# Modal analysis of gunshot

## Main Question or Discussion Point

I am not a weapons designer, this is purely to satisfy my own curiosity :)

I would like to simulate a gun shot in a rifle barrel and determine the mode and freq. of vibration using a FE program (Code_Aster if anyone is interested), but I don't really know what transient load to apply.

What components go into making the barrel vibrate? Shape of course, boundry cond. i.e. barrel to receiver connection etc. - but how would you describe the discharge of a round?

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minger
Ah, I JUST ran through a similar tutorial of this for the new version of ANSYS. Anyways, I'm going to assume that you are trying to model some sort of automatic gun that can cycle enough times such that resonance could come into play.

Regardless, here's what you kind of do. You'll need to do a full fluid-structure interaction problem. You'll have to either use a combusion model, or a source term to model the bullet (it'll be MUCH MUCH easier than actually modeling the bullet). From there, you do a multiphysics run where you're sending pressures, temperatures, and forces to the structural analysis and getting back displacements and temperatures.

From this, you can do a pre-stressed modal analysis and obtain natural frequencies and modes. At this point you can determine which modes may be of interest. You have a definite applied force, so you can then know which modes need to be payed attention to.

FredGarvin
How do you describe the discharge? I would venture a guess that it's probably broadband noise. I can't imagine there being any kind of dominating components to the signal.

minger
In the example that I did, they simply added mass to the domain as a source term, using a step function. We're probably still a little way off to be able to successfully model a gunshot in every way; the idea is to simply get in the ballpark.

p.s. The particular test I ran through included a muzzle brake at the end of the barrel which made some pretty cool pictures.

I may be in over my head here, but I would love to skip the combustion step and see if theres a predefined way of applying the mechanical forces to the barrel - what peaked my interest, was the fact that when you add a sound suppressor to a bolt action rifle (not automatic), you will have to re-sight it to accommodate for the new resonate frequency.

I know I'm just barely scraping the surface of the subject here :)

Minger: If you ever in any way publish anything about that experiment, I would love to see it - sounds very cool

FredGarvin
So you just want a modal analysis of the barrel? Your original post made it appear that you were looking at the gun shot itself.

what peaked my interest, was the fact that when you add a sound suppressor to a bolt action rifle (not automatic), you will have to re-sight it to accommodate for the new resonate frequency.
Where did you hear this? I am interested in this. Knowing what I know of modal testing, I can't see how this would be an influencing factor over other things like say decreased muzzle velocity.

minger
No no, this isn't an experiment that we're doing, rather just a tutorial that I went through getting an introduction to the new multiphysics setup with ANSYS.

As Fred said, doing a simple modal analysis may be enough for you. However, you mentioned transient analysis as well, which isn't really needed for a basic modal analysis.

Now having the actual applied loads can be nice. For example, you may think you're operating right on a natural frequency, however if the applied load is something which cannot excite the resonance then....eh.

So you just want a modal analysis of the barrel? Your original post made it appear that you were looking at the gun shot itself.

Where did you hear this? I am interested in this. Knowing what I know of modal testing, I can't see how this would be an influencing factor over other things like say decreased muzzle velocity.
It is of that principle a barrel tuner works i believe.