram of the reticle looks like there is a disproportionate increase in the subtension between stadia 2 and 3...BUT i've seen those sites even have the wrong reticle advertised. And since you are an engineer, i always defer to you guys when it comes to math apps., no question.

If it's true that the last stadia line in that reticle is 23 MOA or so, i'd call the rep back and see if he can get you one that has a better BDC subtension system (i bet they screwed up the reticle and then made one that's more in line with a normal trajectory). Trying to interpolate accurately between the 2nd and 3rd stadia lines is gonna be quite the challenge.

 

[pandjbox]

When I talked to the Brunton rep, he indicated that he would go to the range or put out a measured target to check the Echo BDC Bar values but as a result of Brunton getting out of the optics business, they had emptied the warehouse of all scopes. So probably no trade. And I agree totally with your observation that interpolating between Bar 2 and 3 would be difficult. The engineer speaking, not an experienced beyond 500yds shooter, I think errors in estimated range and interpolation errors would make the errors on target unacceptable for hunting. In the end if the scope doesn't break, I think I'll be very happy with it and now will be comfortable using it out to 500yds. And by the way, Brunton has indicated that it will no longer warrantee the scopes sold after a certain date. I bought the scope TWO days before that deadline.

Pure speculation, but nearly all of the discounted Brunton 3x9 scopes I've found for sale online are with a BDC reticle. It makes me wonder if they knew they had an issue.