My $477 featherweight (< 5 lb) AR build

[MachIVshooter]

This was an interesting project. Obviously we all know that AR's can be built under 5 lbs, but we also know that lightweight components don't come cheap. My goal was to see if I could get under 5 lbs for under $500. I succeeded.





Yes, I know, if you don't have machining capabilities, this is not doable. I still thought everyone would find it interesting.

It started out with an Ares polymer lower, which a friend of mine bought on sale for $50 with his two billets. He changed his mind about doing a polymer build, though, so gave it to me. I'm counting $50 +$10 S&H he paid, although I actually did not pay for it. He gave it to me as a thank you for letting him use my machines to finish his other lowers, back before ATF 2015-1.

Anyway, I finished it out, but it's been just sitting on my shelf for the last 6 months, waiting for me to decide what to do with it. Last week I decided to start making some LW parts, and the ball got rolling. First was the selector; not modified, but carved out of a hunk of 2219-T6. Remember: A standard selector weighs 17 grams:



That was the motivation I needed. That night I hopped on JSE and ordered up a barrel, gas tube and stripped upper. I was originally going to mill off the FA and dust cover provisions, but then my friend/neighbor/SOT FFL told me he had natural finish Aero Precision LW uppers on hand for $40, so I just grabbed one of those instead. I went with a cheapie fleabay hand guard and aluminum gas block. I am actually fairly pleased with the quality fo the $60 FF keymod guard, although I will switch to a MI or BCM later to shave another 3-4 ounces. But those did not fit my sub-$500 parameter for the initial build, so the advertised 10.2 oz (10.9 on my scale) ebay special is what we're rockin' for the time being.

Anyhow, the barrel showed up yesterday, so I went to town. First I carved 19 grams (2/3 oz) off the upper and 74 grams (2.6 oz) off the bolt carrier:





Then we turned the M4 profile barrel down to .540" both fore and aft of the gas block, taking it from 25.2 oz to 20.1 oz (sorry, no before and after pics there). Last piece of the puzzle was the 2.3" brake to get my 16" title I length. The brake is machined from 6/4 Ti and pinned. Instead of welding and discoloring the Ti, I took a different approach: drilled and tapped the brake #10-32 at the 6 O'clock position, installed it, marked the barrel, removed the brake, spot drilled the barrel .100" with a 3/16" end mill, reinstalled the comp, slobbered the stainless 10-32 socket head with red Loctiite, cranked on the screw in until the head stripped, then cut it off and ground it smooth. You can see it, but pretty much need to be looking for it.



So, the build list is:

-Ares polymer lower: $60 shipped on sale
-PSA Blackhawk LBK: $84 shipped on sale
-Areo Precision raw LW upper: $40 local
-DSA BCG & CH: $89 shipped
-1:9 Chromoly barrel from JSE: $95
-Carbine gas tube: $10
-JSE shipping: $10
-10" FF Keymod hand guard: $60 shipped on the bay
-alumium gas block: $8 shipped ebay
-cheapie flip-up sights: $16 ebay critters
-2.3" piece of 6Al-4V Titanium for the comp: ~$5 (11" piece was $30 shipped)

So that's it! We'll see how she runs today. It still has a standard 3 oz buffer in it, but I'm taking one with me that has no weights to see if it'll run that way. I still plan to do a hybrid aluminum and tool steel trigger as well, but didn't get that far yet.

 

[silicosys4]

Sweet rifle, I like what you've done with it.
Out of curiosity, how many hours of machining do you have into it?

 

[horsemen61]

Nice build

 

[SuperNaut]

Man, my kingdom for a lathe...

I've always thought you could really skeletonize a buffer. I bet it would run with a flat disc of aluminum and two pieces of wire bent into a rounded basket shape.

 

[Arizona_Mike]

Nice build!

I'm a bag fan of going light, esp. reciprocating weight (I run semi-autos without slide weights) but not without adjustable gas blocks!

Mike

 

[MachIVshooter]

Thanks for the compliments, guys!

I did get 90 rounds through it today, and I was pleasantly surprised. Not did it run flawlessly out of the gate, but the recoil impulse unexpectedly gentle. Additionally, I really didn't anticipate much in the way of accuracy from a $95 barrel that I proceeded to cut 1/5 of the metal away from, but I was able to hold 1-1/4" to 1-1/2" 5-shot groups consistently at 50 yards with those cheap flip ups and PMC 55 gr. ball ammo, even after the thing was quite warm.

All in all, every aspect of this build has surpassed all expectations I had at this price point.

Out of curiosity, how many hours of machining do you have into it?

About 7. Took ~2 to make the selector, 1-1/2 on the brake, hour on the barrel, hour on the carrier, hour and a half on the upper. Not that much cutting on the carrier and upper, but it takes a little while to figure out how to hold the parts and get them squared up without any fixtures.

I've always thought you could really skeletonize a buffer. I bet it would run with a flat disc of aluminum and two pieces of wire bent into a rounded basket shape.

Except that the buffer is what bottoms out in the RE and prevents the gas key from hitting the lower. You can run them empty, but they still need to be able to take that hit.

I'm a bag fan of going light, esp. reciprocating weight (I run semi-autos without slide weights) but not without adjustable gas blocks!

This one actually ran 100% with an empty buffer, but the recoil impulse was significantly harsher than with the carbine buffer, and I don't want to subject the polymer lower to that. Not worth it for 2 ounces.

 

[Arizona_Mike]

This one actually ran 100% with an empty buffer, but the recoil impulse was significantly harsher than with the carbine buffer, and I don't want to subject the polymer lower to that. Not worth it for 2 ounces.

If recoil is harder with less weight you have probably caused an over-gas situation by reducing the carrier weight.

Let me know how the Al selector holds out long-term. I went with a Ti-core selector with Al levers.

Mike

 

[MachIVshooter]

If recoil is harder with less weight you have probably caused an over-gas situation by reducing the carrier weight.

Definitely. Just wanted to see how it behaved with a light buffer. Honestly, I was surprised it was so gentle with a standard carbine buffer, as I cut 2.6 oz off the carrier. That said, although I didn't measure, the port on this barrel did look smallish.

 

[bannockburn]

Another amazing AR build. I think you should call this one your "Weight Watchers Special"!

 

[zerobarrier]

That is awesome.

 

[MachIVshooter]

I think you should call this one your "Weight Watchers Special"!

I like it!

 

[MachIVshooter]

Trimmed it down a little more. Turned another ounce off the barrel, ditched the steel RE castle nut and end plate, made an aluminum nut. Cut 40 grams off the hand guard. We now have a base rifle weight of 4.59 lbs (4.74 with sights)

 

[USAF_Vet]

How does the aluminum nut attach to the lower?

 

[barnbwt]

Mach, For a minute I forgot this was a month-old thread, and thought for sure you'd be trolling us with your 16lb steel AR (that or the 1/2 scale, which I'll go out on a limb and guess is less than five pounds...)

TCB

 

[MachIVshooter]

How does the aluminum nut attach to the lower?

It's a jam nut, same as the standard castle nut. I simply omitted the end plate in favor of a 4-40 set screw to retain the detent spring.

that or the 1/2 scale, which I'll go out on a limb and guess is less than five pounds...

1.17 with the baby EOTech and a full mag (and the solid faux barrel)

 

[USAF_Vet]

Ok, so you did thread the receiver for the buffer tube?

I'm looking for a way to avoid tapping the receiver, and am thinking of welding the castle nut on and welding the buffer tube in place.

Edit: got my threads mixed up. I see this was the Ares poly lower, which would already have had the threads. Derp moment.

 

[Dr.Rob]

This thing points itself, it feels like a bb gun.

 

[Arizona_Mike]

You are within 0.27 lb of me for a ton less $$$. Really a nice build. How much weight did you save cutting that groove in the BCM handguard?

Mike

 

[MachIVshooter]

How much weight did you save cutting that groove in the BCM handguard?

I wish it were a KMR. I'd have left that alone. No, this is a $60 fleabay special. Originally weighed 10.9 oz, but ditching the wholly unnecessary locking ring (20 grams) and cutting the top rail (39 grams) was worth 2 oz.

I'm looking for a way to avoid tapping the receiver, and am thinking of welding the castle nut on and welding the buffer tube in place.

Edit: got my threads mixed up. I see this was the Ares poly lower, which would already have had the threads. Derp moment.

You were thinking of the steel lower? Yes, that one is threaded. What are you trying to build? I know people weld a castle nut on those flat spot steel lower kits, but it's not an option with a normal 7075 lower; 7075 is not weldable.

Tapping them is not a big deal, though, if you're at all familiar with running large taps and keeping them straight. A mill and machine vise is best, but a drill press will work pretty well, so long as you can find a way to hold the lower securely in line with the quill.

This fellow demonstrates, albeit at a much smaller scale than the RE thread tap:

https://www.youtube.com/watch?v=oRv6UXpVwrE

Just be sure you are centered over the hole!

You can get the tap for $40 here

 

[Arizona_Mike]

I see V7 weapons systems has a titanium barrel nut for the BCM KMR, a lighter than stock steel slide release lever, and titanium hammer and trigger pins. My build was for kicks (a challenge to see how light I could get) but now that I have a bulged disk, it's my "Sciatica rifle". I'm looking into replacing my aluminum buttplate with a polymer one as well. I tried an A2 buttplate with door but it is actually a couple oz heavier than the Al one. I think an A1 plate may be lighter.

Mike

 

[MachIVshooter]

I see V7 weapons systems has a titanium barrel nut for the BCM KMR

I'd like to see what it looks like.

I'm a steel guy at heart, and I really, really like Ti. Having said that, if weight is a concern and the parameters don't require high melting points or >90,000 PSI tensile strength, 7XXX aluminums are where it's at.

In other words, I'm thinking 7068-T6511 may have been a better choice.

 

[USAF_Vet]

You were thinking of the steel lower? Yes, that one is threaded. What are you trying to build? I know people weld a castle nut on those flat spot steel lower kits, but it's not an option with a normal 7075 lower; 7075 is not weldable.

Yup, that's what I'm doing. I've got a flat spot steel kit coming, and was thinking of welding the nut to the tower rather than threading it, unless I can find someone willing to loan me the proper tap. Purchasing one is about $60 or so, and the point of this project is to keep it as dirt cheap as possible.

Tapping them is not a big deal, though, if you're at all familiar with running large taps and keeping them straight. A mill and machine vise is best, but a drill press will work pretty well, so long as you can find a way to hold the lower securely in line with the quill.

I'm comfortable with large taps, and using a vertical mill to run it. I just don't have a tap that size or pitch.

You can get the tap for $40 here

Cheapest one I've seen. I'll save that link if I ever do any other unfinished receiver builds, but I'm thinking the flat spot kit will be a one off project. Usually I just buy completed or stripped lowers plus kits.

 

[shootr]

WOW! Very cool!

 

[MachIVshooter]

Well, got a chance to run it today after the further lightening; still 100% reliable, including when we threw it on an M-16 lower and ran a couple mags back to back in full auto. I thought the lightened carrier might prove problematic, but it ran at about the same ROF as the all the other uppers that got hot on that lower today.

Also to my surprise, it was still very controllable. Granted, the SOT's lower-a billet 7075 unit with a Magpul UBR stock- probably made the total weight at least 5-1/2 to 6 lbs, but I actually found this upper with it's brake easier to keep on target than the BCM 10.5".

All in all, I am ecstatic that this build has worked out so well. We are somewhere in the neighborhood of 300 rounds without a single hiccup from the get-go, accuracy is at least as good as any other typical carbine, and we now know that it can even take some FA punishment.