I’ve seen the claim several times that gas tube length, dwell time and gas port size can increase or decrease recoil energy.
I don’t mean perceived recoil nor do I mean how sharp or soft the recoil feels but the actual amount of energy generated during recoil. All else being equal, does a midlength rifle actually kick less than a carbine length? Or is it just a softer recoil as the impulse is spread out over a longer time?
If the carbine length does generate more recoil energy, where is the empirical data to show this?
Impulse is instantaneous and is the reaction from the bullet going down the barrel.
2 stages of recoil
Impulse
BCG expending extra energy into your body.
In an ideal case, the BCG will have no velocity and 0 kinetic energy at the end of the buffer tube, therefore no energy to send into your body. Impulse will always be felt, the only thing that reduces it is using lighter bullets.
Longer dwell = more gas = more energy to BCG
Less dwell = less gas = just enough energy to BCG
the midlength is closer to the ideal case.
i disagree- the amount of energy the BCG transfers into the shooter is very dependent on the speed it’s going when it bottoms out on the spring.
if the BCG is flying back faster than it needs to be for reliable cycling, then you’re gonna have an “over-gassed” gun with more recoil- felt or otherwise.
mid-lengths supposedly reduce impulse- felt recoil- by shortening the amount of time the bore is injecting the action with gas. moving the port out closer to the bore means is takes more time to get gas back to the key, and having the bullet pop out the muzzle sooner after passing the gas port then cuts off the already slightly lower pressure gas.
so, on paper, it DOES reduce impulse. personally, i don’t really notice a difference- at least between 14.5s… it’s been many years since i’ve fired a 16" carbine length gun
Naturally. Proven by the equation KE = 0.5mv^2 stick in 0 for v and KE must equal zero. That’s why I say for a system that is gassed just enough, the velocity (and also kinetic energy) of the BCG at the end of the buffer tube will be very close to 0.
I guess it somewhat boils down to semantics. In a physics sense, the impulse occurs before the bullet ever passes the gas port. The expanding gasses act in the direction of the bullet but also in the opposite direction, causing the recoil.
semantics in that initial recoil is caused by the bullet, but followed up with another bump from the carrier… but since the initial recoil is only part of felt recoil, changing bullet weight (or otherwise changing initial recoil-since initial recoil isn’t just from the effort of shoving the bullet down the bore, it’s also contributed to by the propellant blast) isn’t the “only way” to reduce felt recoil. both have to be addressed.
So far, we have that changes in the dwell time, tube length and gas port will affect perceived recoil (felt recoil) but does not increase nor decrease the actual recoil energy or free recoil
Not quite accurate. The BCG cannot expend extra energy into your body. What got it moving in the first place is the energy that generates the recoil. That part of the energy is stored in the momentum of the BCG and compression of the spring.
Shooter’s body provides (along with the mass of the rifle) the resistance needed for the BCG to compress the spring. This will transfer kinetic energy to the body of the shooter even if the BCG stops before it bottoms out the sprinq
No expert but this is what I think…you can’t create energy and recoil is energy.
So, if the rifles weigh the same with the same weight bullet and charged round AND the FPS of the bullet out the barrel is the same with the same FH/Comp’s, they have to have the same recoil. The “felt recoil” can be different but the true actual total energy pushing to the rear will be the same.
The “felt recoil” can be different because the rifle buffer, springs, gas port, gas tube, etc. spread that release of recoil energy over a longer amount of time, instead of just a sharp impact to the rear. Example is a mid-length vs. a M4.
If one gun of the alike guns has more recoil (actual total energy pushing to the rear) then the FPS will be different. The energy that is causing more recoil has to come from somewhere, so it is robbed from the speed of the bullet. Now, how much speed is lost, who knows, but I would think it would be measurable. I reckon this is called, “over gassed”.
The two are one in the same. The propellant blast is the effort shoving the bullet down the bore.
Yes, this is the same principle air bags and crumpling auto frames were designed around.
You are mostly correct. The bullet isn’t robbed of energy per se, rather (assuming same weight bullets) its given more energy to increase the FPS (KE = 0.5mv^2) Conversely, a heavier bullet requires more energy to give it the same velocity of a lighter one. If you’ve ever shot 12ga slugs, you know they kick a lot more than bird shot, its because they are heavier and have the same muzzle velocity (~1200 FPS). This energy is transferred in both equal and opposite directions, which explains the extra recoil.
if there was nothing in the bore, as in a complete vacuum, the propellant blast from a blank would still move the weapon rearward by itself. not by much, unless it was also in a state of inertia, but its still part of the equation.
First of all please excuse the crude drawings, they are not to scale and do not represent the actual part geometry. I’m a visual person myself so I thought this would help.
The first picture represents the rifle and the bullet as two solid, independent bodies. Because the bolt is locked until the bullet passes the gas port, they can be analyzed this way. The blue arrow represents the force of combustion acting on the bullet and the rifle body. This action is equal and opposite. We know that the mass of the rifle body is much more than the mass of the bullet. We also know that the force acting on both bodies is equal. If we consider the equation F=ma (force = mass x acceleration) we can conclude the the acceleration of the bullet will be much greater than the acceleration of the rifle body. Furthermore, we can use the muzzle velocity and bullet weight to figure out the energy given to the rifle body (I keep saying body because I want to emphasize that at this point that the entire rifle can be treated as a solid piece).
I’m sure you’re all familiar with the second picture, there are much better pictures and animations available. I just wanted a visual aid. After the bullet passes the gas port, the gas is sent to BCG, unlocking the bolt. The rifle can now no longer be treated as a solid piece.
