bartholomew's acog ammo conversions

[taliv]

Bartholomew, I was googling for info on acog BDCs and came across this old info you compiled on arfcom (apparently 3 yrs ago)
http://www.ar15.com/forums/topic.html?b=3&f=18&t=229420

I was wondering if you'd updated any of that in the past few years for new models/reticles or gun/ammo configurations.

I think it would make an excellent addition to the reading library sticky here.

More specifically for me personally, I was wondering how you got the info. i.e. if there is a simple calculation could you share it? or is it experimental/measured?

I am about to purchase an acog, but can't figure out which one to buy because my preferred ammo (handloads using M193 pulldowns (~2900 fps from a 16" barrel vs 3132 fps for real M193)) obviously won't really match any of them.

according to my software, there's a drop delta of 3.5" at 300yd and 11.8" at 500 yd between my loads and standard M193 (given a 100 yd zero for both). so if your calculation was basically just trying lots of different combinations of "sight in x" high" until you found one that matched the BDCs closest, then I can do that, but I thought you might have had a sexier method.

(I suppose I could also adjust my loads to match the M193 velocity, but i prefer the somewhat milder load, which is further slowed slightly by a suppressor), and I'm thinking if I can't use the BDCs, then the only advantage an acog has over the aimpoint i'm using now is the magnification/BAC and maybe i'd be better off with something else. but i'd really like to make the BDCs work, even if it means memorizing some holdovers and a funky zero.


thanks

 

[Bartholomew Roberts]

No, that hasn't been updated and unfortunately there wasn't much sexy about it. That chart represented probably about 10 hours of work, so it is unlikely to be updated any time soon.

There has only been one new ACOG BDC introduced since that came out - the TA31 RCO A4 reticle that is designed for 62gr from a flattop 20". All of the other reticle designs use either the original 55gr BDC or the M4 BDC.

The chart also has some errors in it. To name a couple, the original version of the chart was figured using our best guess for the BDC and is off just slightly from the actual figures used. Many of the original values are also figured in yards vice meters. In both cases, it makes a difference but it is a difference that so few people are capable of appreciating that I never corrected it. I figured if you shot well enough to notice that difference, you certainly didn't need my advice

so if your calculation was basically just trying lots of different combinations of "sight in x" high" until you found one that matched the BDCs closest, then I can do that, but I thought you might have had a sexier method.

Nothing sexy... you hit on the exact method. First you figure out the parabola described by the BDC on the ACOG, then you change the distance of the initial sight in height until it matches that parabola best. I started by entering the data for the BDC into a ballistics calculator and then recording the drop every 109yds (100M). I then try to find the sight in distance that best changes the parabola to match those drop figures. Usually it is right around 1" high at 100yds (once you've done that 30 times or so, you start to get a good knack for guessing where to start).

 

[taliv]

thanks for the info. the TA55 is the one i've had my eye on, so i guess it also uses one of those two bdcs. the RCO is interesting, but too hard to come by.

 

[Bartholomew Roberts]

Actually, you might want to verify with Trijicon on the TA55. I am not sure what reticle it uses - I was thinking primarily of the 4x32 and 3x35 ACOGs that are more common.