Getting Your Inputs Squared Away Part: 5
Let’s start now by determining what we need to about the weapon system.
Rifling Twist: If you don’t know already this will have to be identified or measured. Identification is pretty easy nowadays. Most people are shooting off-the-shelf rifles by major manufacturers who advertise their twist rates. AR shooters will very often find the twist rate marked on the barrel somewhere. If neither of these are there for you then take a cleaning rod and set up a patch jag with enough patch material that it fits really snugly in the bore. Mark the cleaning rod at the top with white out so you can tell when it has turned completely 1 time. Get the patch started and push it all the way in. Now slowly pull it out allowing the rod to rotate. When the mark makes one complete revolution mark the rod again. Remove the cleaning rod and measure the distance in inches between the two marks. That number is your twist rate. If you have 8:1 then add “8” in the box. If you have 12:1 then add “12” to the box.
Scope Height: Getting this right is important for a number of reasons. It’s best to use micrometer or some similar instrument that will give you accurate readings down to .01 inches. Measure from the centerline of your scope to the centerline of your bore. You could do the centerline of your scope to the centerline of your bolt as well. How ever you do this step you want center to center vertical distance. My scope centerlines tend to sit 2 inches or less above my muzzle. On AR-10/15 platform rifles with carry handles or PEPR type mounts that number can get up near 3 inches. Keep your scope low if you can.
Rifle Weight: This isn’t as critical but if you want something approaching a useful measure of recoil energy then get it as close to perfect as you can. My rifles vary in weight from 6lbs to 18+lbs.
Reticle: Do you have a ranging reticle? It’s a good idea to use a ranging reticle because it gives you the ability to hold-off minor adjustments that you don’t have time to dial in. All reticles have something in them that will obscure (subtend) some portion of the target behind it. You can measure it or you can check with your scope manufacturer for details. You can even ask me and I’ll help if I can. If you have an MRAD subtended reticle (Mil-Dot, Mil-Quad, G2DMR, etc…) then add a “Y” to the box. If you have a ranging reticle that is set up to subtend minutes of angle then leave the box blank.
Turrets: If your scope turret adjustments are in MOA leave the box blank. If you click off MRAD’s then add a “Y” to the box. This is critical. If you don’t know, ask the scope manufacturer or even me. I’ll help if I can.
Boat Tail Bullet: This dramatically affects the velocity decay rate and actually dopes the ballistic coefficient. If you’re using a flat base bullet or hollow base bullet then leave this blank. If you’re using a proper boat tail bullet then add a “Y” to the box.
Now you’re almost there. The only things left to deal with are the real core ballistics elements. In order to get Part 6 done you’ll need to be ready to get out to the range for chronograph work before you can attempt the next part so grab up 50-100 rounds of your favorite long range ammo, get yourself a small cooler and get a nice full box of cheap-ish blasting ammo. Take that blasting ammo to the range and get your rifle zero’d at 100 yards (that’s my recommendation, see below for more on that). If anything isn’t right, you need new rings or whatever, get that done. Your weapon system has to be field ready before we move on. Once we finish the next bit and some range work you’ll be ready to get out and start making hits at long range. You’re going to need a chronograph for Part 6. They’re not that expensive. 80 bucks at Midwayusa.com for a Chrony. You’ll also probably need a tripod that you can mount your chronograph on. A decent tripod is 20 bucks. A great one is infinity dollars. Pick one that you like.
A note on Zero Range: 100 yards is fine for most people. I shoot out to a mile with a zero range at 100 yards. Military combat snipers may find it more useful to zero a 7.62x51mm NATO rifle at 400 or 600 yards but that is not due to any desire for precision. Combat means fast fast fast and the targets shoot back. For civilian shooters we’re going to dial a solution more in the way an assassin might operate. We’re not in a rush for the most part. Setting up for 100yrds will let you zero at a range you can walk to quickly and that you can see well from the bench through the scope. There are other benefits. Suffice it to say, my recommendation for civilians is 100yrd zero.
Stay tuned for Part 6.