Open letter to BDC designers

[1168]

In the year+ that has passed since I first posted this rant, I’ve made more of an effort to learn about commercial and military BDCs, and the specifics of bullet flight. I’ve shot ACOGs, PA PLx, Steiner P4Xi, ELCANs, VCOG, some dialy scopes, and of course Eotechs and Aimpoints. I’ve done this with a variety of 5.56, 223, and 7.62 barrel lengths, projectiles, and types/brands of ammo. Regular guns and belt-feds. Played with few brands of calculators, quite a bit. I also shot a grenade launcher a few times, but its lessons don’t carry over to this subject.

***TLDR: Doesn’t matter what cartridge the optic manufacturer writes on the box. Its probably workable if you have a gun/ammo between 2500 and 3000fps. There’s a good chance the reticle doesn’t even exactly match the advertising***

Pretty much my entire adult life, I’ve known that any little thing will make a BDC useless. Barrel length, projectile, HOB (Height Over Bore), the number of floats in a Mardi Gras parade, apparent vs actual height of movie stars, moon phase, etc. Simply not true. Within reason, most well-made ones are adaptable. In fact, within some limitations, you can change ammo weights without touching the dials and be fine.

So, what are those limitations? What is reasonable? A BDC made for 300PRC (I don’t think that is a thing) will probably not work for 55gr Tula and an 11.5” barrel. But one made for 55gr/24” might work with 62gr/14.5”. Here, I am talking about killing E-types, B/C steel, or even F-Types. Hits are hits, from 100-600 meters or yards. If you’re shooting past that, or even routinely past 400, you should probably be dialing. Because wind. If you’re shooting tiny targets with high-precision ammo where a minute or two of drop or drift matters, you should probably be dialing. If you are shooting 2-4 minute (10+shots, most of us aren’t shooting 1MOA ammo, even if you think you are) ammo at 2 minute or more UKD (UnKnown Distance) targets, and time is critical, a good BDC is rather what you need.

WTF am I talking about? Given the size of targets, the change in temp after zeroing, the dispersion of ammo, etc, most BDCs are close enough to literally not notice on most guns with most ammo and almost all shooters. The arc simply does not change as much as we think, within the usable ranges of most combinations. As an experiment, use a ballistic calculator with the zero set to zero yds and the HOB set to 0 inches to compare some of your favorite loads. Set max distance to 600 and intervals to 100 in order to not be overwhelmed. Now, pick projectiles and use Litz BCs to compare them. For every step in bullet weight (55,62,69,77), use a delta of 0-200fps. So, in Sinister’s example, with a 77gr ammo zero, lets say at 200yds, with 2600fps, there are a lot of 55gr loads that will get you hits at the distances you would cheap out on, at like 2800-3000fps. This is just for math purposes. As an extremely general rule, 100fps steps gets you similar drop ballistics between weights (like dropping from 62 to 55gr), and are common differences. Extremely general. Notice that I said drop, not drift. So the dude that handloads to near max with 62 or 68gr projectiles might be able to find some ballistic similarities in cheap factory ammo. Which is what I have found.

Remember being a kid and learning to throw rocks? Did we weigh them?

So, how does one use this nonsense? Find an app that does this shit. As our barrels get shorter, and our bullets heavier, a little extra HOB seems to help. On my 12.5”s I’ve been a little surprised at how few inches exist between prediction and observation of impacts. Both the PLx 1-8 (1.94”) and the VCOG 1-8 (2.05”) with 55gr reticles have worked quite well for me in SBRs with 62-69gr ammo. And I’ve checked the PLx with garbage 55gr after zeroing with 69 or 68gr ammo, and found that it works to the limit of that ammo. I would suggest using a calculator that has reticles in it to see if they will work, and then truing them at longer range, or at least figuring out the best strategy. Because if I’m trying to shoot out to 600, a one minute deviation means more to me at 500 than a two minute deviation at 300.

Turns out, its all OK, because its all so adjustable to the actual arc. If you’re a Military guy, just ignore all of this and refer to TC 3-20.40.

[AndyLate]

I truly appreciate a long post that is worth the time to read it!

Andy

[One More Time]

My PA ACSS 1-8 does the best with 55 or 62 gr.
It's way off with with 5.56 77 gr.

I use Strelok+ that's pretty handy for seeing what the drops are and has a slider for 1-8 power that adjusts the reticle and ranges.
According to Strelok it's pretty close to the the other two when using 7 power.

So in practice you would range at full power and adjust to 7 for the correct drop for A zone hits.
I keep it simple and zeroed it for 62gr Mk318, 55 gr is close enough to not worry about.
Since 55 and 62 is what I shoot with this rifle it works out.

But for anything else I would rather have a tree type reticle for holdovers.

[1168]

My PA ACSS 1-8 does the best with 55 or 62 gr.
It's way off with with 5.56 77 gr.

I use Strelok+ that's pretty handy for seeing what the drops are and has a slider for 1-8 power that adjusts the reticle and ranges.
According to Strelok it's pretty close to the the other two when using 7 power.

So in practice you would range at full power and adjust to 7 for the correct drop for A zone hits.
I keep it simple and zeroed it for 62gr Mk318, 55 gr is close enough to not worry about.
Since 55 and 62 is what I shoot with this rifle it works out.

But for anything else I would rather have a tree type reticle for holdovers.

My PA 1-8 is FFP, so I can’t try that. What I’ve found with Strelok: When evaluating a reticle and ammo combo, I put in all the most accurate data I can, including HOB, which is usually around 3” for me. I find that using 1.93” mounts seems to make a lot of reticles work better with SBRs and heavier bullets than 1.5” mounts. Although, thats not why I use 1.93” mounts. BUT, for zero offset, I use a vertical of -2 or -3…that puts me low at most or every point in the reticle.

Then I look at the reticle, zoom in, and turn the virtual elevation dial until it matches at the longest range I plan to shoot at. This usually gets me really close at most of the other distances. Then I go back to data input and try different zero offsets to match that. What I end up finding is that if I get a perfect zero at 200, I’m completely off at 600. Thats probably where a lot of the “BDCs don’t work” comes from. But, if I’m willing to accept zeroing 2-4” high at 200, I’m getting less error from the scope as I stretch it.

As an example (near worst case), here’s a 77SMK at 2600fps, 3” HOB, 3” high at 200 on a silhouette at 600yds: F3E4B377-882A-4299-AA39-759BB6B89AE0.jpeg

Of course, with calculators, you have to shoot it to be sure, but it sure helps figure out where to start.

[One More Time]

14.7" barrel, 2.81" HOB at .5" high at 100 yards.
I noticed that the 77gr keeps up better until 800 yds.
55 and 62 seem to drop out around 500.

I suppose its not that far off.

Better on 7


If all I ran was 77's through it could work out.
I had it on my 18" for a bit that gets fed 77's but never ran it out past 100 yds.
That gun wound up with a better scope that took me a bit of looking around for a reticle I liked that didn't have a BDC which is a NF 2.5-10x42 with the MOAR reticle.
I liked the SWFA 2.5-10 but the BDC killed it for me for that gun.

Although I may end up getting it and use it on the 14.7" instead of the PA 1-8

[sinister]

In the year+ that has passed since I first posted this rant, I’ve made more of an effort to learn about commercial and military BDCs, and the specifics of bullet flight. I’ve shot ACOGs, PA PLx, Steiner P4Xi, ELCANs, VCOG, some dialy scopes, and of course Eotechs and Aimpoints. I’ve done this with a variety of 5.56, 223, and 7.62 barrel lengths, projectiles, and types/brands of ammo. Regular guns and belt-feds. Played with few brands of calculators, quite a bit. I also shot a grenade launcher a few times, but its lessons don’t carry over to this subject.

***TLDR: Doesn’t matter what cartridge the optic manufacturer writes on the box. Its probably workable if you have a gun/ammo between 2500 and 3000fps. There’s a good chance the reticle doesn’t even exactly match the advertising***

So, what are those limitations? What is reasonable? Here, I am talking about killing E-types, B/C steel, or even F-Types. Hits are hits, from 100-600 meters or yards. If you’re shooting past that, or even routinely past 400, you should probably be dialing. If you are shooting 2-4 minute (10+shots, most of us aren’t shooting 1MOA ammo, even if you think you are) ammo at 2 minute or more UKD (UnKnown Distance) targets, and time is critical, a good BDC is rather what you need.

WTF am I talking about? Given the size of targets, the change in temp after zeroing, the dispersion of ammo, etc, most BDCs are close enough to literally not notice on most guns with most ammo and almost all shooters. The arc simply does not change as much as we think, within the usable ranges of most combinations. As an experiment, use a ballistic calculator with the zero set to zero yds and the HOB set to 0 inches to compare some of your favorite loads. Set max distance to 600 and intervals to 100 in order to not be overwhelmed. Now, pick projectiles and use Litz BCs to compare them. For every step in bullet weight (55,62,69,77), use a delta of 0-200fps. So, in Sinister’s example, with a 77gr ammo zero, lets say at 200yds, with 2600fps, there are a lot of 55gr loads that will get you hits at the distances you would cheap out on, at like 2800-3000fps. This is just for math purposes. As an extremely general rule, 100fps steps gets you similar drop ballistics between weights (like dropping from 62 to 55gr), and are common differences. Extremely general. Notice that I said drop, not drift. So the dude that handloads to near max with 62 or 68gr projectiles might be able to find some ballistic similarities in cheap factory ammo. Which is what I have found.

Remember being a kid and learning to throw rocks? Did we weigh them?

So, how does one use this nonsense? Find an app that does this shit. As our barrels get shorter, and our bullets heavier, a little extra HOB seems to help. On my 12.5”s I’ve been a little surprised at how few inches exist between prediction and observation of impacts. Both the PLx 1-8 (1.94”) and the VCOG 1-8 (2.05”) with 55gr reticles have worked quite well for me in SBRs with 62-69gr ammo. And I’ve checked the PLx with garbage 55gr after zeroing with 69 or 68gr ammo, and found that it works to the limit of that ammo. I would suggest using a calculator that has reticles in it to see if they will work, and then truing them at longer range, or at least figuring out the best strategy. Because if I’m trying to shoot out to 600, a one minute deviation means more to me at 500 than a two minute deviation at 300.

Turns out, its all OK, because its all so adjustable to the actual arc. If you’re a Military guy, just ignore all of this and refer to TC 3-20.40.

Reviving a dead horse to refresh knowledge.

The new hotness the last few years is higher rings or mounts for a better heads-up position on the stock. I have since had neck fixation surgery as a result of decades wearing the Dome of Obedience while doing free-fall and static-line parachute jumps with ignorant loads.

Where standard 1.4-inch rings or Unimounts would have been "Normal" back in the 20th Century, fitting IR aimer-illuminators to the 12 o'clock of your Picatinny rail or tube fore-end might now call for a 1.93 or 2.3-inch (or higher) to get a clear view without your gadget (PAQ-4, PEQ-2, PEQ-15, Attilla, Vital, NGAL, etc.) blocking the lower 1/3 of your scope.

This may help stretch the service life of older scopes (the P4Xi, 1-6 Razor Gen 2, T5Xi, etc) as the slightly increased height, legacy BDC reticle, and evolving bullet lengths-weights norm-out closer with the higher Height-over-bore.

[Hank6046]

Reviving a dead horse to refresh knowledge.

The new hotness the last few years is higher rings or mounts for a better heads-up position on the stock. I have since had neck fixation surgery as a result of decades wearing the Dome of Obedience while doing free-fall and static-line parachute jumps with ignorant loads.

Where standard 1.4-inch rings or Unimounts would have been "Normal" back in the 20th Century, fitting IR aimer-illuminators to the 12 o'clock of your Picatinny rail or tube fore-end might now call for a 1.93 or 2.3-inch (or higher) to get a clear view without your gadget (PAQ-4, PEQ-2, PEQ-15, Attilla, Vital, NGAL, etc.) blocking the lower 1/3 of your scope.

This may help stretch the service life of older scopes (the P4Xi, 1-6 Razor Gen 2, T5Xi, etc) as the slightly increased height, legacy BDC reticle, and evolving bullet lengths-weights norm-out closer with the higher Height-over-bore.

You mention a rise in height over bore could lead to slightly increased accuracy with a BDC and I think that is interesting, I might have to test this out when we don't have fog, freezing rain and 2.5 feet of snow on the ground.

As you revive this old thread I think about one of the first real lessons I learned through a rifle class after getting out and that is that "Perfect is the enemy of Good Enough", I realize that my ACOG stadia lines aren't exactly going to be a perfect representation of where my round will go as I send it down range at 400+ yards, but I like the understanding that it might be a good reference point for to utilize when engaging targets anywhere from 300-500 yards.

[BobinNC]

Closed letter from BDC designers...

Mil/MOA reticles exist.

This ^^^