Category Archives: Ballistics

Understanding Gun Precision

I’ve written a number of posts over the years in which I test the precision of various firearms. Some readers have asked about the particular methodology I use.

When testing guns for accuracy it is common practice to look at the Extreme Spread of a group of 3 or 5 test shots. I will explain why this is a statistically bad measure on a statistically weak sample. Then I will explain why serious shooters and statisticians look instead at some variation of circular error probable (CEP) when assessing precision.

It is easy to fool yourself with Extreme Spread, and it’s even easier to fool others.
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Hit by my own bullet!

Here’s a .22LR bullet that went through my paper target at 50 yards and then somehow ricocheted back to hit me on the forehead. (It then bounced onto the bench in front of me, leaving me scratched and bewildered, but nothing more serious.)

.22LR bullet that hit me

.22LR bullet that hit me

I’m well aware that bullets can do surprising things, and I’ve even researched some of them in depth. I’ve seen videos of people supposedly getting hit by ricochets of their own bullets fired into conventional backstops, but this is the first time I can offer a first-hand account of such an event.

I was doing more precision testing with a rifle in my machine rest, which meant that I was firing tight five-round groups very quickly. The last shot in one group made an unusual impact sound in the earthen berm behind the target, and as I lifted my head from the scope I heard a snap in the trees to my left and then something like a small pebble hit me in the forehead hard.

I was alone on a private range, and as I rubbed my head feeling for blood or a welt (and finding neither), I saw this slug on the bench in front of my machine rest. It was too hot to hold, so I set it aside while I finished shooting my test plan. After photographing it back in my shop I weighed it at 40.7gr, so if it shed any lead during its trip it appears to have made up for it by catching dirt in its crevices.

Does this story come with a lesson? Well, for one thing, wear eye protection! If this had hit me in an eye it would have produced a serious ocular injury. Another: Unlikely things can happen! I would still say it’s extremely unlikely for an unjacketed, subsonic bullet fired into an earthen berm to ricochet a full 180 degrees and cover another 50+ yards. (And, in that rare event, the bullet would not have enough energy to cause serious injury, unless it hit someone in the eye.) But however unlikely something may seem, just realize there’s probably somebody who’s going to have an astonished look on his face when it eventually does happen. Do what you reasonably can to make that the worst consequence!

Suppressed Subsonic Sound Levels

This post follows the introduction to shots, pops, and sound pressure levels. Virtually all firearms create pressure levels above 140dB, which is the limit established in MIL-STD-1474D to avoid unacceptable hearing damage. Hence, we put suppressors on our guns to bring their peak noise down to “hearing-safe” levels.

We may further reduce the nuisance and noise associated with gunfire by shooting subsonic loads to avoid the loud and unmistakable sonic crack created by supersonic bullets in flight.

Small subsonic loads, like a 40gr .22″ bullet leaving a suppressed rifle muzzle at just 1000fps, make peak sound levels that are roughly the same as manually cycling the bolt of the gun shooting them: about 120dB. (Without a suppressor the same loads meter about 148dB.)

Is Barrel Length Still a Factor with Suppressors?

Yes. Even with low-pressure .22LR ammunition we can see something interesting: Barrel length has a significant effect on muzzle report. Shooting the same loads through a rifle and a pistol (barrel length with AAC Element II suppressor 9″ vs. 25″ for the rifle) the muzzle report is about 6dB higher from the pistol.*

I also ran a variety of subsonic 300BLK loads through two different AR-15s using the same suppressor (an AAC Cyclone): one gun with an 8″ barrel, the other with a 16″ barrel. The shorter barrel produced peak sound pressures 3-7dB higher than the longer barrel (depending on the powder load, as we will see below).

Are Suppressors Effective on Autoloading Actions?

Yes. A common question is whether a suppressed autoloading (i.e., semi-automatic) gun is louder than one with a locked action. Modern autoloaders use gas pressure and/or momentum from the discharged round to eject the empty case and load a new one. This usually occurs while the bore still contains a significant amount of propellant pressure. I.e., some of the same propellant that produces the muzzle report comes out of the breech.

Once we add a suppressor can the breech report exceed the muzzle report? It turns out that it can if a gun is poorly tuned, but that with typical guns and loads designed for them it does not. For example, I tested both a .22LR pistol (the Buckmark) and 10/22 rifle (Feddersen-barreled Ruger). Whether I let the actions cycle normally or held the bolts closed, the peak sound levels were the same, with one exception: 60gr Aguila ammo – which is very exceptional ammunition: Its bullet is 50% heavier than almost any other .22LR, and it is loaded in a .22 Short case. As a result, the case unplugs the breech before the bullet even leaves the muzzle! Since there is no suppressor on the breech, on the rifle this releases pressure of 128dB (vs 121dB from the muzzle with the breech locked) and on the pistol it produces 130dB (vs 127dB from the muzzle with the breech locked). However, it is possible to tune these guns to this unusual round by using heavier bolts and/or springs to prevent the action from unlocking before the bullet has left the barrel.

Similar mismatches can be produced with other actions. In fact, just attaching a suppressor to a centerfire autoloader that wasn’t designed for one can be such a nuisance that many designs and components now allow for the gas system to be adjusted. But as another test: I ran a wide range of subsonic loads through my 300BLK AR-15 with its standard gas system in place (i.e., autoloading), and then with its gas port completely blocked (i.e., locked breech). The peak sound pressure levels were identical in each scenario.

Does Powder Load Make a Difference?

Yes. The standard subsonic 300BLK load uses a 10.4gr charge of a relatively slow powder in order to provide enough gas volume to cycle a wide range of guns. I have experimented with other loads to see how light a powder charge I can use while still cycling my guns and producing the same muzzle velocity (about 1000fps) with the same 220gr bullet. (The only way to do this is to use faster burning powders.) I hypothesized that lighter charges would also reduce noise. Testing with the LxT1 sound pressure meter confirms this:

Powder Charge 8″ barrel dB 16″ barrel dB
A1680 10.4gr   140 137
IMR4227 9.0gr   138 132
Steel 7.7gr   137 130

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Introduction to Shots, Pops, and Sound Pressure Levels

This is a Larson Davis Class 1 sound level meter. With a response time of under 30 microseconds and sensitivity up to 177dB, this is the sort of instrument needed to accurately measure the sound levels produced by transient events like gunshots.
Larson-Davis LXT1-QPR sound level meter
Since I didn’t want to sink over $3,000 into a sound meter, I was able to rent one from Aimed Research, which has become my go-to company for ballistic research equipment and expertise.

I’ll be posting the results of my research shortly. This post explains the basic science needed to fully understand the methods and results.

We use a decibel amplitude scale to describe sound pressure using the formula dBSPL = 20 log10(peak pressure/ambient pressure). Since we will only be talking about sound pressure levels going forward we will assume that all dB values refer to dBSPL.

We will be looking at explosive noise events that we will call “pops:” sounds dominated by a single, rapid peak in air pressure. The human ear is not very good at assessing peak “loudness” of short pops.* But the magnitude of that peak can predict both the audible distance of the pop and its potential to damage hearing of nearby listeners.

The distance at which we measure a sound is as important as the dB value, because sound pressure decays linearly with distance. On the decibel scale, this means that the same sound measured from twice the distance will be 6dB lower. As is customary, unless otherwise stated, all dB measurements will be given for a distance of one meter from the source of the sound.

For reference, here are some typical pops and pressure effects.

Category Event dB
Bad Stuff 99% lethal overpressure   205dB
1 pound TNT   203dB
Tissue damage observed   185dB
Gunshots .338LM 25″ bbl w brake   177dB
.338LM 25″ bbl suppressed   145dB
.223 16″ bbl   166dB
.223 16″ bbl suppressed   132dB
.22LR 16″ bbl   148dB
.22LR 16″ bbl suppressed   126dB
Sonic Crack .308   152dB
.223   148dB
.22LR HV   144dB
Gun Actions AR-15 bolt release 123dB
10/22 bolt release   119dB
AR-15 dry fire   113dB
10/22 dry fire   105dB
Steel Hammer Nail into wood 133dB
Iron anvil   133dB

*For example, I’ve never been able to hear the “first-round pop” that tends to occur with cold suppressors. But in my test data I did find many cases where the first round peak was 3-4dB higher than subsequent shots.

Feddersen 10/22 Accuracy with Gemtech, CCI, Aguila, SK+

Test Ammo - Gemtech, CCI SV, SK+, AguilaI haven’t been able to find any decisive reviews of Gemtech’s 42gr .22LR subsonic ammunition. I finally picked some up under $4/box and decided to wring it through my Feddersen-barreled 10/22.

Since my last precision testing of 10/22 rifles, I have also refined a testing process capable of higher sample volumes, so I decided to compare the Gemtech to these other subsonic flavors presently abundant in my stockpile. (Of course, since Gemtech’s ammo is supposedly optimized for use with a suppressor, this test was conducted with an AAC Element II screwed to the muzzle of the 16″ Feddersen match barrel.)


One thing that has made precision testing much easier is this universal machine rest I developed: After every shot it returns the gun to the exact same position (which can be confirmed by the 32x scope on top), so it’s easy to shoot a string quickly and with zero shooter error.

I’ve also become a little more disciplined with respect to fouling the barrel: When shooting a clean barrel, or changing ammunition types on a fouled barrel, I ignore the first five shots. Different rimfire ammo use different lubricants, and it takes some number of shots before the bore is consistently coated in the new lubricant. Five shots isn’t really adequate to fully stabilize the bore. (A good bolt action rifle will show that ten or twenty shots are required for it to settle in.) But at the level of precision one can get out of an autoloading rimfire, five shots seems “good enough.”

At 50 yards (the test distance shot here), muzzle velocity variance doesn’t really come into play. But it certainly does at 100 yards and beyond. It was easy to prop a chronograph in front of the machine rest and record the velocity of every round fired during the testing.


Another thing that helped streamline analysis was OnTarget’s TDS software. It can’t (yet) auto-detect multiple shots on a single target, but it does auto-detect the points of aim, and it makes marking the shots and groups fast and easy.

I took advantage of the latest statistical tools available from The aggregated data and analysis are in this Excel workbook.

Summary results, here linked to TDS-marked targets, show that (in this gun) Gemtech’s ammunition is better than Aguila but worse than CCI SV:

Ammunition CEP Radius (MOA) Average fps fps Standard Deviation
SK Plus 0.37 1045fps 14.7
CCI SV 0.50 1039fps 15.2
Gemtech 0.58 1022fps 14.5
Aguila 0.67 1015fps 10.0

Subsonic .22 caliber

Following some renewed interest in my old post on The Missing Subsonic .22LR Market, I figured it would be worth posting some of the notes I made during my last journey through that topic. The purpose of that post was to wonder why Aguila is the only manufacturer in the world making .22LR bullets significantly heavier than 40gr. To explain why that’s of any interest let’s take a few steps back.

Why the obsession with subsonic bullets? The answer is, “Peace and quiet.” A typical center-fire rifle shot meters about 165 dB a few feet from the muzzle. A good silencer reduces that by 30-35 dB, making it hearing-safe to shoot, but by no means “silent,” nor even quiet. The largest component of the sound signature of a silenced rifle shot is the bullet’s “sonic crack”: A supersonic projectile creates pressure waves that reverberate along its entire flight path, and in the case of a rifle bullet these create roughly as much sound as an unsuppressed .22 rimfire shot. In fact, people working in the target pits hundreds of yards downrange from a firing line are generally required to wear hearing protection because the sonic crack of bullets passing close overhead can still cause hearing damage.

So at some point after one buys a rifle silencer one begins to wonder how to make it even quieter. Having absorbed most of the muzzle blast in the silencer itself the next step would be to eliminate the sonic crack. This requires changing the rifle load so that the bullet’s velocity is reduced from its usual 2500-3200fps to a subsonic velocity — 1100fps under standard atmospheric conditions.

“No problem. Let’s just double or triple the bullet weight,” one might naively say. Unfortunately it’s not that easy. A 55gr .223 lead bullet, which shoots about 3000fps, is typically 3/4″ long. A 150gr bullet driven by the same load would be subsonic, but to be reasonably aerodynamic it would also be almost 2″ long, and this is where we run into some inconvenient facts about ballistics. We’re talking about rifles, and why are they rifled? In short: to keep the bullet pointed in the right direction. Rifling imparts spin to bullets fired through a barrel, and it turns out that the twist rate of the rifling is somewhat particular to the bullets a gun is designed to shoot. A bullet has to spin at a certain minimal rate to maintain stability throughout its flight. If it doesn’t spin fast enough then aerodynamic forces will cause it to yaw or even tumble, which disrupts all of the careful work that went into making a rifle that could send a bullet over a long and accurate flight path.

Spin Stability

One of the most accessible spin models for small caliber bullets is the “Miller Stability Formula” which estimates gyroscopic stability based on four parameters:

  1. Rifle twist rate
  2. Muzzle velocity
  3. Bullet length
  4. Bullet weight

Holding all else constant, bullet flight stability increases with higher muzzle velocity or twist rate (either of which cause it to spin faster), and decreases with shorter or lighter bullets (both of which require the bullets to spin faster to sustain stability).

According to the Miller formula a 2″ .223 bullet weighing 150gr and shooting 1000fps would require 1:4″ rifling (that’s one full turn in just 4″ of barrel length!) to stabilize. Neither bullets nor barrels are made anywhere near those specifications. The fastest rifling you can find in .22″ barrels is a sharp 1:6.5″, which at subsonic speeds is barely sufficient to stabilize the heaviest .22″ rifle bullets on the market, which are 1.16″ long 90gr boat-tail target bullets.

Granted, at subsonic velocities aerodynamic profile isn’t as important. So what are the heaviest bullets we can shoot with a .22 rifle? Lead has a specific gravity of 11, which means that for a given volume it weighs 11 times as much as water. Water weighs 253gr per cubic inch, so a .22″ diameter slug of lead (which has a volume of .038 cubic inches per inch of length) weighs 106gr per inch. If we are willing to fire a ballistically inefficient flat-nosed, flat-base, unjacketed lead slug then, according to the Miller Formula, a standard 1:9″ barrel can just barely stabilize about a 90gr (.9″ long) bullet at 1000fps.

Extreme Shock makes pricey 100gr tungsten-powder slugs that are slightly denser than lead and will stabilize in 1:7″ barrels at subsonic velocities. If you don’t mind spending over $1/round that could be a decent route to go. But if you try lighter or pointier bullets you run into other problems related to case volume: Hodgdon has published two subsonic loads for .223 Remington. One of them uses a tiny amount of fast-burning powder, which turns out to be very problematic. After two out of six test loads I made fizzled and left the bullet stuck in the barrel Hodgdon techs noted that their TiteGroup subsonic recipes are very sensitive to variation. They now recommend only using the exceptionally bulky TrailBoss powder, which will reliably get the bullet out of the barrel, but (at least in my experience) not at very consistent velocities.

My conclusion: Why bother? Due to the stability constraints in .22 caliber with conventional bullets you can’t go much past 70gr anyway. Forget trying to get a .223 autoloader to cycle on such light loads. Meanwhile, for a fraction of the price you could be shooting Aguila’s 60gr .22LR, or any number of grades of 40gr .22LR.

.22 diameter bullets

10/22 Precision Rematch

KIDD 10/22 in Archangel stock, Ruger 10/22 with Feddersen barrel in Vantage stock

These are both Ruger 10/22 style semi-automatic rifles built for shooting .22LR with maximum accuracy. On top is an $860 rifle built entirely by KIDD Innovative Design. The receiver and trigger are milled from aluminum, and the bolt from hardened steel. The single-stage trigger is also a crisply machined assembly that adjusts down to a pull of just 1.5 pounds. The lightweight barrel is guaranteed to group inside of half an inch at 50 yards. The gun here is screwed into a comfortable $100 ProMag Archangel Target stock

Do you have to spend $1000 to get an accurate .22 rifle? Expert barrel maker Fred Feddersen says one of his $170 barrels will turn an off-the-rack Ruger into a gun that can compete with any custom autoloader. So the second gun shown is a standard Ruger 10/22 receiver and bolt onto which I swapped Feddersen’s barrel. Of course I don’t think I can really shoot that well with a standard trigger, so to be fair I bought another $200 KIDD trigger assembly for it. The gun is shown here screwed into a beautiful $175 Tactical Solutions Vantage laminated stock.

I tested these for precision last year. This time, with a few more ammo types, I also tested all ammo both with and without a suppressor.


Both guns were cleaned and then shot through the following sequence of 40gr subsonic target loads:

  1. 30 rounds SK Plus
  2. 25 rounds SK Match
  3. 15 rounds Eley Match
  4. 40 rounds CCI SV
  5. 40 rounds Aguila SuperExtra

All rounds were fed from the same transparent Ruger 10-round box magazine. This time the KIDD ran with no hiccups whatsoever. The Feddersen-barreled Ruger, shot second, had one failure to feed with the CCI and one with the Aguila.

The raw data and calculations can be downloaded here. The targets are shown at the end of this post. Summary analysis:

Precision of KIDD vs Feddersen rifles on different ammunition
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XCR Short-barrel Precision

XCR equipped with a precision stainless 11

A precision-obsessed friend was lamenting the apparent inability of his Robinson XCR to shoot tight groups. Although it was not designed specifically for precision, I am a fan of the gun’s design. Particularly given its ease of changing barrels, I began to wonder how much we could improve on the standard chrome-lined light-contour barrels. So I sent two Lothar-Walther match-grade 1:8 rifled stainless steel blanks to Robinson and waited (seven months) for them to return as heavy-contour 11″ 5.56mm XCR barrels.

This is not a gun that is easy to shoot precisely: It is light, and its single-stage trigger breaks at over 4 pounds, which I know robs me of accuracy. In order to remove shooter dispersion from the equation I tested various configurations in a custom machine rest. I ran a range of ammunition through two standard 11″ barrels and, sure enough, 10-round groups would typically exhibit a mean radius in the vicinity of 1.5MOA. The standard 16″ light barrel, interestingly, printed 10-round groups with MR just above 1.0MOA shooting light bullets (both XM-193 and Wolf Classic!), but didn’t do as well with heavier bullets in match-grade loads (despite its 1:9 twist).

With the new precision barrels the rifle prints 10-round groups with mean radius consistently below 1.0MOA, like these:

XCR Precision 11

Note that from the short barrel 75gr .223 loads run about 2260fps. On the high end, 55gr 5.56mm clocked 2775fps.

There is some vertical stringing evident, which varies with the upper, and which suggests there is further room to tighten the design. And, as mentioned above, these groups were achieved with a machine rest. When I shoot off of bags 10-round groups open up by roughly another 0.5MOA. Suffice it to say that with a good barrel and shooter this gun is capable of respectable accuracy.

Barrett REC7 Precision

Barrett REC7 with NF-F1 and LAR

We’ve been questioning what sort of precision we can expect from a piston-driven rifle with a chrome-lined bore. Those are both features believed to reduce accuracy (vs direct-impingement and an unchromed bore). We tested our Ruger SR-556 shooting ten-round groups using the same suite of ammo as in our Savage .223 Precision test and got CEP across the board about 1 MOA (with the exception of the bottom-of-the-barrel Wolf Classic, which produced CEP of 1.5 MOA).

This weekend we got a chance to try a little harder with a Barrett REC7, a premium rifle. Shown above, we fitted it for this test with an LRA bipod and NightForce F1 scope. Five shooters took 10 shots each with two different types of ammo. Given the conditions the shooter variation was minimal. The ammo makes the difference, and this gun sets a new precision benchmark for its type:

Load CEP (MOA) 90% Confidence
Federal Gold Medal Match 77gr 0.50 (0.45, 0.57)
American Eagle XM193 0.96 (0.86, 1.13)

Flash Rounds: Tracers Fired Backwards

Tracers loaded forwards and backwardsA traditional tracer bullet like the M62 carries an incendiary payload in its base. We’ve done a lot of experimenting with bullets fired “backwards” (i.e., base first), and found them to be both accurate and effective, even if their blunt shape produces excessive long-range drag.

TracerExplosionSo what happens when you fire a tracer base first? First of all, the tracer doesn’t ignite in flight. But if it hits something hard, like a rock or steel target, the jacket ruptures and the impact energy usually ignites the tracer compound all at once, producing a satisfying flash and puff of smoke, leading us to call them “flash rounds” or “smoke rounds.”

Once again our friends at Aimed Research provided a high-speed video of a flash round hitting a metal target:

Granted, these are nothing like a “true” incendiary round, which sounds like a cherry bomb exploding. Following is a video of a real incendiary bullet hitting the same metal target. It has a larger payload of fuel (magnesium and/or aluminum) and oxidizer (barium nitrate).