Would changing the weight of gun components affect recoil?

[sophiecentaur]

Will the momentum of the projectile be the same, if you only change the mass of the BCG (or the mass of the gun)?

Not strictly, of course - you are right. But the KE of the projectile is by far the greater share, because the ratio of the masses is so high. 40g vs 2kg (= 1/50) is a ballpark value, giving the bullet fifty times the amount of KE.

And what if you trigger the gun to accelerate as the recoil occurs? A precisely timed jerk of the barrel would counter it and the reference of the person holding the gun the jerk would start as the barrel starts moving and the bullet launch would be opposite so the gun would quickly return to stop at its original location. The gun would jerk away then back to rest.

I guess that wouldn't really be accurate for a human to hold...

That could be a novel idea but what could you accelerate it with? Rocket power? You'd have to deal with a recoil from that action as well, which could upset the timing of when you fire. (A bit like the autofocus on early digital cameras, which meant you always missed the action!.) Plus, you would need the action to be perfectly balanced to avoid a kick, to spoil your aim.
It strikes me that a system (present ones?) that deals with the recoil after the bullet has left, will upset the aim least.

 

[Bystander]

40g vs 2kg (= 1/50) is a ballpark value

More like 4-5 g vs 4-5 kg (maybe a little lighter).

 

[sophiecentaur]

Which has been pointed out multiple times in this thread.

If one wishes to examine "what" recoil systems for field artillery are designed and required to handle, recoil is energy. If one wishes to examine effects of recoil of the GAU-8 upon aerodynamics of an A-12, one is going to be more concerned with momentum (loss of airspeed).

Field artillery has a serious problem because the guns need to be light enough to haul around the place. Land based, fixed guns are probably the least problematical (if the concrete emplacement is strong enough). They still need to have built in symmetry or great torsional strength, to stop the bucking tendency.

 

[sophiecentaur]

More like 4-5 g vs 4-5 kg (maybe a little lighter).

Useful information and makes my point even better. Thanks.
Yes - 40g would be a bit like an elephant gun!

 

[CWatters]

Well, according to wikipedia, recoil is momentum.

http://en.wikipedia.org/wiki/Recoil#Perception_of_recoil

That seems clear. However the OP mentions the weight of moving parts within a gun not the weight of the gun itself. Overall conservation of momentum applies but with several bits moving it seems unlikely that the recoil velocity is a simple single event...

The gas from the barrel of the AR15 travels down a tube from the barrel, and hits what's called the "gas key." The gas key is connected to the BCG (bolt carrier group.) When the gas hit's the BCG's gas key, that's what starts the rearward travel of the parts.

This suggests that for part of the time the BCG moves backwards FASTER than the butt is recoiling. If one part of the gun recoils faster (temporarily) the rest of the gun must recoil slower (at least temporarily). So it might go something like this...

Bang
Butt and BCG start moving backwards at some velocity V determined by conservation of momentum
Bullet passes gas key allowing gas to travel backwards, BCG starts moving backwards faster than the butt so the butt must slow.

eventually the BCG/Buffer will hit the very back of the tube

At which point the butt must speed up again.

Clearly overall conservation of momentum must apply but I suspect the weight of the BCG may effect what the velocity profile looks like. Exactly how would be tricky to work out. Probably easier to measure it?

 

[dshield55]

So, now I have a whole bunch of questions that I need answering.

If kinetic energy compresses a spring, we have the formula (1/2)mv^2=(1/2)kx^2. But if energy is related to momentum then the kinetic energy formula for that is (p^2)/2m. Either way, the momentum of an 8oz bolt system going twice as fast the 16oz bolt system is going to have completely different effect on how far the buffer spring is compressed. Although the momentums are the same, the energy of the 16oz system would be double the energy of the 8oz system and the 16oz system is going to compress the spring slightly more.

One of my previous questions that I felt like wasn't answered is whether I could assume that if the gas setting was the same and the spring was the same, would the higher mass system compress differently than the lower mass system. And apparently it doesn't.

So is the recoil still less for the heavier slower system? Does it take longer for the heavier mass to compress the same spring being that it will compress it further than the lower weight system?

One of the objectives of custom AR15s that have an infinitely adjustable gas system is to tune it just right so that the spring is only compressed far enough to cycle the weapon and pick up a new bullet but not get so far compressed so as to hit the rear of the receiver extension tube all the parts are traveling down. If the heavier system could compress the spring further than the lighter system, would it then follow that I could apply less gas to a heavier system than a lighter system in order to get them to travel the same distance?

 

[twoslit]

Which has been pointed out multiple times in this thread.

If one wishes to examine "what" recoil systems for field artillery are designed and required to handle, recoil is energy. If one wishes to examine effects of recoil of the GAU-8 upon aerodynamics of an A-12, one is going to be more concerned with momentum (loss of airspeed).

Recoil has a direction, so it is a vector quantity. Momentum is a vector. Energy is not a vector. To equate recoil with energy cannot be correct.

 

[dshield55]

Recoil has a direction, so it is a vector quantity. Momentum is a vector. Energy is not a vector. To equate recoil with energy cannot be correct.

If recoil is a vector, couldn't it also be a force? Force and Momentum are both vectors, right?

For instance, for the purpose of this question, I didn't want to deal with the recoil associated with the bullet and gasses leaving the front of the barrel. Conceptually, I can see that as being a momentum issue. But, for the purpose of this question dealing with recoil associated with the moving parts of the action, wouldn't it be a force causing the recoil and not momentum?

The reason I think it would be a force is that I'm visualizing the moving parts compressing the spring as they move rearward and I think it's the force of the compressed spring that is causing the gun to dig into your your shoulder and support hands causing the recoil, not necessarily the momentum of the parts moving. Could that be correct?

 

[sophiecentaur]

That's a statement about what you do with the momentum. I could design a recoilless rifle - it shoots two bullets, simultaneously in opposite directions. But I think the shooter - and the person behind him - would not think this is an acceptable solution.

A Bazooka is an alternative. Leave the momentum transfer to the rocket propulsion. (You can still 'singe' the guy behind you, if you are not careful)