Turret tape inputs explained
Every number the builder asks for, what it means, where to find it, and why it matters. Your tape is only as good as these.
A turret tape is just math made physical. It turns your load's trajectory into yardage marks on your dial, which means the tape can only ever be as accurate as the numbers you feed it. None of these inputs are mysterious once you know what they are, so here's the complete, field-by-field guide.
1. The basics: rifle, caliber, ammo
Rifle and caliber are just labels. They print on the tape and help you keep things straight if you shoot more than one rig. Ammo is the load name. You can type it, or pick a bullet from the database, which auto-fills the ballistic numbers below. These don't affect the math, but accurate labels save you from sticking the wrong tape on the wrong scope.
2. Ballistic Coefficient (BC)
BC is the single most important number for how your bullet flies downrange. It measures how efficiently a bullet overcomes air resistance, or how "slippery" it is. A higher BC means the bullet sheds less speed, drops less, and drifts less in the wind. A low-BC bullet bleeds velocity fast and drops hard at distance.
You don't calculate BC. The bullet manufacturer publishes it, so find it on the box, the manufacturer's website, or the load data for your projectile. It's a small decimal, usually somewhere between about 0.2 and 0.8. Enter the number exactly as published, and pay attention to which standard it's referenced to. That's the next field.
3. Drag Function: G1 vs G7
A BC only means something relative to a standard projectile shape, and there are two common ones:
- G1 is referenced to an older, flat-based, blunter shape. It's the most commonly published BC and works fine for shorter, flat-based bullets and shorter ranges.
- G7 is referenced to a long, sleek, boat-tail spitzer, which is what most modern long-range match and hunting bullets actually look like. For those bullets, a G7 BC stays more consistent across the bullet's whole speed range and usually predicts long-range drop more faithfully.
The rule that matters: match the drag function to the BC you entered. If the manufacturer gives you a G7 BC, select G7. If they give a G1 BC, select G1. The two numbers are not interchangeable. For the same bullet, the G1 BC is noticeably larger than the G7 BC, often roughly double. Enter a G1 number but select G7 (or the other way around) and your tape will be wrong. When a maker publishes both, a true G7 BC is usually the better choice for a modern long-range bullet.
4. Grain Weight
Bullet weight in grains. It's mostly used for downrange energy figures, and it's part of how you identify your bullet. Use the actual weight of the projectile you're shooting (for example, 140 gr).
5. Muzzle Velocity (fps)
How fast the bullet leaves the barrel, in feet per second. It's the starting point for the entire trajectory, and it's the input people get wrong most often, because the number on the ammo box was measured from a different barrel than yours. Chronograph your actual load if you possibly can. A velocity that's off by 100 fps can move your 500-yard mark by a foot or more. If you reload, use your measured average. If you shoot factory ammo, a chronograph reading from your own rifle still beats the box.
One more note. Velocity changes with temperature, because powder burns differently when it's cold or hot. If you chronograph in summer and hunt in deep cold, expect some shift. It's another reason to verify at distance.
6. Zero Range
The distance at which your rifle is sighted dead-on, where your point of aim and point of impact meet. The builder solves for the exact launch angle that puts you on at this distance, then references every come-up mark from there. Enter the zero you actually confirmed on paper (commonly 100 yards, though some hunters prefer 200). Be honest here. If your true zero is 100 but you enter 200, every mark is off.
7. Sight Height (in)
The vertical distance from the center of your bore to the center of your scope's tube, in inches. Because your line of sight sits above the barrel, this affects the trajectory, especially up close, and it affects the zero solution. For most setups it's roughly 1.5 to 2.0 inches. Measure it once with calipers or a ruler, from the center of the bore up to the center of the scope tube. It doesn't change unless you remount or swap rings, so you only need it right once.
8. The atmosphere, and why it matters more than you think
This is the part shooters underestimate. Your bullet flies through air, and the thicker that air is, the more it drags on the bullet and the more it drops. Air density isn't fixed. It changes with altitude, temperature, and humidity. The same load sighted at a sea-level range on a cold morning can hit noticeably differently at 9,000 feet on a hot afternoon. Up close it's irrelevant. Past 400–500 yards it can be the difference between a hit and a clean miss. The builder asks for three atmospheric numbers:
- Altitude (ft) is your elevation above sea level where you'll shoot. Higher means thinner air, less drag, and less drop. You can look this up for your location, and the engine works out your local barometric pressure from it automatically, so there's no separate pressure field to fill in.
- Temperature (F) is the air temperature where you're shooting, in degrees Fahrenheit. Cold air is denser and gives more drop. Hot air is thinner and gives less.
- Humidity (0–1) is relative humidity entered as a decimal, not a percentage. Half humidity is 0.5, not 50. It has the smallest effect of the three, but enter it the right way.
Don't have exact numbers? Reasonable estimates for your area get you close. And because conditions change, DialWRX lets you re-dial a tape you already bought for new atmosphere for free. Build it for today, regenerate it for that high-country hunt later. Want the deeper version of how this feeds the math? See how the DialWRX ballistics engine works.
9. Your scope's turret
These tell the builder how to lay marks around your specific dial.
- MOA or MIL is the unit your turret adjusts in, and it must match your reticle's unit. Not sure which to run? Read MOA vs MIL: which turret should you run?
- Clicks Per Unit is how many clicks make one full MOA or MIL. Most MOA scopes are 4 (1/4 MOA per click) and most MIL scopes are 10 (0.1 MIL per click). It's printed on the turret or in the manual.
- Units Per Rotation is how many MOA or MIL one full turn of the turret covers (commonly 15–25 MOA or 5–10 MIL). This tells the tape where the dial wraps and whether your longest shots land on a second rotation.
- Turret Diameter (in) is measured across your elevation turret. Calipers are easiest. Otherwise wrap a strip of paper around the turret, mark the overlap, measure that length, and divide by 3.1416. This is what sizes the tape to wrap your dial exactly, with no trimming.
10. Range and layout
Max Range sets how far out the tape goes, and the spacing between marks controls how busy it looks. There's also an option to trim transonic marks. Beyond where your load slows toward the speed of sound, drop predictions get unreliable, so trimming keeps fantasy numbers off the far end of your tape. The rest of the appearance options (colors, tape height, wraps, yard or meter labels) are pure style and never change the math, and you can restyle a purchased tape any time, free.
11. Print at 100% and apply
Your data is only useful if the tape comes out the right size, so print at 100% scale. Never use "fit to page," which shrinks it and ruins the wrap length. For the toughest tape, print on vinyl sticker paper (a few dollars at Amazon or Walmart) so it peels and sticks right on. On plain paper? Lay a strip of clear tape over the print before you cut it out. Either way, wrap the strip snugly around the elevation turret and finish with a small piece of clear tape across the ends so the seam won't lift in the field.
Build it free, then trust it
Enter your numbers, preview the tape free, and only pay when it's exactly right. Get the inputs honest, especially velocity and the atmosphere, and your tape will dial true the first time.
Related guides
How the DialWRX ballistics engine works