Here’s some high speed video comparing why free floated barrels move during firing. The results shouldn’t be very surprising, but the cause is a lot more simple than some might think.
The movement is not due to barrel fluting or piston/op-rod operation, but the simple act of a muzzle device placing uncontested force at the very end of the barrel. Because the barrel is free floated, it only has support where it meets the receiver. As a result, we see the barrel move a bit. When this same rifle has a muzzle device that does not place such force on the barrel, we do not see any movement.
you essentially have the favorite college engineering class example, a cantilevered beam with a downward load applied to the end. For a given brake, barrel length, barrel cross section and cartridge combination, you’ll have repeatable deflection. I didn’t include barrel material because all steel has the ~30M for its modulus of elasticity. If you change any of those, you’ll alter its response. Fluting could have an effect, it depends on what it does to the barrels moment of inertia. In simplified terms, if you have a fluted barrel with the same or greater moment of inertia than the barrel with the circular cross-section, you’ll get the same or less deflection, respectively. If you were to take a “normal” HBAR profile and flute it, you’d get greater deflections than if you had a larger outer diameter HBAR and fluted that.
The application is what most of us want to know about –
1) Does this create more dispersion of the group?
2)Given the barrels cross section, are some brake choices too much of a good thing?
Point being, recoil reduction or keeping the sights on target is good, but are there negatives in other circumstances, such as quick burst groupings at 200m?
Of course, one way is to film a dozen of these screwed onto a M4gery, the new profile barrel without M203 cuts, a recon profile, and HBAR in 16″ and 20″, standard and freefloat. At least 72 different combinations.
I don’t think that’s going to be a quick sunny Saturday afternoon at the range kind of thing. Probably have to cut down on that a bit.
My immediate thought of the title was relating it to, “pistol whippin’.” Lol
Mike my brother is a mechanical engineer, you sound just like him.
My take on this is that so long as the barrel has returned to center before the next round leaves the chamber, there is no issue. I would say measure and time the deflection, see if there is any possibility of deviation shot to shot, and if so, know your shot to shot deflection.
Intuitively, there has got to be a longer settling period for longer barrels, but longer barrels tend to live on longer, more methodically employed weapon systems, where the variation down range might be greater, but more time for settling is allowed in any event.
I think I am also looking at fluting in the positive sense (being additional ridges that add rigidity, rather than relieves that lose mass) so in this sense wouldn’t the fluted barrel whip less than a standard barrel?
I agree that the return to center is the most important part. I can’t see fluting adding rigidity without having to also add to the barrel’s initial diameter. I agree that fluting will reduce mass, it may even increase surface area, but unless it changes the barrel’s “I”, the moment of inertia, it won’t do anything for stiffness. Man, if I just had 10 minutes on my CAD system where I used to work, I could hammer all of it out.
Are you looking at it as a heavy barrel that has mass removed? I think I’m looking at a fluted barrel as being thicker than a Gov’t profile barrel, with the flutes milled in. That’s what I mean by adding rigidity, as compared to a gov’t profile, not as compared to a heavy barrel. A matter of starting point, if you follow me.
Having watched it again, it seems that the barrel is where it needs to be when the shooter is ready to send another round, but I’d like to see if full auto catches up to the whip.
Current whip (visibly) stops after less than 20 milliseconds, for the bolt to cycle and the hammer to fall again would take between 75 and 120 milliseconds. It may be vibrating, however, I cannot see that with my current equipment.
In a slow fire setting the barrel whip isn’t a problem as long as it consistent. It was a known issue in some of the high power airguns I used to hunt prairie dogs with and we still made solid hits out to ~70 yds.
The fa consideration on an m4 platform is interesting.
An old Mech. Engr. who hasn’t done shock and vibration analysis since college ~ ’60; nor fired an AR-15. But if memory serves me, for a given diam., the stiffness (resistance to flexing) is related to total cross section area and distribution within that area – area moment of inertia. And add to Mike W’s properties – “end support”. Generally, the more area at the periphery(eg. an I-beam), the siffer. For a given diam., fluting reduces both the total area and area disributed towards periphery. Also the end support – pinning – of the beam effects deflection. A cantilever (pinned – rigidly suported – at one end) will deflect more than rigidly pinning at both ends. A firearm barrel is generally a cantilever.
I believe I heard an interesting exception to being a cantilever is the Dan Wesson barrel. That skinny barrel should deflect more than a standard barrel – less cross section (smaller diameter). But grouping was found to be better. Eventually it was found that the barrel was virtually pinned at both ends: The encompassing heavy shroud, relative to the light barrel, pinned the muzzle end. So that skinny barrel was pinned at both ends.
But as I said, hadn’t looked at that stuff in many a decade. Sorry for the many words! Old engrs. never die, they just lose their sliderules.