Chronos High Speed Video Camera for Shooting Analysis

Frequent readers know that I have been looking for high-speed video to go mainstream for years – ever since Casio dipped its foot into the consumer market with its F1 camera in 2008.

The sort of bullet-freezing high-speed video that has become familiar to YouTube audiences still requires a Phantom or Photron camera that runs well into five figures. However, there is finally a “pro-sumer” level high-speed video camera that fills the niche between 3-figure “action cameras” and those 5-figure professional cameras: The self-contained Chronos 1.4 is launching at $3,000 and offers 1.4Gpx/s throughput on a rolling 4-second buffer, which ranges from 1280×1024 @ 1,057fps to 640×96 @ 21,649fps! Aimed Research recently gave me an opportunity to test the beta version of this camera. I’ll show some of the cool things this camera can do over the next few posts.

Note that this was a beta device, and I didn’t have enough time to learn to optimize the camera at its limits. For example, here’s a 60gr .22 bullet leaving a barrel recorded at 9,000fps. With some more tweaking I expect I could have gotten the shutter speed low enough to show the bullet as a solid in each frame instead of a blur:

Subsonic 22 leaving barrel recorded at 8923fps

With time resolution in the thousands of frames per second we can see a lot of hidden phenomena. The following video of the same 10/22 rifle shooting the same 60gr .22LR bullet was recorded at 2,356fps. One surprising thing we can clearly see here is that the bolt bounces off of the breech when returning to battery.

Another thing we can see is something we heard during recent sound level testing: The unusual 60gr .22LR load is 50% heavier than the bullets for which this action was designed, and the case is 0.2″ shorter. As a result, the case clears the chamber less than 1ms after the bullet leaves the barrel, which causes a significant amount of pressure (and sound) to vent out the breech of the barrel. (A standard 40gr round doesn’t open the breech until 3ms after the bullet leaves the barrel.) If we wanted to tune this gun for this unusual cartridge, video like this would really help us confirm how changing bolt mass and spring rates affects the action.

Hiperfire 24C Trigger – Because Hammer Force Matters

As a precision shooter I’ve never found a trigger that I thought was too light.  Two of my AR-15s have CMC 3.5-pound drop-in triggers, and two of them have Timney 667S 3-pound triggers.

The drawback to light triggers on the AR platform is that there is a limit to how strong the hammer spring can be.  Ammunition with notoriously “hard” primers (e.g., military or Russian loads) will often fail to fire with these match triggers.  I even experience occasional failures to detonate match primers (e.g., FGM205M).  Timney addresses this problem by offering a heavier hammer spring, but that increases the trigger pull of their 667S to 4 pounds.

Hiperfire came out with a brilliant trigger system they call the HIPERTOUCH 24 in which the hammer pivot point is offset and supplemented with two adjustable springs so that the hammer force is lighter when it’s on the sear but heavier when it’s striking (similar to the way compound bows use cams to make the drawn weight lighter than the release weight). Now that it can be found under $200, it is my top pick for AR-15 triggers.

Hiperfire 24C trigger

The result is a 3 pound trigger that hits almost as hard as a standard military trigger.

I tested this against my existing triggers to confirm their claim:

Trigger Pull Weight
(pounds)
Hammer Energy (inch-ounces)
Mil-standard       8.5
18
Timney Heavy       4
18
CMC Match       3.5
11
Timney Light       3
10
Hiperfire 24C       3
16

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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.)

Testing

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 uses 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.

Analysis

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 ballisticaccuracy.com. 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  

2017 Product watch

Either I’ve stopped thinking ahead or markets are finally catching up: I’ve found existing or possibly imminent sources for most of the innovative products on my wish list:

High speed video is creeping into the mainstream: The just-under-$1000 Sony RX100MV can record a few seconds of HD (sort of) resolution at up to 1000fps.

Usable thermal imaging is widely available under $1000, and digital “night vision” is getting close to supplanting perpetually too-pricey image intensifiers. I found enough businesses working in this space that I’ve decided to wait to see what comes to market, rather than pushing for the particular integrated consumer product I have in mind.

In the consumer gun industry:

I haven’t written about them, but Flat Line bullets have brought revolutionary monolithic projectiles for long-range shooting to the masses.

Two years ago I asked for a three-mode trigger. I still haven’t seen what I specified, but the Echo trigger is a clever variant I hope to try soon.

Mantis has promised a second generation of their IMU that mounts to gun rails and interfaces with smartphones, allowing us for the first time to conveniently quantify and analyze recoil effects on firearms.

Bullpups continue to storm the mainstream. If you don’t like any of the increasing number of native bullpup autoloaders (AUG, Tavor, RDB, RFB, PS90 etc.) you can find conversion stocks for many popular platforms like the M1A, Ruger 10/22, Saiga, et. al. The KSG pump-action shotgun can actually be found at retail. From Europe we have clever single- and double-shot bullpups that should eventually be available at reasonable prices in America.

I don’t know that it’s in development, but at least someone thought it was worth patenting a new sealed-gap revolver.

Inexplicably, however, I’m still waiting for more heavy subsonic .22LR ammunition!

Everyday Carry Knives: Kershaw SpeedSafe

I’ve had a Kershaw Cryo G10 for two years, and during that time I’ve pretty much stopped carrying any other knife. It clips and rides more comfortably in the corner of my pocket than any other. I love the SpeedSafe assisted opening feature. I haven’t abused it, but I can at least tell that the steel will take and hold a decent edge. And since it can be replaced for under $30 I don’t feel the need to baby it.

I took advantage of a sale this week to add three more SpeedSafes to my collection. Top to bottom is a Filter, Brawler, Cryo 155Ti, and my two-year-old Cryo:

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.

Testing

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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