Muzzle Flash

Hey everyone, I noticed something on the range yesterday that rekindled something I though about during my active military time.

I’ve noticed that my AR-15 build as well as many other weapons I’ve fired gets a bigger and brighter muzzle flash as the barrel gets hotter. I usually noticed it whenever I’d go to the cyclic rate of fire on the trusty 240B but many other weapons do it as well.

I was wondering if anyone else has noticed this phenomenon and might be able to shed some light on the subject as to why this happens.

My guess is that, because muzzle flash is caused by hot pressurized gases meeting cool air, the hotter the barrel gets the bigger and brighter the flash will become.

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I would guess that it would be due to the shrinking of the barrel (heat contracts steel) increasing the drag on the boolit as it goes down the tube which would seal the bore tighter creating a larger flash. Never chronographed a hot machine gun so I have no proof that the boolits were getting faster due to restriction but it seems to make sense on a physical science level.

Cheers,

Craig6

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Interesting… never thought about this, and definitely cannot discuss about AR15…
But…
In my case, I’ve noticed bigger flash shooting Walther PPQ than other handguns. And it did not depends on shooting duration.
Mostly I shoot Fiocchi and S&B (9mm). With 115gr never had muzzle flash. With S&B 124gr I noticed flashes every single shot on PPQ. Never on SIG, CZ, 1911.
Even during or after “Vegas drill” (45 rounds) I shot with my CZ there was no flushes.

I took photos of this phenomenon when visiting Range with my son.
Walther PPQ, S&B 124gr ammo:

I updated the topic with a pic of my AR with a SuppressorCo muzzle brake running 5.56 through an uncomfortably hot barrel. Lol

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That could definitely be a contributing factor. I’d be interested to see chrono numbers between hot and cold bore shots. I might have to get with a buddy and test that out.

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I had a rich Uncle that used to indulge my “experiments” but ever since my ID card turned from Green to Blue, I don’t get to play anymore :unamused: I’d be interested in the chronograph used for the experiment as they don’t record machine gun fire well unless you happen to know someone with a doppler range :sunglasses:

Cheers,

Craig6

To be technically correct, steel expands when heated. The effect of expansion on the inner diameter of a barrel (cylinder) is that the bore gets smaller. Physics don’t change.

Sorry to be “that guy”.

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JUst to enforce what you said, the expansion of the barrel’s bore also allows more gas to stream past the bullet, thus a bigger, brighter flash.
AlthoughI rarely fire my rifles at full “Cyclic Rate”. I will fire until the barrel is hot and check the bullet spread between the cold/cool barrel and the hot barrel.

Larry

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Watching IraqVeteran8888 on YouTube do an AR meltdown demonstrates this. As the gun gets hot, the muzzle flash increases, and changes color I might add. I would say, the hot barrel, provides less resistance. The lower resistance, means less powder is required to push the bullet down the muzzle. That’s why velocities increase. That leaves a certain amount of unburned powder, which result in a fireball.

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Hmmm. There seems to be disagreement on whether the barrel’s ID gets smaller or larger, and or more slippery as it heats up.

At this point I’ve heard two schools of though. The barrel swelling as it gets hotter tightens the bore, or loosens the bore? I would think as outside diameter gets bigger, the inside diameter would tighten because the metal swells internally and externally.

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OK, I think we need some other input. First the bore of a hot gun is not slicker, the bullet moves easier through a hot barrel due to expansion of the barrel as a whole the O.D. swells along with the I.D. swelling. The bullet has a lower resistance due to the fact that the bullet is not engageing the rifling of the barrel. The blast that is seen is the unburned powder and gas.
It is very elementary, Watson.

Larry

You are absolutely correct. When I think about hot vs cold and expansion and contraction I always default to freezing water makes it expand. Water in it’s natural state is fluid, steel obviously is a solid.

So heating the steel would expand the bore and the external bit of of the barrel as well loosening it as @45IPAC states meaning more gasses escaping with the boolet as opposed to being held in check and being consumed in the tube. That said @Gunner502 makes an interesting observation about internal and external expansion especially if you consider that the barrel is not heated equally.

Perhaps it is both, when a barrel is heated with say 10 rounds it enlarges internally but the external barrel is still relatively cool. At that point in time the cartridge would have a fair pressure spike on firing creating MORE CUP and an increase in muzzle velocity which would result in the “hot barrel loss of accuracy”. If rounds continue to be fired the barrel becomes the same temp on the inside as the outside, since the outside of the barrel has more surface area it would draw the internal dimension of the barrel out creating a loose condition where in gasses escape around the boolit as it goes down the tube.

I’ve been working on camshaft numbers all day and my head hurts, so if I’m wrong again it won’t be the first time today :upside_down_face:

Cheers,

Craig6

So you’re positing that the id and the od of the barrel both swell to a larger diameter.

I can see how it might do that, but can also see that as the steel gets hot, it would tighten the barrel also. But gas changes at the end of the barrel seem to indicate that more is escaping out the end.

Could it also be that the super heated barrel is burning or re burning gases that should have been cleanly expended? Like a car’s catalytic converter, and it has little to do with barrel size?

In order to move this topic along I would suggest a visit to the Ruger or S&W tech and pose the question. The fact remains that a portion of the barrel can not expand and the rest contract during the same heat cycle. The heat is moved from the interior to the exterior, heat expands.

Just an opinion of an old Mechanical Engineer.
Larry

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I’ll see if I can contact an expert on the subject tomorrow.

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This is turning into the gun store debates that I miss so much prior to the advent of the internet. It will come down to somebody with a chronograph and a barrel they wanna burn up and a couple hundred rounds of ammo. Then we will argue over the finer points of fireballs, color, velocity and distance. Ya’ll just took me to a happy place :heart_eyes:

Cheers,

Craig6

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image

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Alright guys, I messaged ruger and we’ll see if they get back to me. I also think I’m going to do a little experiment tomorrow measuring the ID and OD of a steel pipe before and after heating it with a torch. I’ll post what I find.

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Sounds like coronavirus to me. High fever, cough, no social distancing from other firearms in your safe?:thinking:
I’ve been quarantined way to long! My range is like my doctors office, by appointment only and the waiting time is awful.

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