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Discussion Starter #1
Hey guys, I was thinking last night about how to make the 17 lighter and maybe lessen the recoil/jarring effect by reducing the reciprocating mass.

I know that it's a battle rifle and if I was using it as such, I'd leave it as FN intended, but I like to tinker with stuff as a hobby, so please don't try to convince me to leave it alone, as that's not what I'm inquiring about. Let's try to keep this thread free of rants about "it's perfect as FN intended" and "don't mess with it". Think of this discussion as "how to do it right" (if that is even possible), rather than "is doing it the right thing to do". Thanks!

What do you think about having a bolt carrier made of titanium or aluminum? I wonder which would be lighter? Advantages or disadvantages of each?
The aluminum one may need a titanium or steel cap or such on the front where the piston strikes it. Thoughts?

I would think less mass would require a lighter recoil spring too. Does a x% weight reduction translate to the same x% spring pressure reduction?

Any machinist out there that could make one to test? Any idea about cost?
 

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Aluminum would be lighter, but titanium stronger. For aluminum you'd probably want a 7075 or 7178 with a T6 hardness, and probably a good hard coat anodizing for better wear resistance. Not familiar enough with the FN design to offer much more than that.
 

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Me and a buddy of mine had thought about the same idea. I think it's doable, but would involve a lot of testing. More testing then we wanted to invest in. From your post, it seems you have the same concerns we did. The journey to find out if you need heavier springs, different jet size, or just how to tune it in just right would require a fair amount of ammo. You might get lucky and find it quickly - but before people buy a different bolt carrier - they are going to want to know that it's been tested for thousands of rounds I think. Would it make receiver screws cant or damage them if the speed of the bolt carrier increased ? Would the bolt carrier wear evenly ? I think it's a great idea, but seems to be a project left to a company that has more financial resources to undertake. Financial liability for any damage done to the rifle if we missed something came into play too. I really wish someone would put the research and development into it and produce on though - and that's assuming it would function just as well as FN's OEM bolt carrier. It could possibly solve the issues people have with lower end scopes and electronics used on the rifle.
 

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Cost prohibitive. I am sure you can find a manufacture out there that would make you one for the 17 though.

Titanium is 45% lighter than steel. So you would save about 12oz or so. You would have to rework the entire reciprocating system though.
 

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Hey guys, I was thinking last night about how to make the 17 lighter and maybe lessen the recoil/jarring effect by reducing the reciprocating mass.

I know that it's a battle rifle and if I was using it as such, I'd leave it as FN intended, but I like to tinker with stuff as a hobby, so please don't try to convince me to leave it alone, as that's not what I'm inquiring about. Let's try to keep this thread free of rants about "it's perfect as FN intended" and "don't mess with it". Think of this discussion as "how to do it right" (if that is even possible), rather than "is doing it the right thing to do". Thanks!

What do you think about having a bolt carrier made of titanium or aluminum? I wonder which would be lighter? Advantages or disadvantages of each?
The aluminum one may need a titanium or steel cap or such on the front where the piston strikes it. Thoughts?

I would think less mass would require a lighter recoil spring too. Does a x% weight reduction translate to the same x% spring pressure reduction?

Any machinist out there that could make one to test? Any idea about cost?
You have two things to consider: conservation of energy and conservation of momentum. Energy is equal to one half of the mass times the square of the velocity. Momentum is equal to mass times velocity.

The gas system imparts a specific amount of momentum and a specific amount of energy to the piston which then transfers almost all of that to the bolt carrier. Because momentum and energy are fixed, the relationship between mass and velocity are inverse. In other words, if you lighten the bolt carrier it will travel much faster. Lightening the recoil spring will increase the velocity even more.
Momentum and energy are both absorbed into the system over the time it takes to slow the bolt carrier. Slowing the bolt carrier happens due to friction between moving parts (which we try to reduce to avoid wear) and the force exerted by the recoil spring. Ideally, the velocity when the bolt carrier reaches the buffer at the aft end of the upper receiver will be as relatively low. The connection between the upper receiver and the buffer must dissipate both the remaining energy and momentum. The entire time the impulse (change in momentum over time) is being delivered to your shoulder.
Your idea of reducing both bolt carrier mass and recoil spring spring constant will lead to a very quick recoil impulse with a sharp slap delivered to your shoulder and quite possibly shear the screws at the aft end of the receiver. A shorter recoil impulse feels much sharper to the person doing the shooting.
If you reduce the mass of the bolt carrier you will have to increase the recoil spring spring constant or find some other way to slow down the bolt carrier. Increasing the spring constant of the recoil spring will result in an increased velocity for the bolt carrier as it slides forward to strip the next round. That again results in greater energy when the bolt slams the bolt closed, which results in wear to the bolt head and barrel extension. The firing pin will have more energy and momentum as well, which means you risk slam fires unless you change the firing pin spring.
It’s not that FN is perfect or that their products are shrines that shouldn’t be defaced, but after as much time as they have made firearms they have developed some institutional knowledge of the balance between bolt carrier mass and recoil spring strength that must be struck when designing automatic weapons.

Please let us know how it goes. Experimenting is great, but for your own safety you should first study and understand the physics.


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Like HK said, if you lighten the BC you will have timing issues. You'll need a new recoil spring likely a stiffer one, possibly a different gas jet size too. Doable for sure but a considerable amount of testing will be required. There's also the OEM BC's weight and inertia which actually gives the SCAR it's felt recoil impulse. If you lighten the BC you might end up not liking how it feels, not sure a few ounces of shaved weight is worth it. It is your rifle, tinker away and good luck...
 

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Discussion Starter #7

Cost prohibitive. I am sure you can find a manufacture out there that would make you one for the 17 though.

Titanium is 45% lighter than steel. So you would save about 12oz or so. You would have to rework the entire reciprocating system though.
Yeah, I'm not sure titanium makes much sense for AR bolt carriers, since manufacturers make lightweight steel ones already. 45% lighter, saving 12oz, sounds great! Just need to find someone to make it.
 

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Discussion Starter #8
You have two things to consider: conservation of energy and conservation of momentum. Energy is equal to one half of the mass times the square of the velocity. Momentum is equal to mass times velocity.

The gas system imparts a specific amount of momentum and a specific amount of energy to the piston which then transfers almost all of that to the bolt carrier. Because momentum and energy are fixed, the relationship between mass and velocity are inverse. In other words, if you lighten the bolt carrier it will travel much faster. Lightening the recoil spring will increase the velocity even more.
Momentum and energy are both absorbed into the system over the time it takes to slow the bolt carrier. Slowing the bolt carrier happens due to friction between moving parts (which we try to reduce to avoid wear) and the force exerted by the recoil spring. Ideally, the velocity when the bolt carrier reaches the buffer at the aft end of the upper receiver will be as relatively low. The connection between the upper receiver and the buffer must dissipate both the remaining energy and momentum. The entire time the impulse (change in momentum over time) is being delivered to your shoulder.
Your idea of reducing both bolt carrier mass and recoil spring spring constant will lead to a very quick recoil impulse with a sharp slap delivered to your shoulder and quite possibly shear the screws at the aft end of the receiver. A shorter recoil impulse feels much sharper to the person doing the shooting.
If you reduce the mass of the bolt carrier you will have to increase the recoil spring spring constant or find some other way to slow down the bolt carrier. Increasing the spring constant of the recoil spring will result in an increased velocity for the bolt carrier as it slides forward to strip the next round. That again results in greater energy when the bolt slams the bolt closed, which results in wear to the bolt head and barrel extension. The firing pin will have more energy and momentum as well, which means you risk slam fires unless you change the firing pin spring.
It’s not that FN is perfect or that their products are shrines that shouldn’t be defaced, but after as much time as they have made firearms they have developed some institutional knowledge of the balance between bolt carrier mass and recoil spring strength that must be struck when designing automatic weapons.

Please let us know how it goes. Experimenting is great, but for your own safety you should first study and understand the physics.


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That's great info. Thanks for sharing. I understand what you're saying about the spring, but are you considering adjusting the gas jet to have just enough pressure to push the lighter BC? I already have a collection of PMM gas jets and I use the Mototech 3 position regulator. I'd start with the smallest jet and set the regulator to the lowest setting and that should get the BC moving very gently. Since the BC is getting just enough gas pressure to push it back would a lighter spring be better to push it back into battery, since the weight it's pushing back is less?
 

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That's great info. Thanks for sharing. I understand what you're saying about the spring, but are you considering adjusting the gas jet to have just enough pressure to push the lighter BC? I already have a collection of PMM gas jets and I use the Mototech 3 position regulator. I'd start with the smallest jet and set the regulator to the lowest setting and that should get the BC moving very gently. Since the BC is getting just enough gas pressure to push it back would a lighter spring be better to push it back into battery, since the weight it's pushing back is less?
It may require tuning both spring and jet. Keeping in mind the BC is in the middle of two opposite forces, gas and recoil spring. If one overpowers the other the cycle timing is off. I would think having a machinist make the BC would be the easiest part. Finding that happy balance between the aforementioned opposing forces will take patience and ammo. Like I said in my initial reply, the mass of the BC will change along with it inertia and felt recoil impulse.

It may or may not need I lighter spring, I have no idea. If you can adjust down gas flow and expansion perhaps if not a lighter BC will slam the buffer at a much higher velocity.
 

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Discussion Starter #10
Me and a buddy of mine had thought about the same idea. I think it's doable, but would involve a lot of testing. More testing then we wanted to invest in. From your post, it seems you have the same concerns we did. The journey to find out if you need heavier springs, different jet size, or just how to tune it in just right would require a fair amount of ammo. You might get lucky and find it quickly - but before people buy a different bolt carrier - they are going to want to know that it's been tested for thousands of rounds I think. Would it make receiver screws cant or damage them if the speed of the bolt carrier increased ? Would the bolt carrier wear evenly ? I think it's a great idea, but seems to be a project left to a company that has more financial resources to undertake. Financial liability for any damage done to the rifle if we missed something came into play too. I really wish someone would put the research and development into it and produce on though - and that's assuming it would function just as well as FN's OEM bolt carrier. It could possibly solve the issues people have with lower end scopes and electronics used on the rifle.
Maybe it's something we could all work on together. I've got a pile of ammo ready too. Got it back when it was cheap. Maybe we could split the cost to buy an extra BC to send to a manufacturer to copy, if they needed one. Happy to share costs with anyone wanting in on the project. I'd volunteer to get it running and figure out the jet size. Then I could ship it to you and have you run it as much as you wanted to test it too. The more testers the better IMHO.
 

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Discussion Starter #11
It may require tuning both spring and jet. Keeping in mind the BC is in the middle of two opposite forces, gas and recoil spring. If one overpowers the other the cycle timing is off. I would think having a machinist make the BC would be the easiest part. Finding that happy balance between the aforementioned opposing forces will take patience and ammo. Like I said in my initial reply, the mass of the BC will change along with it inertia and felt recoil impulse.

It may or may not need I lighter spring, I have no idea. If you can adjust down gas flow and expansion perhaps if not a lighter BC will slam the buffer at a much higher velocity.
Yeah, I think the gas jet part will be easy, since most of us already know how to do that. It's that pesky spring that has me concerned. Maybe I could talk Wolff Spring into making some varying weights of springs to test. Other than that, clipping off 1 coil at a time and testing the result is the only option I know of. Good thing OE springs are relatively inexpensive.
 

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That's great info. Thanks for sharing. I understand what you're saying about the spring, but are you considering adjusting the gas jet to have just enough pressure to push the lighter BC? I already have a collection of PMM gas jets and I use the Mototech 3 position regulator. I'd start with the smallest jet and set the regulator to the lowest setting and that should get the BC moving very gently. Since the BC is getting just enough gas pressure to push it back would a lighter spring be better to push it back into battery, since the weight it's pushing back is less?
As others have also pointed out, there are lots to consider.
Any bolt carrier group must be going slowly enough to allow the magazine to lift a shell in time, but must have enough inertia (which is literally momentum) to move the shell forward against friction from the shell or follower beneath it, the feed lips guiding it, and drag of the bullet tip against the feed ramp.
Reducing the gas port will definitely slow the rearward velocity. At some point it will fail to travel far enough back or fast enough to reliably eject. So, then you reduce your recoil spring or open the gas port. You already have reduced momentum because you’ve lightened the carrier mass. Reducing the recoil spring will reduce the velocity too so now you have even less momentum. At some point that leads to unreliability.
I always like to take these problems to the extremes. If you close the gas port you have no reciprocation. Your recoil will be identical to a single shot rifle.
Or the other extreme where you have the heaviest bolt assembly. Recoil will be spread out over a longer time and possibly feel less, especially if the bolt carrier doesn’t reach all the way to the buffer. Some refer to that as “continuous recoil” or “constant recoil “ because from the time the bolt unlocks until it locks back up the rearward force on the is constant. There’s no slap at the rearward end of travel (although it is nearly impossible to avoid a slap when the bolt returns to battery).

You almost certainly can make a SCAR 17 rifle’s recoil profile smoother by juggling mass, spring constants, and port size. The total impulse of the rifle will always be the same, but how it’s delivered over that brief time can make a significant difference to how enjoyable it is.


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Here's an idea for a prototype so you'll have an idea of what you may be getting into. If FN/MGW/HDD can provide an OEM BC, weigh it with a very accurate scale then start drilling holes to lighten it.

As for springs, Wolff isn't the only game in town, just search for a spring manufacturer. You can also get a tension/compression gauge to determine what the OEM spring is rated at.

I would strongly suggest documenting ANY changes made religiously. Nothing worse than finding the Holy Grail then forgetting how you got there.

As for aluminum as a choice, I would think the piston would dimple the "business end" over time which would affect timing all over again.
 

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Yeah, I'm not sure titanium makes much sense for AR bolt carriers, since manufacturers make lightweight steel ones already. 45% lighter, saving 12oz, sounds great! Just need to find someone to make it.
The lightweight steel ones are not full mass or full auto capable. They also have to be tuned with a lighter recoil spring buffer. I only used the AR example to show that titanium BCGs are being made.

With everything else that will have to be modified on the 17, you may find that the 12oz saved by using titanium for the bolt group will wind up being lost with the other modifications that will have to be made.
 

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Here's some spring info...

 

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Discussion Starter #19
The lightweight steel ones are not full mass or full auto capable. They also have to be tuned with a lighter recoil spring buffer. I only used the AR example to show that titanium BCGs are being made.
Yeah, that was helpful. I hadn't noticed the Ti ones before.
 

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Discussion Starter #20
Here's some spring info...

Oh, that will be very helpful. Thanks!
 
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