smg762 said:

with 7.62 SAWs that are as light as an SA80....what would the recoil be like on full auto?

I had the opportunity to shoot the Evolys in 5.56 mm and 7.62 mm, and the recoil was very, very smooth (and I'm only 5'7" & 150 lbs).

To the point that one of the senior officer shooting the Evolys told us "the 7.62 mm is so light & easy to shoot, why do we need a 5.56 mm?"

So, the soft recoil, basically, means that the forward bolt stroke starts from the zero initial velocity, and thus the power reserve to feed the cartridge under less than ideal conditions (such as increased internal friction due to ingress of dirt, sand, lack of lubrication etc) is minimal. Or am i wrong here?

mpopenker said...

the soft recoil, basically, means that the forward bolt stroke starts from the zero initial velocity,

I would expect that the forward bolt stroke starts with zero initial velocity on every machine gun ever made. Did you mean zero spring load?

As far as I understand it, the bulk of felt recoil comes from impact with the buffer at the end of bolt travel, which is essentially a very much stiffer spring than the return spring. Using a hydraulic buffer ought to mean that you have put a damper into the system so the resistance from that element is based on velocity rather than position like it would be with a spring. As such you can apply it in parallel to the return spring without running the risk of stopping the bolt short, provided it's tuned right. So the bolt could have a preload from the spring at the point where it is held by the sear and consequently have enough power in the spring to feed cartridges even when fouled.

RovingPedant said:

I would expect that the forward bolt stroke starts with zero initial velocity on every machine gun ever made. Did you mean zero spring load?

Not necessarily

When bolt group bottoms up the spring and hits the rear of the receiver, it is usually an elastic impact, and bolt group receives some of its remaining recoil energy back, to start with some initial velocity (imagine a steel ball falling on a steel plate and bouncing back)

Hydraulic or friction buffer (as opposed to the spring buffer) eats up (dissipates) most of its excessive energy, so the return spring starts the bolt group back from the zero, or close to that. Only the power stored in the return spring is available to operate the feed, lock and fire cycle.

For example, in a typical Kalashnikov AK assault rifle bolt group rebounds back from the rear trunnion at the initial velocity of 3-4 m/s under normal conditions, adding some significant surplus KE to the amount of energy stored in the return spring. This is one of the reasons why AKs are so reliable, but also results in an 'excessive' felt recoil. TANSTAAFL.

mpopenker said...

Not necessarily

When bolt group bottoms up the spring and hits the rear of the receiver, it is usually an elastic impact, and bolt group receives some of its remaining recoil energy back, to start with some initial velocity (imagine a steel ball falling on a steel plate and bouncing back)

 

This is something that would happen under automatic fire only? Otherwise it will be stopped by the sear?

mpopenker said...

Hydraulic or friction buffer (as opposed to the spring buffer) eats up (dissipates) most of its excessive energy, so the return spring starts the bolt group back from the zero, or close to that. Only the power stored in the return spring is available to operate the feed, lock and fire cycle.

Which would be true when the bolt is held by the trigger/sear arrangement, wouldn't it?

mpopenker said:

       RovingPedant said: I would expect that the forward bolt stroke starts with zero initial velocity on every machine gun ever made. Did you mean zero spring load?

Not necessarily When bolt group bottoms up the spring and hits the rear of the receiver, it is usually an elastic impact, and bolt group receives some of its remaining recoil energy back, to start with some initial velocity (imagine a steel ball falling on a steel plate and bouncing back)

A steel ball falling on a steel plate reaches a velocity of zero before bouncing back.  The same is true of a machine gun bolt.  Initial velocity is zero.

mpopenker said:

For example, in a typical Kalashnikov AK assault rifle bolt group rebounds back from the rear trunnion at the initial velocity of 3-4 m/s under normal conditions, adding some significant surplus KE to the amount of energy stored in the return spring. This is one of the reasons why AKs are so reliable, but also results in an 'excessive' felt recoil. TANSTAAFL.

The AK is firing from a closed bolt, while nearly all MGs (and the Evolys in particular) are firing from an open bolt (but you already know that). That means that the first round is always fired with zero bolt initial velocity and the return spring should be designed with this point in mind.

Yes a steel ball falling onto a steel plate does reach a velocity of zero, right before gaining back some of the velocity it just imparted to the steel plate from an inelastic collision (or is this technically an elastic collision? I'm not an engineer i just play one on tv) and the restitution rate of the two materials in question.

It actually can gain back a fairly substantial amount of the energy it just dumped into whatever it hit depending on the restitution rates of the two materials in question as well as other relevant factors which I poorly understand other than knowing in an abstract way that they exist.

If we're talking about the AK and pkm as they currently exist, both designs do in fact impart extra energy to the return stroke from their collision with the rear trunnion. Whether another design would do so or not and to what degree would heavily depend upon the specifics of that design, however in these two cases Max is entirely correct.

This is one of the stealth advantages of the pkm operating mechanism with it's pull out then push forward belt feed mechanism. It actually lets you shorten the receiver quite substantially without compromising reserve power.

The chicoms make 308 non disintegrating belt pkm clones that still pull then push feed using the signature pkm claw as it is and it's not totally impossible to make a disintegrating link belt for a pull then push feed gun.

When you consider that it will probably be easier to make lightweight polymer pull out disintegrating links it's not entirely out of the realm of possibility that true next generation belt feds could use pull out links.

roguetechie said:

If we're talking about the AK and pkm as they currently exist, both designs do in fact impart extra energy to the return stroke from their collision with the rear trunnion. Whether another design would do so or not and to what degree would heavily depend upon the specifics of that design, however in these two cases Max is entirely correct.

No, Max is entirely incorrect.  Max said that the bolt starts with "some initial velocity."  

Even the PKM bolt starts with zero initial velocity.  https://youtu.be/01mrTVUJ_Bo?t=45