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.
Reply With Quote
Bookmarks