Designing A Rifle From Scratch(ish)

[Xlucine]

9 hours ago, Sturgeon said:

 

Sitting pretty at an estimated 6.8 pounds unloaded right now. 

 

Gotta say, rifles are really easy to design in SolidWorks.

 

How's the handguard made? Extruded aluminium with the lug by the magazine welded on?

[Sturgeon]

1 hour ago, Xlucine said:

 

How's the handguard made? Extruded aluminium with the lug by the magazine welded on?

 

Same way Geissele or any other handguards are made. Extrusion with post machining.

[Xoon]

1 hour ago, Xlucine said:

 

How's the handguard made? Extruded aluminium with the lug by the magazine welded on?

 

2 minutes ago, Sturgeon said:

 

Same way Geissele or any other handguards are made. Extrusion with post machining.

CNC milling?

 

Why not plastic?

[Sturgeon]

10 minutes ago, Xoon said:

 

CNC milling?

 

Why not plastic?

 

Rigidity for when you're mounting lasers and other auxiliary/night sights.

[Xoon]

1 minute ago, Sturgeon said:

 

Rigidity for when you're mounting lasers and other auxiliary/night sights.

Why not thicken the plastic?

[Sturgeon]

29 minutes ago, Xoon said:

Why not thicken the plastic?

 

See: G36

[Xoon]

21 minutes ago, Sturgeon said:

 

See: G36

Good point, I guess no-one has had problems with aluminum? 

[Sturgeon]

14 minutes ago, Xoon said:

Good point, I guess no-one has had problems with aluminum? 

 

No, aluminum alloys are pretty stable.

[Xoon]

40 minutes ago, Sturgeon said:

 

No, aluminum alloys are pretty stable.

That's good. 

 

Since the handguard is made by extruding the aluminum, have you considered making it a circular instead of hexagonal to improve its strength and reduce the material usage? 
Also, instead of using ovals to perforate the handguard, I think a 60 degree staggered formation of circular holes would improve the strength again.  Something like this:

 

Please correct me if I am wrong

[Sturgeon]

9 minutes ago, Xoon said:

That's good. 

 

Since the handguard is made by extruding the aluminum, have you considered making it a circular instead of hexagonal to improve its strength and reduce the material usage? 
Also, instead of using ovals to perforate the handguard, I think a 60 degree staggered formation of circular holes would improve the strength again.  Something like this:

 

Please correct me if I am wrong

 

The holes are M-LOK slot mounting points, and the handguard has to be octagonal for the slots to work.

 

[OnlySlightlyCrazy]

It's worth noting that many manufacturers, even some well regarded, choose to include M-Lok only on those surfaces and angles they deem most useful, using other (presumably cheaper or stronger) cutouts in other locations.

 

Imo, this artificially limits the modularity of the rifle, which is one of the AR-15's many claims to fame. From what little I know, the ability of the rail to hold zero is - for aluminum or steel rails - determined mostly by the length and rigidity of the barrel nut. Flex of the rail itself is, if memory serves, less of a concern. That said, post 2005 any new rifle design does need to bear in mind that the laser, not the optic, is the primary aiming device for 50% of the life of the weapon, if not more, so paying additional attention or accepting additional weight in service of that fact is wise. 

[Sturgeon]

6 minutes ago, OnlySlightlyCrazy said:

It's worth noting that many manufacturers, even some well regarded, choose to include M-Lok only on those surfaces and angles they deem most useful, using other (presumably cheaper or stronger) cutouts in other locations.

 

Imo, this artificially limits the modularity of the rifle, which is one of the AR-15's many claims to fame. From what little I know, the ability of the rail to hold zero is - for aluminum or steel rails - determined mostly by the length and rigidity of the barrel nut. Flex of the rail itself is, if memory serves, less of a concern. That said, post 2005 any new rifle design does need to bear in mind that the laser, not the optic, is the primary aiming device for 50% of the life of the weapon, if not more, so paying additional attention or accepting additional weight in service of that fact is wise. 

 

During the development of IMR, they initially used Mk 14 rails which have M LOK at 3, 6, and 9, but found they wanted the full 7 sides M LOK'd. That's why Seahorse has a MK 14 rail, because it was based off an earlier stage of development.

[Sturgeon]

I am sure it hasn't escaped @OnlySlightlyCrazy's notice that my handguard is essentially a clone of the Geissele MK16:

 

[Xoon]

1 hour ago, Sturgeon said:

 

The holes are M-LOK slot mounting points, and the handguard has to be octagonal for the slots to work.

 

Now I know why all modern rifles use slots instead of holes.

 

Out of curiosity, how is the handguard mounted to the rest of the gun?
Just two fasteners? Why not have one on the top and one on the bottom instead of two at the bottom?
Or have the handguard slide in on the rest of the gun, and secure it with one fastener? 
And what type of fastener do you use? Hex key? 

[Sturgeon]

14 minutes ago, Xoon said:

Now I know why all modern rifles use slots instead of holes.

 

Out of curiosity, how is the handguard mounted to the rest of the gun?
Just two fasteners? Why not have one on the top and one on the bottom instead of two at the bottom?
Or have the handguard slide in on the rest of the gun, and secure it with one fastener? 
And what type of fastener do you use? Hex key? 

 

The handguard is secured around a precision barrel nut. The fasteners are just to index it.

I am planning to use Torx for everything.

[Collimatrix]

4 hours ago, Xoon said:

 

CNC milling?

 

Why not plastic?

 

That handguard is rather long.  It could be made of injection-molded plastic, but injection molding something that long while still holding good dimensional tolerances is challenging.  The mold halves have to close exactly on the part.  Those mold halves would be something on the order of 300mm long.  At that length, slight differences in temperature between the mold halves are enough to cause distortion and misalignment due to differential thermal expansion.

 

I think there are plastics that could be rigid enough and stable enough to be used in a handguard.  PEEK, for example, has excellent mechanical properties and a melting point that's 100 degrees or so higher than nylon 6.  A polysulfone might work, as might PPS.  But these are fairly expensive plastics that have additional challenges in processing that would slow down production.

[Xoon]

38 minutes ago, Sturgeon said:

 

The handguard is secured around a precision barrel nut. The fasteners are just to index it.

I am planning to use Torx for everything.

Ah, I see. Why not use a hex key? Easy to manufacture, works with power tools with no problems, and easy to store in the rifle. In case of a total loss of hex keys, just raid a local IKEA for a century's worth of hex keys. 

 

38 minutes ago, Collimatrix said:

 

That handguard is rather long.  It could be made of injection-molded plastic, but injection molding something that long while still holding good dimensional tolerances is challenging.  The mold halves have to close exactly on the part.  Those mold halves would be something on the order of 300mm long.  At that length, slight differences in temperature between the mold halves are enough to cause distortion and misalignment due to differential thermal expansion.

 

I think there are plastics that could be rigid enough and stable enough to be used in a handguard.  PEEK, for example, has excellent mechanical properties and a melting point that's 100 degrees or so higher than nylon 6.  A polysulfone might work, as might PPS.  But these are fairly expensive plastics that have additional challenges in processing that would slow down production.

3D printing could work. But is there no plastic handguards? 

[Collimatrix]

16 minutes ago, Xoon said:

 

 

3D printing could work. But is there no plastic handguards? 

 

There are plastic handguards, they are almost all injection molded.  Injection molding has potentially amazing cycle times.  We're talking less than a minute per part in a process that can be run nearly continuously.

 

3D printed plastic parts are weaker than injection molded ones, even if they're made from the same plastic.  3D printing is also a lot slower.  It's really not cost-competitive for mass-manufacture.

[Sturgeon]

1 minute ago, Xoon said:

Ah, I see. Why not use a hex key? Easy to manufacture, works with power tools with no problems, and easy to store in the rifle. In case of a total loss of hex keys, just raid a local IKEA for a century's worth of hex keys. 

 

 

Because allen screws suck dick.

[Lord_James]

1 hour ago, Collimatrix said:

 

That handguard is rather long.  It could be made of injection-molded plastic, but injection molding something that long while still holding good dimensional tolerances is challenging.  The mold halves have to close exactly on the part.  Those mold halves would be something on the order of 300mm long.  At that length, slight differences in temperature between the mold halves are enough to cause distortion and misalignment due to differential thermal expansion.

 

This might be stupid, but couldn’t you make that 300mm long handguard in 3x 100mm long pieces and fuse them together? It might not look pretty, but it should get the job done without having to worry to much about warping. 

[Sturgeon]

 

Currently having a bit of a mass ratio problem due to the use of my firing-pin-in-bolt arrangement, one which @Collimatrix correctly predicted. My mass ratio goal was 5.6:1, same as the AK, but currently it's standing at 4.4:1. In order to raise it to 5.6:1, I would need to add 113 grams of mass to the bolt carrier, or remove 20 grams from the bolt. Removing that mass from the bolt does not seem feasible, at least not without some radical changes, so the bolt carrier seems to be the only way to go. 113 grams is rather a lot though, and would make the entire rifle go up in weight by a quarter pound (obviously). Plus, there's a fairly significant space constraint in my receiver. It's not obvious I have the space to make my bolt carrier much longer, at least not without compromising the overtravel.

 

So it's at a bit of an impasse until I redesign something or figure something else out. I have a couple ideas I can try, though.

[Sturgeon]

3 minutes ago, Lord_James said:

 

This might be stupid, but couldn’t you make that 300mm long handguard in 3x 100mm long pieces and fuse them together? It might not look pretty, but it should get the job done without having to worry to much about warping. 

 

Remember that your handguard/rail is a mount for multiple sighting systems. So it must be very rigid. It doesn't seem like you could achieve that with your arrangement.

[Meplat]

6 hours ago, Sturgeon said:

 

See: G36

Obligatory image dump.

[Sturgeon]

But no, Meplat, according to all Right Thinking People on the internet that doesn't happen!

@Willy Brandt be triggered

[Meplat]

Just now, Sturgeon said:

But no, Meplat, according to all Right Thinking People on the internet that doesn't happen!

@Willy Brandt be triggered

Amazing that it took the Germans, what, 10 years to admit it "might" have a problem?

 

I took those images back ~2003..