Designing A Rifle From Scratch(ish)

[Sturgeon]

Current BCG configuration:

 

[Sturgeon]

First charging handle is a bit too gamer:

 

[Sturgeon]

This may be surprising, but the charging handle is always the biggest bitch in the whole process of designing a modern infantry rifle. There are just too many factors involved to ever come to a good compromise. You basically get to just live with a set of tradeoffs that leave everyone unhappy in some way.

Having said that, here is where the charging handle of the F-4 is at:

 

It's little more than a fixed knob running in a slot in the side of the receiver, but it does charge the weapon and provide forward assist functionality. I was originally going to make the charging handle non-reciprocating, but my initial plan for doing that was ill-conceived, so right now it's reciprocating. I will probably redesign the whole thing, but we'll see. I might just cop out and make it a right-side reciprocator like I was thinking of doing.

 

Right now, I'm leaving it be and getting to work on the bolt hold open.

 

[Sturgeon]

Oh, well the good news is that I got down to 3.05 kg / 6.72 lb unloaded, no magazine and the SOCOM profile barrel.

[Sturgeon]

Happier with this one:

 

[Oedipus Wreckx-n-Effect]

It looks like the SnagHook3000.

[Sturgeon]

Still not terribly happy with it, but here's the mud guard I came up with:

 

[Sturgeon]

So the rifle is now basically done. I'm not happy with the charging handle but I know from experience that I'm just not going to be. So what's left is to make the buffer and spring, assembly of the fire control group, and some glamor shots.

[Sturgeon]

 

Here's the (nearly) finished product.

[Oedipus Wreckx-n-Effect]

Is there space on the receiver for a modestly sized Punisher skull?

[Sturgeon]

On Facebook someone asked me why I didn't design a monolithic upper rail like the SCAR, ACR, MCX, and other noodlewaffe. I posted a pretty detailed reply, and figured I'd copy it over here too:

 

OK, so good question that has a fairly detail-centric answer. Like the SCAR and the Remington ACR, this rifle has a receiver that's intended to be made of an aluminum alloy extrusion, which for those unfamiliar is where you squeeze metal (or any other material for that matter - noodles are made this way for example) through a die into long shapes of regular cross-section. The shapes can then be cut into shorter lengths to make all manner of things. Beams, supports, building material, noodles, rifle receivers, and what have you.

This is potentially a very cheap way to make rifles that can have comparatively low startup costs, but it comes with a couple disadvantages. The first and most obvious disadvantage is that extruded shapes must be of regular cross section Whatever initial 2D shape you make the die is what you get. Rifle receivers are *mostly* uniform in cross section, but in many places (such as the barrel mounting area) they really, really do not want to be. The SCAR and all other rifles therefore need to bolt on features near the barrel and stock area (that's why they all have visible screws - mine would too IRL but for the sake of my PC's graphics unit I omitted them). This disadvantage is something pretty much all extruded receiver guns get to tackle, there's not really any getting around it.

The second disadvantage, though, is one the shorter receiver of my design addresses: Warpage. One of the big problems with shoving a bunch of heated metal through a die is that while you get an elongated uniform shape, it can be tricky to get it straight enough to be within tolerance. The longer your receiver, the worse this problem is. The receiver I am using is better production optimized in that respect than a SCAR's receiver, as it is some seven inches shorter and would therefore be significantly easier to make a bunch of segments within tolerance enough to be rifle receivers and there would be less waste. Not to mention you'd just get more receivers per foot, being shorter.

The third disadvantage is not something that was really evident when the SCAR was first designed (early 2000s). Today, there's a lot of value put in the ability to mount handguards of different lengths - and especially of longer lengths - to a rifle. Most SCAR owners who actually use their rifles don't just stick with the original 7" long rail, for example, they use aftermarket rails to extend the handguard to a more modern length. The SCAR was not really ever designed for this (and it shows), and adding a handguard of this type results in a lot of material overlap and excess weight (which is bad). Later I'll do an apples-to-apples comparison of the SCAR with this rifle and you'll see just how big a difference it can make, if we're talking rails of the same length.

Now, you could have a rail of a fixed length, say 7" like a SCAR, and then a low profile mounting point for additional sections of rail past that. However, you need a place to mount the barrel, and that feature has to be precision anyway. Adding a *second* precision feature to the front of the handguard (as those additional rail sections need to be rigid too, for laser mounting, etc) adds a lot of weight and cost to the gun. By making the front trunnion and barrel nut the central precision mounting point for everything in my gun, I'm economizing those factors to a large degree. The "bulkhead" design of my trunnion also allows me a lot more flexibility in how I approach the design of the gas system and other features, which allows further savings (my gas system is lighter than a SCAR's, for example).

The ACR and MCX also tackle this problem, but differently. Instead of using one extrusion as both a receiver and handguard like a SCAR does, they use two extrusions. One provides the receiver and an extended length upper rail for optics mounting, and the other provides all the side and bottom rail surfaces. Neat, right? This is a good idea, but like everything it comes with a compromise, which is that you need extrusions that are within tolerance for the total length of each finished component, so you effectively have increased your sensitivity to tolerance just like a SCAR's receiver would be, despite the fact that you're only retaining a small segment of upper rail forward of the front trunnion. This design also results in considerable wasted material, as you're cutting whole chunks off your extrusions. None of these things are a big deal, but in my estimation we already know how to make barrel nuts as precision rail mounting points that are just as suitable for mounting forward optical devices as monolithic rails so... Why not just do that?

So there you go. That's why I designed it that way! Hope it wasn't more than you bargained for! ;)

[Sturgeon]

I made this model just so I could compare it to the SCAR 16S:

 

[Sturgeon]

More brought over from Facebook:
 

Here's a version with a 7" long rail for direct comparison to the SCAR-L, [name]. I also added a photo of a stripped SCAR 16S on a scale. Adjusted weight for the SCAR-16S (for comparable barrel lengths and without the front or rear sights) is 3.08 kg / 6.79 lb. The weight of my design in this configuration would be about 2.73 kg / 6.01 lb. It gets interesting when you start talking long rails, too. An MREX rail supposedly adds net 3.7 ounces to an FN SCAR, bringing this up to 3.18 kg / 7.02 lbs. Going from the 7" rail segment to the 13" rail on my rifle brings the weight up by about 4.4 ounces, to 2.85 kg / 6.29 lbs. I attribute this to the removal of relatively heavy Picatinny rail segments and mounting hardware with the SCAR, but I'd have to have an afternoon to play around with one and an MREX rail to say for sure. In both cases, my design is about three-quarters of a pound lighter, anyway.

 

 

 

 

 

 

[Sturgeon]

Here it is in 10.5" barreled configuration:

 

2.62 kg / 5.77 lbs unloaded

 

 

[Sturgeon]

Just needs the LPK:

 

 

3.07 kg / 6.76 lbs in this configuration.

[Sturgeon]

Here's a shot of the inside:

 

BCG:

 

 

 

I am probably going to redesign the charging handle tomorrow, as I'm not entirely satisfied with it.

[Xlucine]

I have a suggestion:

[Sturgeon]

2 hours ago, Xlucine said:

I have a suggestion:

I already (seriously) considered that and it wouldn't work very well for detail reasons.

[Sturgeon]

I am declaring a fatwa on charging handles.

[LoooSeR]

5 hours ago, Sturgeon said:

I am declaring a fatwa on charging handles.

Put a robotic charging handle. Cars have robotic transmissions, why shouldn't guns have them too?

 

 

 

 

 

 

 

 

 

 

 

[Sturgeon]

After four major iterations, the charging handle is finished:

 

 

[comrade kalashnikov to the rescue again]

 

Only thing left to do is throw the LPK in and we have a complete gun.
 

Current weights (unlikely to change meaningfully)

 

SCAR-alike config: 2.78 kg / 6.14 lb

 

14.5" / 13" w/ ACR stock: 3.10 kg / 6.84 lb

 

10.5" / 8.5" w/ ACR stock: 2.68 kg / 5.90 lb

[Oedipus Wreckx-n-Effect]

12 hours ago, Sturgeon said:

I am declaring a fatwa on charging handles.

 

is the final reciprocating?

[Sturgeon]

4 hours ago, Oedipus Wreckx-n-Effect said:

 

is the final reciprocating?

 

Yes.

[Sturgeon]

 

 

 

BCG:

 

 

10.5" lightweight barreled version

 

 

"SCAR-alike" config for weight comparison:

 

 

 

 

 

 

Final weights (no magazine, no ammunition)

SCAR-alike config w/ ACR stock: 2.74 kg / 6.03 lb

14.5" / 13" w/ ACR stock: 3.06 kg / 6.74 lb

10.5" / 8.5" w/ ACR stock: 2.63 kg / 5.80 lb

 

I plan to do a sort of "lessons learned" writeup sometime next week.

[Sturgeon]

Left side: