Category Archives: Ballistics

Heavy Metals for Bullets

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As a preface to an upcoming series on subsonic rifles, I have compiled the following information regarding metals for bullets.

To produce good ballistics a bullet needs several characteristics.

  1. Internal: It has to remain solid at the temperature and pressures of firing, to avoid melting in the gun. It also must be softer than the barrel through which it is fired so that it conforms to the rifling of the barrel (which is critical to ballistic stability) and so that the barrel can be shot repeatedly without degrading.
  2. External: It should be as dense as possible, since density not only increases stability but also reduces the energy lost to air resistance during flight.
  3. Terminal: It needs to be tailored for some terminal objective. Depending on the target, we may want a bullet to explode, expand, penetrate, stop, or do some combination of those things.

Following is a list of elements that are of interest in meeting these objectives, ordered by density and noting their rough current cost:
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Sight in your Suppressor

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Summary: The most efficient baffle suppressors have internal asymmetries that can affect the flight path, and hence point of impact (POI) of bullets fired through them. Asymmetric suppressors can be “sighted in” to find the index that produces the most benign POI shift.

My first full-power rifle suppressor was Gemtech’s M4-02. I was baffled by its persistent tendency to not only double the size of my groups but also to drop them more than 3MOA from zero.

I have since learned that changes in point-of-impact (POI) when attaching a silencer to a rifle are considered normal. My first assumption was that POI shift was primarily due to changes in the barrel’s harmonics. So I spent a day trying to work up a load on my precision .223 bolt gun that would give me the same POI as my unsuppressed groups. I found loads that gave better groups with the suppressor, but they were still consistently 3MOA low. After some discussion with experts I concluded that harmonic changes are probably not the dominant source of POI shift, especially with thicker barrels.

Then I heard about AAC’s M.I.T.E.R. (Multi-Indexing for Targeting at Extended Range) System: A patent-pending suppressor and mount that allows the user to quickly change the index of the can in increments of 1/5th of a turn. AAC’s manual for the SPR silencer, which uses the MITER mount, has a helpful chart showing how rotating the can’s position can change the POI of groups. A review of this silencer even noted that POI shifts exceed 3MOA in some positions.

Asymmetric Blast BaffleWhy would changing the suppressor’s index reliably change the point of impact? It turns out that the most efficient baffle suppressor designs require asymmetries in the baffles to encourage the muzzle blast to redirect itself away from the bore line and into the baffles, instead of straight out the end of the can. If you look at the first “blast baffle” in such a can you will often see something like the divot shown in this photo. It is well known that the bullet isn’t the first thing out of the muzzle: The bullet pushes a column of air ahead of it, and as it leaves the muzzle the bullet is even overtaken briefly by the high pressure propellant gases behind it. As you sight in a silencer with an asymmetric blast baffle you will probably notice that the orientation of the divot correlates with the POI shift. So it appears that the divot is causing some of the propellant to nudge the bullet in a consistent direction.

How can you “sight in” your asymmetric suppressor? Essentially you want to shoot groups as you slowly change the orientation of the can around a full 360 degrees. But you should never shoot a silencer that isn’t fully tightened down. For sighting in my 1/2″-28TPI suppressors I bought this bag of very thin steel washers to allow me to shim it out roughly a fifth of a turn at a time. After zeroing I shot five-shot groups at each orientation, using two different types of bullets: flat-based 55gr, and boat-tailed 69gr. The resulting group centers and sizes are depicted in the following chart (which links to a spreadsheet with the raw data).

Obviously with my heavy barrel the orientation of this silencer is a critical factor in POI shift. Adding two shims before screwing on the can gives me groups that are only half an MOA from the bare barrel’s zero, with impacts just a little high. (With no shims this can produces the 3-4MOA drop shown on the chart.)
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Shooting the Short-Barreled XCR

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Robinson Armament XCR-L 11-inch with HTG Eureka-4 suppressor, EOTech 512, AR-Tripler in LaRue 649-S mount

I spent some time at the range with a chronograph, my short-barreled XCR, and my newest 5.56mm suppressor: HTG’s Eureka-4. The Eureka is 4.6″ long and weighs only 15 ounces. It extends the gun’s overall length only 2.8″ more than the standard flash suppressor it replaces, making it ideal for a compact, CQB-style carbine like this.

George Vais, who patented and developed the HTG suppressors, assured me that the Eureka is designed to absorb the higher muzzle pressures of shortened 5.56mm barrels, which some people fear will erode the blast baffles of cans not designed for that abuse. Granted, my 11″ barrel isn’t remarkably short given that some people run these rounds through 7.5″ barrels. At 11″ even the standard flash suppressor does a decent job moderating the muzzle blast, so shooting this XCR is no harsher than any other carbine I have shot in this caliber.

With the silencer attached, the gas piston valve can be turned to its lowest setting (“S”) and the gun still cycles reliably. This is thanks to the longer backpressure curve that a silencer maintains. However, a consequence of putting rounds designed for 20″ barrels through a system this short is that a significant amount of propellant gas blows back through the ejection port, which is not the most pleasant thing to have venting in front of your face. If I had to do this all day I’d probably wear goggles to keep it from irritating my eyes.

Without the suppressor mounted I turn the gas valve to its highest setting (“4”), as is suggested during the first few-hundred round “break-in” period. Factory 55gr .223 loads that chronograph 3150fps out of my 24″ bolt gun leave the unsuppressed 11″ barrel at just over 2600fps. I was surprised to discover that the gun’s action can tap a lot of power out of the shots, even though with this barrel the propellant doesn’t reach the gas hole until 1.5″ from the muzzle: Turning the gas system from “4” to off — essentially making this a bolt-action gun — the bullets gain an extra 250fps! (Of course, this extra velocity is accompanied by a more violent muzzle blast.)

Screwing the silencer on increases muzzle velocity even further (thanks to an effect known as “freebore boost“): With the gas system off and the silencer on this gun shoots factory 55gr bullets over 2930fps, which is not bad for a barrel assembly measuring just 14” long overall. Turning the gas system back on to “S” drops velocity just below 2900fps. Out of curiosity I kept the silencer on and stepped the gas valve up a notch to “1,” which pounded the action harder, ejected the case further, and dropped muzzle velocity to 2830fps. Always one to test the limits, I then cranked the gas back up to “4” and fired one round, at which point the action crushed the neck of the spent case and somehow produced a triple-feed malfunction!

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The Missing Subsonic .22LR Market

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Precision shooters know that keeping bullet speeds out of the transonic region preserves accuracy. The pressure dynamics around the sound barrier can upset a ballistically efficient bullet on its way to the target.

It turns out that the round nose and stubby heel of a typical .22LR bullet make it remarkably aerodynamic at subsonic speeds. Consequently, match-grade .22LR ammunition is typically designed to leave a rifle barrel under 1000fps.

Competitive rimfire shooters aren’t the only ones who have discovered advantages to subsonic ammunition. Anyone who fires a well suppressed gun will note that even if the muzzle blast is fully contained in the baffles of a silencer a supersonic bullet makes a significant amount of noise of its own: As it travels down range the supersonic pressure waves in its wake produce a “sonic crack.” Since .22LR cans are so light, cheap, and efficient, there are a lot of suppressor owners opting for subsonic ammunition to keep shooting sessions as quiet as possible.

Anyone who has pulled a subsonic .22 bullet has probably been surprised at how much empty space is in the case. It takes less than one grain of powder to propel the standard 40gr .22 lead bullet to the sound barrier. With all that extra room in the cartridge, why not add some more mass to the bullet? After all, holding all else equal, mass is your ballistic friend: It increases ballistic coefficient, which increases a bullet’s effective range by helping it retain velocity and resist atmospheric disturbances. Extra mass at the same speed also increases energy, which enhances terminal ballistics.

Aguila 60gr SSS .22LRAt some point you’re bound to notice a peculiar offering in the .22 marketplace: Silver boxes of Aguila-brand subsonic .22LR ammunition with some odd-looking 60gr bullets. Based on all of the preceding observations, you might justifiably exclaim, “Ah ha! There’s a great idea! I’ll put those in my rifle and enjoy all of the benefits of subsonic shooting for pennies a round, but with improved ballistics!”

And you would be right, except for one problem: Virtually every .22LR barrel is made with 1:16” rifling, and that is not adequate to reliably stabilize those longer 60gr lead bullets. In fact, I have looked long and hard to find anyone who makes a .22LR barrel with faster rifling that is also threaded to accept a suppressor. (The closest you can come is to buy an aftermarket specialty barrel from a place like Green Mountain, and then pay another $100 to get someone else to thread its muzzle. Or buy a .22LR conversion kit for a .223 rifle, many of which have 1:9 twist threaded barrels.)

Do some more research and you will also conclude that Aguila does not enjoy the most stellar reputation in rimfire ammunition. And yet they are the only company that makes .22LR bullets heavier than 50gr (and there are only a tiny number of other specialty loads heavier than the standard 40gr).

.22LR is by far the most popular consumer cartridge. Every .22LR shooter with a silencer, and many without, would love to be able to buy reliable and accurate 60gr+ bullets, as well as threaded barrels with sufficient twist rates to stabilize them.

So my open question to the firearms industry is: Where are the reputable bullet manufacturers selling cases of plinking, varmint, and match-grade 60gr .22LR ammunition? And where in the vast marketplace of .22LR guns and parts are the 1:12 twist .22LR barrels with threaded muzzles to shoot those bullets?

M25 and M62 .30 caliber Tracer Bullets

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As a new handloader I thought it would be fun to add tracers to my inventory. Tracer bullets that have been pulled from surplus military ammunition can be picked up for $15/100 at gun shows or online.

I bought one bag each of M25 and M62 tracers, but since I couldn’t find any detailed information about them online I decided to analyze them myself. Ten of each are shown here, M62’s on the left. Both tracer types are marked with an orange tip. (The black residue on the bottom of the bullets is the remnant of sealant used on military ammo and comes off after a spin in a vibratory cleaner.)
MilSurpTracerBullets
MilSurpTracerBulletBases

M62 tracers are designed for .308 (7.62x51mm NATO) rifles. Overall length is 1.33″, and they have a single .05″ cannelure which leaves a bearing length of roughly .60″.

BulletSectionsM25 tracers were supposedly designed for .30-06 rifles. They are nearly identical to the M62’s except for a second cannelure that allows them to be crimped more deeply in a case. This photo shows whole and sectioned samples of M62, M25, and (for comparison) 168gr BTHP bullets.

TracerCompoundUnderBaseCupI weighed the M62’s at about 143 grains and the M25’s at about 145 grains. On both bullets the base of the copper jacket is crimped around a 1 grain copper cup about .20″ in diameter. The cup is seated almost .1″ deep from the base of the bullet on M62’s, and slightly more shallow on the M25’s — .06-.08″ from the base, which could account for the extra weight on the M25’s given that the overall length on both is the same. Ahead of the cup, and behind the lead core, is .40″ of tracer compound with a dimple in the center, shown here.

The tracer compound is brittle and powdery, and weighs only about 10 grains. It burns with a bright red flame for about 4.5 seconds and (in static tests at least) leaves behind 3-4 grains of ash.

In order to help conceal a shooter’s location these tracers are designed to not fully light up for the first 100 yards of flight. At muzzle velocities over 3000fps that’s just a tenth of a second from firing to the appearance of the full flame. I did a number of tests to try to determine the tracer compound’s ignition temperature as well as the mechanism by which this delay is achieved, but was unable to get consistent results.

First I took a tiny amount of tracer powder and put it directly on a thin nichrome wire linked to a dimmer switch and a thermocouple. I slowly raised the temperature to 900F but nothing happened. I shut down and then rapidly hit the heater with full current and got an immediate ignition. In later tests I opened the back of a tracer and put the compound in direct contact with a thick copper wire heated to 800F and still got no ignition!

In an attempt to more accurately simulate the conditions of being pushed from behind by the hot gases in a firearm I took an intact bullet and anchored its base directly to the top of a copper wire, which I slowly heated with a propane torch. It reached 540F and after some time at that temperature the entire tracer pellet ignited and ejected out of the back of the bullet, pushing the bullet 5 feet into the air and leaving the copper cup still on the test stand. That’s not what’s supposed to happen either, but it does suggest a potential failure mode: If the tracer compound ignites from anything other than its base a shooter will see a red flash and then nothing as the compound leaves the bullet and burns much faster dispersed in the air. This also confirms (as did later tests) that the tracer compound provides negligible thrust — in this extreme case accelerating the bullet less than 10fps.

[Update: Though they don’t significantly propel the bullet, burning tracers do extend the bullet’s range. In a summary of BRL research on .50 caliber bullets Robert McCoy notes, “The tracer adds heat and mass flux into the wake, which raises the base pressure and lowers the base drag. For the APIT, M20 projectile, the tracer reduces the total zero-yaw drag coefficient by approximately 7 percent, at all speeds tested.”]

Fortunately, it’s not hard to get a proper ignition of these tracers: Hitting them briefly with a propane flame will reliably ignite them, as shown in this video. Here is a photo of three burned tracers along with two of the recovered base cups.

Recoil Reduction Products

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I own two over-under field shotguns.  These are light guns optimized for carrying on long walks hunting birds.  But a lighter gun means heavier recoil.  I like to take my field guns trap shooting, but it can be painful to shoot more than a few rounds with 12-gauge or even 20-gauge loads.

Competitive shooters with customized guns and release triggers laugh at the idea of using field guns for sporting clays.  “If you want less recoil you have to add more weight to the gun,” they insist.  Adding weight is certainly one way to dampen recoil.  But I’m a practical guy and I don’t want to turn my light field guns into heavy competition guns.

Fortunately there are other ways to control recoil:

  1. Fit.  This is the primary factor affecting the shooting experience of a bird gun.  If the gun doesn’t fit the shooter then the shooter won’t be able to maintain a correct shooting posture: Cheek “welded” to the stock, butt snugly in the shoulder pocket, and dominant eye aligned with the front sight bead.  If you can’t lock the gun against your cheek and shoulder then the recoil of every shot will slap you (often leaving visible bruises).  If your eye isn’t aligned with the sight then you will probably miss the target.  (The shooter’s eye serves as the rear sight on a bird gun.  It doesn’t take much sight movement to change the point of impact at 20-40 yards!)
  2. Action.  A semiautomatic action will absorb a significant amount of recoil.  Of course that doesn’t help if you’re talking about making a bolt or break action easier to shoot.
  3. Dampers.  You can dissipate recoil energy using damping mechanisms.  Almost every gun comes with a rudimentary damper in the form of a rubber recoil butt pad.  Here I review some more advanced dampers.

Edwards Recoil Reducer

Edwards Recoil Reducers are light cylinders containing a spring-buffered counterweight that absorbs and even redirects (depending on installation angle) recoil.  I got one delivered from Brownells for $60.  It looks and feels like a rugged device.  Edwards has been making these for over forty years and backs each reducer with a lifetime warranty.

Installation in a wooden stock can be a bit of a project.  Take the recoil pad off of any shotgun and you’ll discover a lot of empty space in the stock.  Before you can properly install the reducer you probably need to fill that space with some combination of dowels or other wood trimmed for a tight fit.  Then with a 7/8″ forstner bit you can carefully drill out a hole as high and parallel to the gun’s barrel as possible.  Push the recoil reducer into the hole, make sure it’s snug, and screw the recoil pad back on to hold it in place.

Installation in a hollow plastic stock is simply a matter of unscrewing the recoil pad, positioning the reducer, and filling the remaining space with sprayfoam, as shown here.

After installing the Edwards Recoil Reducer in one of my guns I took it right back to the trap field — still sore from shooting four rounds two days earlier.  The recoil reduction was immediately obvious.  I’m still working on a recoil gauge to quantify peak forces, but to me it felt like close to half of the recoil was gone.  Even after four more rounds with the gun I would have been comfortable continuing to shoot.

Graco GraCoil

Graco’s GraCoil is a butt plate that contains an adjustable piston that compresses up to 5/16″ to absorb recoil.  Compression damping is also what a good recoil pad is supposed to do, but pads aren’t adjustable and they can’t get too mushy before they impact handling.  The GraCoil spring can be tightened just enough that it doesn’t move under the pressure of your shooting stance, but then immediately starts to compress to absorb the recoil of a shot.

GraCoil plates also include a mechanism to enable significant adjustment to the position of the butt pad.  This allows for significant improvements in the fit of a shotgun (which, as noted above, is an essential feature!).  I opted to buy the GraCoil Model GC15-LP which also includes a length-of-pull adjustment.

The GraCoil needs to be ground to fit a particular stock, something I didn’t feel like attempting.  Total cost of the GC15LP with factory installation is $375.  MPC Sports will sell and install the same unit for $325.  I went with the latter vendor.  I carefully reviewed the proposed work with their gunsmith over Email and then mailed my stock to their shop in Atlanta.  The completed piece was back in my hands just a week after they received it.

After tweaking the tension on the piston and getting the butt pad in just the right place shooting with the GraCoil is so easy and natural I really could break clays all day long.

Freebore Boost Effects

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When I first took my .223 suppressor (Gemtech M4-02) to the range with a chronograph I was surprised, on reviewing the data, to find that it was doubling the standard deviation of muzzle velocity. Baffle suppressors are known to increase muzzle velocity in an effect known as “freebore boost.” Basically they extend the barrel length, which gives the propellant extra time to accelerate the bullet. So I was expecting to see higher velocities, but not higher variance: When I added the can to the end of the barrel average velocity went up about 50fps, but the standard deviation of velocity also went from about 22fps to 45fps!

When it comes to marksmanship anything that increases variance reduces accuracy. Are suppressors inherently bad for accuracy? After some correspondence on the SilencerTalk.com forum I decided to run a controlled experiment. I went back to the range with a stopwatch and a Mastercool infrared thermometer.

Muzzle Velocity versus Suppressor Temperature

What I found is that muzzle velocity is strongly correlated (R2 = .60) with suppressor temperature. I.e., once your can is hot your bullets will go faster. Several people hypothesized that this is simply because hot air is less dense, so it offers less resistance to the accelerating bullet. Another hypothesis is that the propellant burns more completely or more rapidly in the superheated air it encounters in the baffles, which increases the rate at which it can propel the bullet.

How hot does a suppressor get? For this experiment I shot Federal XM193 5.56mm (55gr Boat-tail ball) ammunition. On average each shot heated the 16 ounce suppressor by 10 degrees F. During one phase of the experiment I shot 30 rounds in under 90 seconds, which raised the suppressor temperature from 110 to 410 degrees F. During that extended string of shots my standard deviation was only 19fps — which is about the same variance I see when shooting without a suppressor. It took ten minutes for the suppressor to cool back down below 150F. (Ambient conditions were 62F, 25% humidity. Surface temperature in the sun was 88F. Winds were 10-15mph.)

If I may summarize liberally from the results of the experiment: A baffle suppressor essentially has two states, which we could call “Hot” and “Cold.” A suppressor is Hot when either (1) shots are fired in quick succession or (2) its temperature is above about 150F. In the first case I imagine that the baffles are still full of hot propellant vapor; in the second case the can is radiating enough heat to keep ambient air at this elevated temperature. A suppressor goes Cold as soon as ambient air seeps back in.

Using this equipment (16″ bbl and 6″ suppressor) the muzzle velocity out of a Hot suppressor is around 50fps higher than out of a Cold one. And if we look at the standard deviation of muzzle velocity when shooting Cold it comes out the same as when shooting Hot.