Accuracy Testing a Rifle, Part Two
September 07, 2016
In the first part of our accuracy-testing column, we covered the importance of building a good shooting position and how we do it here at Guns & Ammo. In this second part, we're going to talk about how we select ammunition and other ancillary equipment in our quest to squeeze every last bit of accuracy.
We favor testing rifles with match ammunition because we often don't have the time to develop optimal handloads for each gun, and match ammunition is, by design, intended to shoot well out of most rifles. We see no need to penalize a rifle by testing it with hunting ammunition that it may not like and then publishing the accuracy data as a comprehensive indicator of how the rifle will perform. It is unfair for both the rifle and the ammunition.
The distance between the bullet's bearing surface and the rifling is one of the biggest contributing factors to how accurate that load will be. Some bullets want to just kiss the lands, and others want to be .080 inch away (or more) for optimal accuracy. Many bullets are temperamental this way and will only shoot well once we find the distance they like. For a personally owned rifle, the best bet is to buy different boxes of ammunition and see what does the best while still providing the terminal performance we need. This takes time and a wide variety of loads before we find what our rifle prefers.
We favor match ammunition because the bullet's ogive is shaped to efficiently center itself in the barrel's throat. If it enters the rifling straight, it will also exit the muzzle straight and give us the best accuracy. This is why bullet manufacturers spend so much time and effort making match bullets. They do well in a wide variety of chamber dimensions and are the fastest way to get a good idea of how a rifle will shoot without handloading.
The absolute best bullets to shoot in a rifle for a quick accuracy test are match bullets with a flat base. The flat base gives a nice, consistent surface for the gas to push against as opposed to the cone-shaped boattail. Short, flat base bullets are much less likely to yaw in the throat than the longer, heavier bullets with their aggressive boattails.
We publish group sizes, extreme spread (ES) and standard deviation (SD) for each load tested. Group sizes are a popular and easily quantifiable takeaway that would be foolish to exclude. Including ES and SD is also a common practice, but we do so in the hopes that the reader doesn't overvalue this information.
It's important to remember that a 100-feet-per-second increase in velocity will only move a bullet's impact by about an inch at 350 yards. If you do most of your shooting inside 350 yards, don't sweat the ES and SD too much. In terms of rifle shooting, as much as I want to see low numbers in both of those categories, they really don't have much influence until we get out past 500 yards. We're better served by focusing our effort on finding what load our rifle shoots best (or creating our own) if we do most of our shooting inside 350 yards.
For those shooting factory ammo at longer distances, ES and SD have greater significance. Knowing this, G&A generally uses only Oehler 35P or MagnetoSpeed chronographs. We fire 10 rounds across the chronograph for each load to collect the ES and SD. We've selected these two chronographs for testing purposes because they're the most accurate when checked against Doppler radar. There are cheaper and/or easier-to-assemble chronographs available on the market, but the information they provide isn't as accurate. If ES and SD matter to you, use either the Oehler or the MagnetoSpeed to generate meaningful data.
Three or Five?
A sure way to start a fight with rifle shooters is to ask how many rounds should be fired for each group. Some say three, some argue for five, and yet others are adamant that we should all be shooting 10 rounds per group if we're serious about knowing our rifle's accuracy potential. Ten is ridiculous.
The truth is, the rifle decides how many rounds it should fire per group. Specifically, the barrel contour and chambering are what tell us how many rounds the rifle wants to fire.
Hunting rifles with their light barrel contours should shoot three rounds per group. There just isn't much steel to mitigate the effects heat has on the barrel once we start shooting. Heat is the great destroyer of a barrel's accuracy, and skinny barrels heat quickly. Once a barrel heats up, group sizes increase, sometimes dramatically.
If we insist on shooting five rounds per group out of a light hunting barrel, we're using that barrel for a purpose other than for what is was intended, so shame on us. Hunting barrels were never meant to keep the Mongolian horde at bay; they exist to give us one to three accurate shots at a time. Skinny barrels get tested with three-shot groups.
Tactical and competitive rifles have much heavier barrel contours, so these should get five rounds per group. The heavier barrel is much better equipped to handle the effects of heat, which is why tactical and competitive rifles have heavy barrels in the first place. If terrorists swarm the neighborhood, grab the rifle with a heavy barrel. These rifles can handle the heat, so they get five rounds per group.
We should also consider the rifle's chambering when we're deciding how many rounds to shoot per group. The larger the case capacity, the more aggressively we'll have to manage heat. For example, a .223 Remington burns about 23 grains of powder per cartridge fired. This doesn't generate a lot of heat, so we can get away with shooting five-round groups out of a barrel with a traditional hunting contour.
However, a 7mm Remington Magnum burns about 70 grains of powder per cartridge fired. That's a significant amount of heat for all but the heaviest of barrels. Getting consistent three-round groups out of a hunting contour with this cartridge will require patience and time to let the barrel cool.
The overriding principle with how many shots per group is to shoot as many as the barrel will consistently handle. We always want to use and test the rifle as the manufacturer intended. Hunting rifles can usually take three per group; varmint and tactical/competitive rifles can usually handle five.
Not every rifle we test is equally set up for success. Rifles that come with match-grade stainless barrels have an advantage because they are frequently lapped prior to leaving the factory. Barrels that had the chamber hammer-forged along with the bore also have an advantage much like the premium stainless models. In both cases, the bores are usually very smooth and lack any burrs or scratches from the factory chambering process.
If you want to give your rifle the best chance to do well for your own accuracy test (and it doesn't have a barrel like those described above), first put 100 rounds down the barrel with whatever ammo is the cheapest. This knocks the hemorrhoids out of the bore. Once it's had a good cleaning, let the testing begin.