One of the coolest parts of my job here at DonnyFL, Saber Tactical, and Karma Airguns is how much testing we get to do. Every platform, every caliber, and every slug presents its own set of challenges and learning opportunities. Over time, I’ve come to appreciate that slug testing is equal parts art, science, and statistics. Too often in our community, results get presented without enough context, or worse, through the lens of ego or promotion rather than physics and statistics. This article is meant to bring balance to that discussion, rooted in real-world testing and statistical analysis.
Why I Test at 50 Yards First
When I’m tuning a rifle for slugs, I always start my group testing at 50 yards. Some people might jump straight to 100, but there’s a reason I don’t. At 50 yards, environmental factors like wind and thermals play a smaller role, which means I’m isolating the rifle, the tune, and the ammo—not Mother Nature.
Shooting at this distance gives me a clear picture of basic initial information to include if a slug is being stabilized and consistent velocities that would be exaggerated at longer ranges. If a setup can’t stack shots at 50, it’s not going to magically perform at 100. If you read enough forums and online communities, there is a common misconneption that slugs can “go to sleep” and right themselves at a distance. I am no ballistician, but I have yet to see an airgun that shoots 1-inch groups at 50 yards (2 MOA), but then groups at 1 inch at 100 yards (1 MOA). So if it isn't good at 50 yards, I am going to assume it isn't going to be good at 100 yards.
Once I’m satisfied that the rifle is shooting tight, consistent groups at 50, then I stretch it out to 100 and beyond to validate long-range stability. This two-step process saves time, air, and ammo, and it helps me eliminate false positives from chasing groups in the wind.
The 100-Yard Verification Standard
Testing at 50 yards is useful, but 100 yards is where slugs either prove themselves or fall apart. I’ve seen slug designs hold together beautifully at closer ranges only to fall apart past 75 yards. Your goal should be consistent one-inch(ish) groups at 100 yards, which is true Minute of Angle performance. Even with a 3–5 MPH wind, a slight wind variance can knock a slug out of an MOA group at 100 yards. I am good at reading wind, but it is impossible to predict the future of the almost one-third of a second of 100-yard flight time from muzzle to target and account for what the wind will do the second I pull the trigger.
When testing at 100 yards for groups, avoid compensating with wind holds. Instead, learn what your slug is naturally doing with wind drift and vertical drop based on your calculated or measured BC with the same point of aim. The FX True Ballistics chronograph gives you this data, but you must apply it correctly. What drives me crazy is seeing posts showing “stacked” 100- to 300-yard groups without mentioning conditions. Unless those shots were made in a perfectly windless environment, what you’re seeing is statistical cherry-picked variance rather than repeatable precision. Again, I too have been guilty of showing the best case scenario, but I also try to show multiple 5-shot groups in sequence to show the statistical consistency over many shot groups.
Bore Fit and Barrel Dynamics
This is where it is important to point out that a lot of airgunners who first get into slug testing find a sense of frustration and blame the lack of good results on the slugs or the speed at which they are shooting them. Slug performance starts with bore fit.
The Karma Airguns Airguns I have tested prefer .218 slugs while FX rifles with their 1:16 twist barrels often favor .217 or .2175. All Karma Airguns, including the SLS, EQ, and Red Panda, use a 1:17.7 twist rate across their barrel systems, which has proven to stabilize a wide range of slug weights very effectively. Every barrel can have a slight difference, even within the same make and model of airgun. Slight variances in machining, tolerances, or the way the rifling formed can result in very different slug preferences. This is why some rifles print single-hole groups with one slug size while others scatter them unpredictably.
If a slug is too large, it creates excessive friction and increased barrel leading. Some barrel systems may also have a very slight choke at the end, which isn’t ideal for slug shooting where an unchoked barrel performs best. An oversized slug can wreak havoc on the leading at the muzzle, and if you let that leading go too long, you’ll eventually be forced to clean it out with harsher abrasives like a brass brush. We’ll discuss leading later on, but be aware that an oversized slug can make things go bad quickly.
Ballistic Coefficients and Real Numbers
When you first start getting into airgun slugs, you will hear a lot of talk about Ballistic coefficient (BC) ratings. In simpliest terms, BC is a metric used to describe the external ballistics of a projectile's ability to carry energy over distance with a flatter trajectory and a level of wind deflectiion.
So BC is another area where careful testing is critical when testing slugs and their effectiveness to compare one slug over another. The AirMarksman / Corbin slugs I’ve been running have been producing some outstanding numbers. The AirMarksman Ace 40.1 grain are getting well above .23 BC G1 across my barrel systems as well as .15+ BC G1 with the 33 grain hunting slugs. I’ve also found that Karma Airguns really prefer .218 slugs while the FX 1:16 barrels are liking the .217 and .2175 sized slugs. Zan slugs are another top pick, especially the Zan ELR 35.5 grain, which has proven to be one of the best performers I have tested in Karma Airguns like the EQ and the Red Panda.
Numbers like these are not just for bragging rights. They directly inform ballistic solvers like the Element Optics app or the FX True Ballistics chronograph system. These tools calculate ballistic solutions over long distances, letting you see exactly how wind drift and vertical drop will affect your shot. Without true BC data from your rifle and slug combination, you’re just guessing past 100 yards. As our sport matures and new models are introduced in the broader precision shooting sports, you will start hearing more about custom drag models and other models to better predict the ballistic solution for a given slug shot from a given airgun at a given velocity and tune.
Ego vs. Physics
But what about all these posts about airguns always shooting sub-MOA groups? I’ll admit it—I’ve been guilty of showing my best groups after I have tested a multitude of tunes. Sometimes I’ll spend hours tuning a gun and then post the best group it produced. The groups are real, but the question is: is that sample size statistically significant? We all like to show what the gun is capable of at its best, but the truth is that those miracle groups are outliers, not the baseline.
That reality is what makes repeatable MOA groups at 100 yards with airgun slugs environmentally significantly impacted. A really good slug-shooting airgun might average 1.5-inch groups at 100 yards, 3 inches at 200 yards, and 6 inches at 300 yards over a large sample. That’s just math. Yet our culture often rewards the exceptions instead of the averages.
As a simple thought experiment, plug a mild 5 MPH wind into the Element Optics app and watch what happens to drift at 100, 200, and 300 yards. Even the best wind readers cannot predict those micro-variations. Wind is like waves on water: you can see the big sets coming, but the little ripples in between are what destroy consistency.
Statistical Variance and Real Expectations
The heart of slug testing and real data collection is statistics and the statistical significance of any data shared. One of my favorite science-based contributors to the precision shooting community is Bryan Litz from Applied Ballistics, and he has covered this in great detail, but it applies to all projectiles to include airgun projectiles.
A five-shot group is a snapshot. A 50-shot group tells you the truth or at least a more statistically significant truth. This is where the study of statistics gives us an important tool: the normal distribution curve, more commonly called the bell curve.
The bell curve shows how results cluster around an average, with fewer results appearing as you move farther from that average in either direction. In slug testing, let’s say you have an airgun tuned to shoot consistent sub-MOA groups at 100 yards, meaning it will often group right around one inch. If we plot out 100 or more shots on paper and measure the group diameters, they won’t all be exactly one inch. Instead, most will cluster tightly around that one-inch mark, but some groups will come in smaller and some larger.
Here’s how the math plays out. In a normal distribution, about 68 percent of the results fall within one standard deviation of the mean. That means if your average group size is one inch, roughly two-thirds of your groups will measure between about 0.8 inches and 1.2 inches. About 95 percent of results fall within two standard deviations, so nearly all of your groups will be between 0.6 and 1.4 inches. The last 5 percent of results are outliers, and that’s where you see the “wow” half-inch groups, or the occasional flyer that stretches a group to 1.5 inches or more.
This is why those jaw-dropping tiny groups plastered on social media, while real, do not represent repeatable performance. Statistically, they are the exceptional tail ends of the distribution, not the average. Over a long sample size, the bell curve always wins. If your rifle averages one-inch groups, that is the number you should build your ballistic expectations around, not the half-inch miracle that happened once in 50 tries.
By understanding the bell curve, airgunners can stop chasing unicorn groups and instead focus on building confidence in the repeatable center of the curve. That is where true marksmanship lives.
Closing Thoughts
Airgun slug testing is a blend of art, science, and statistics. Bore fit, twist rate, BC, and wind all matter, but so does understanding what your data really means. Don’t let ego or marketing fool you into believing that one tiny group defines a rifle. Look at the averages, learn to respect the statistics. Tuning for slugs can be frustrating, but if you enter into this testing process as being part of the journey, you will find even more enjoyment when you find those MOA groups, knowing all the variables you needed to learn and take into consideration to get to that destination of success.
