A Flash Course In Physics

Muzzle Blast Basics

By Will Dabbs, MD

No matter how you slice it, day or night, muzzle flash looks super cool. This SBR from Blacksword
delivers it spades.

In 1686 Sir Isaac Newton published his Principia Mathematica Philosophiae Naturalis, a seminal treatise on physics that included his three laws of motion. In the great man’s own words, they are: (1) Every object persists in its state of rest or uniform motion in a straight line unless it is compelled to change that state by forces impressed on it; (2) Force is equal to the change in momentum per change in time. For a constant mass, force equals mass times acceleration; and (3) For every action there is an equal and opposite reaction.

These three basic rules govern the way everything in the universe moves. From the locomotion of a flagellated bacterium to the unimaginably vast forces driving planetary mechanics, it all boils down to these three fundamental laws. And, in addition to all that other stuff, Newton’s laws also determine how our guns work.

Super-short 5.56 SBRs produce ballistic performance close to that of this Rock Island Armory
.22 TCM 1911, but in a bigger package. Quite the fire-breathing dragon, no?

On A Personal Note

I once built a gun I christened “The Pocket AR.” Sporting a novel takedown upper assembly from Cry Havoc Tactical and a ridiculously short 7.5″ barrel from Model 1 Sales. This diminutive flamethrower will stow inside a briefcase and has been surprisingly reliable. However, the muzzle flash in rapid fire will loosen your dental fillings while simultaneously igniting nearby brush. In an effort at making this thunderous little gun even more effective, I inadvertently crossed into The Dark Side of Sir Isaac’s three inviolate rules.

The barrel on this SBR is so short the handguard telescopes around it. Back when I had the muzzle tipped with a standard M4 flash suppressor this wasn’t a problem. However, when I swapped it out for a Hiperfire 556C muzzle compensator all heck broke loose.

The Hiperfire 556C does a simply splendid job of redirecting all the white-hot plasma to the top and sides to counteract the recoil impulse of the gun. This phenomenon is based on the same principle driving the rocket launching Elon Musk’s red Tesla roadster on its odd trek toward Mars. Jet a little high-speed gas out the snout of your gun and it will inevitably push back.
Newton’s laws predict these prodigious mechanical forces. A little bit of mass embodied within this hot gas, combined with a whole lot of acceleration directed appropriately, creates a force pulling the gun forward while pushing the muzzle down. The end result was a controllable little rifle and a corresponding catastrophic failure of the handguard.

Will threaded a Hiperfire 556C on the snout of his Pocket AR. The good news? It got more controllable.

Will’s homebuilt takedown SBR with its 7.5″ barrel. Yes, you read the barrel length correctly.

The bad news about the Hiperfire? After about 10 rounds the plasma had chewed the
handguard to pieces. It’s an effective brake. But, as Will found out, it needs to
run outside the handguard! Oops.

The Issue Of Scale

We get a little bit jaded to the power we wield when we head out for some Saturday afternoon recreational blasting. The sheer volume of energy involved is breathtaking. Pressures in a high-pressure scuba tank run around 3,500 psi. By contrast, the pressure in the chamber of a 5.56mm rifle can reach 62,000 psi. In the case of my Pocket AR, all the redirected muzzle blast quite efficiently deconstructed my handguard while literally scorching the hair off my support hand.

The Hiperfire 556C sports a series of gills directing the muzzle blast as effectively as might a rocket nozzle. When the resulting plasma jets are focused on the 6061-T6 aluminum of a typical modern black rifle handguard, the immense forces involved can and will eventually rip the material to pieces. In this particular case it also left the backside of my left hand smoother than might a proper bikini wax.

Nobody got hurt and it did spawn some fairly entertaining prose (after all, I just got to use the words “bikini wax” in a gun magazine!). However, had I wrapped my support mitt around the far end of the handguard the result could have been much more colorful. My soft pink anatomy likely would not have fared well enfolded around the equivalent of a 62,000-psi blowtorch.

Holy donuts! This remarkable shot was captured from an M203 grenade launcher
firing standard 12-gauge birdshot through a 40mm adapter. Imagine a parade of
those flaming rings going toward a target!

Really big military boomers need big brakes to harness all the commotion.

Speed Kills

A 55-gr. M193 5.56 round pushed through Gene Stoner’s original 20″ M16 barrel will chronograph a bit north of 3,000 fps. Cut the tube by two thirds and the velocity drops to around 2,000 fps. By contrast, a 40-gr. .22 TCM round fired through a 5″ Rock Island Armory 1911 produces roughly the same 2,000 fps. As the effectiveness of these small-caliber rounds is driven primarily by their velocity, this means a really short-barreled AR packs roughly the same downrange thump as a .22 TCM 1911 — at several times the weight and bulk.

The muzzle flash on a really short-barreled 5.56mm rifle can be visible from the International Space Station and — if you’re close enough — will reliably clear your sinuses. Such theatrical visuals are dependable crowd pleasers anywhere two or more gun nerds are gathered. However, the same downrange pandemonium will also utterly disassemble your hand should you let it slip into uncharted territory.

Stubby little guns are cool and the Hiperfire 556C is remarkably effective. However, keep it clear of hands and handguards.

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One thought on “A Flash Course In Physics

  1. Larry Koehn

    Get a muzzle device that directs ALL noise and blast away from the shooter. Who needs a brake or a compensater on a 22 caliber rifle? My wife loves my full size AR because there is no recoil. I love my 28 inch overall AR pistol. It has a 12.5 inch barrel and has a 2 in.welded titanium blast deflector that is a medium flash hider that directs all noise and blast straight out the muzzle of the gun. It is a Helios device available from vseavenweapons.com. 129 bucks. Get you one.

    Reply

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