Accuracy Testing a Rifle, Part One
July 14, 2016
Every month, Guns & Ammo tests rifles for accuracy. The results are a pretty common source of quantifiable information that our readers have expressed an interest in, so we make sure to put our best effort into getting you the most comprehensive information possible.
Like any activity that gets a lot of time and attention, we've accumulated a library of testing techniques that work, and we've identified several pitfalls that shooters must avoid if they hope to ever know a rifle's true potential.
Accuracy testing begins with building a good shooting position. Too often, we see shooters so focused on shooting and what's happening at the target that they never learn how to create a solid shooting foundation. Without that foundation, we introduce variables into our marksmanship equation that degrade our overall performance.
Where possible, we're going to try to avoid confining the conversation to testing equipment and focus on teaching shooting-position principles.
The Big Three
There are three main principles associated with optimal shooting positions: Get stable, address pre-ignition vibration and test your position. There's a conversation associated with each, but as long as we understand and apply these three principles, we're well on our way to really knowing how our rifles will perform and not limiting that performance with our own inability.
When I was growing up, Pops always taught me to rest the rifle's forend on solid objects when possible. Wise council, that. A solid support under this area of the rifle is widely recognized as a key component to any good shooting position.
Not all forend supports are created equal, however. Once, I was out in Texas on a sheep hunt when I noticed one of my fellow hunters using his backpack under the forend to confirm his zero and compare group sizes prior to starting the hunt. The setup wasn't a bad one, but the V-shaped forend allowed the rifle to wobble unless it was held in place with muscular tension.
Not surprisingly, the rifle was zeroed just fine, but it wasn't shooting quite as well as the others. This wasn't the rifle's fault; the position just wasn't as stable as it could be.
When we do accuracy testing here at G&A, we want our position to be so stable that the rifle stays in place even if we walk away from it. Simply laying the rifle across supports is definitely better than offhand shooting, especially in the field, but it will cost us tenths of an inch when we're accuracy testing. Our goal is to eliminate everything that costs us even one-tenth of an inch, and it starts with the forend support.
Good forend supports will come in the form of either sandbags or bipods. Benchresters and F-class shooters have some amazing setups, but these are specialized devices that probably cost more than we're willing to spend.
Sandbags are my preferred forend rest because they not only greatly assist stability, they also do an excellent job of eliminating pre-ignition vibration (we'll talk more about that later). Good forend sandbags should make as much contact with just the forend as possible and wrap around the sides of the rifle to prevent it from wobbling; eight inches of contact is better than 2 inches.
Sinclair International has an excellent selection of sandbags for those wanting a finished product or who accuracy test often. For you economy and casual shooters, a pillowcase half-full of sand is one of the best rests I've used (check with "the boss" first so you don't accidentally grab one of her "good ones").
Bipods are also a solid choice for accuracy testing because they firmly attach to the rifle and keep it from wobbling around, regardless of the forend's width or shape. As long as the bipod's legs are on dirt, grass or a padded surface such as a shooting mat, they can help us produce exceptional accuracy. Bipod legs placed directly on concrete or wood (even if the legs have rubber feet) will open group sizes anywhere from .2 to .7 MOA due to pre-ignition vibration.
The most neglected aspect of accuracy testing is the rear bag. It is an absolutely essential item and is mandatory here at Guns & Ammo. Any rifle without a rear bag must rely on the shooter's muscular tension to aim, and muscular tension introduces error.
My favorite rear bags are wedge-shaped and allow elevation adjustments by moving fore and aft on the stock toe. A rear bag that also wraps up along the sides of the stock is beneficial because it helps kill some of the rifle's vibration.
The important concept to remember here is that the rear bag is essential and that its two main purposes are to give us stability and allow us to make fine point-of-aim elevation adjustments once we are behind the rifle. Never leave home without at least one.
Once we build our position, we begin the process of eliminating pre-ignition vibration (PIV) from the rifle prior to accuracy testing. PIV occurs when the trigger releases the sear and the hammer strikes the back of the firing pin (e.g., ARs) or when the firing pin snaps home (e.g., bolt guns).
In both cases, we have metal slamming into metal at a time when we need to be the most precise. PIV is what causes our crosshairs to jump when we dry-fire our rifle. We'll take a detailed look at the phenomenon in a later article, but for now we'll just focus on how to get rid of it.
Sand and the human body are good vibration dampeners. Forend sandbags that have lots of contact with the rifle and wrap up around the sides of the forend do a better job of eliminating PIV than smaller bags, and that's why we favor them.
When settling behind the rifle, we like to get so close to the gun that our relaxed body pushes the rifle forward into the sandbag or bipod. When using a bipod, this technique is referred to as "loading." The slight forward pressure should require no muscular tension to maintain, yet it does a great job of eliminating PIV. We can load a sandbag just as well as we can load a bipod. Loading is our first and best tool for eliminating PIV.
Likewise, the rear bag can also be called up to help eliminate PIV. Rear bags that allow us to pinch the bag into the sides of the stock help deaden vibration due to the increased contact between sandbag and rifle. This doesn't necessarily mean we need one of the rear bags that have "ears"; we can simply use the parts of the bag that squish up on the sides when the stock's toe sinks into it.
A rifle will tell us if it's unhappy with our position when we dry fire. G&A's rifleshooters like to crank the magnification up above 15X and watch for movement of the crosshair when the trigger breaks. If we're not loading directly down the centerline axis of the bore, our off-axis pressure will combine with PIV to make our crosshairs hop off our point of aim. Hopefully, the distance is small, but even a .2-MOA shift means we're not shooting to the full potential of the rifle/ammunition combination.
We should only begin accuracy testing once we've eliminated all reticle hop. If we can't eliminate reticle movement during dry fire, we're not really evaluating the rifle as much as we are the shooter. If you've never worked to get rid of reticle hop and have just focused on getting comfortable behind the gun prior to shooting, expect to see anywhere from a .2- to a 1-MOA improvement after applying these principles.