Custom Aero Precision M4E1 Defensive Rifle Build: How To

I don’t know why, but I find assembling AR-15 uppers and lowers relaxing. There’s a certain satisfaction to taking a pile of various parts and pieces and building them into something which meets your needs. Building an AR-15 is a rather simple and straight-forward process. It doesn’t require a great deal of skill, just the ability to systematically install parts correctly and verify they are functioning properly. Occasionally trouble-shooting is required, but typically this is due to substandard parts or a simple error during initial assembly. While I have built a “Poverty Pony” or two in my time, I tend to look for high quality parts from a manufacturer with a good reputation. This provides a bit of peace of mind, and typically a rifle which performs very well.

For this article, I decided to focus on Aero Precision. Aero Precision is not a gun manufacturer per say, rather they offer a host of parts and pieces enabling you to build an AR-15-type firearm yourself. This can be as simple as buying a complete and assembled upper and lower receiver, by purchasing individual pieces or anything in-between. They offer stripped receivers, barrels, handguards, trigger groups, bolts and bolt carrier assemblies, receiver extensions and a whole lot more. Basically, everything a home builder might need to assemble a good quality AR rifle.

For Precision Long Range, Check Out Aero's M5E1 Rifle

Everyone has their own personal likes and dislikes when it comes to rifles. That’s the great thing about the AR series, it is extremely easy to customize. So, I decided to put together a rifle to meet my needs for recreation and personal protection using Aero Precision components and offerings. My goal was to build a short, quick handling 5.56x45mm carbine which was lightweight, reliable and accurate. I also wanted it to have certain features I deem desirable, such as a lightweight handguard, M-LOK mounting system, 1 turn in 7 inches barrel twist with a 5.56x45mm NATO chamber, ambidextrous charging handle and a crisp match trigger.

Why pick 5.56x45mm NATO when so many newer options are readily available? While I am a fan of cartridges such as the 6.5mm Grendel, 300 AAC BLK, 9x39mm, .458 SOCOM and .50 Beowulf, and believe they have their place, my favorite AR cartridge remains the mundane 5.56x45mm NATO. Why? It’s very accurate, relatively efficient and very economical. Plus, the cartridge’s relatively small external dimensions, light weight and low recoil impulse are all advantageous. Toss in the simple fact the AR-15 was designed around this cartridge, parts and magazines are well refined, inexpensive and widely available and it makes a lot of sense. I probably shoot more 5.56x45mm NATO ammunition a year than any other centerfire cartridge.

Once I had decided on a cartridge, I next needed to select the proper barrel. I am a big fan of 16-inch carbines, but decided to do something a bit different this time. Instead of 16 inches, I decided to drop down to 14.5 inches. This would shorten the overall length a bit, but would require a pinned and welded muzzle device. While I typically go with a flash suppressor, on this build I decided to try a brake to reduce muzzle movement. I’m a big fan of Mid-Length gas systems, so added that to the list. A low-profile gas block would be required to fit under the handguard.

The handguard is a key part of the rifle. I was looking for something lightweight but not flimsy. Some designs on the market today trade robustness for a savings in weight. I was looking for something which was both light and rugged. I wanted M-LOK slots for mounting accessories and M1913 rails only where I might actually use them, such as for mounting a front sight. While I have no problem with standard receivers, I was looking for something which offered a bit more. Basically, I wanted a step up in features on both the upper and lower receivers.

Luckily, this is not a problem when shopping on Aero Precision’s website, and I found what I was looking for in one of their M4E1 complete upper receiver assemblies. This was a fully assembled barreled upper receiver assembly ready to drop onto a lower. The foundation of the assembly is an Aero Precision M4E1 upper receiver which features angular machining which gives it a bit of flair and helps to set it apart from a standard GI upper. This is a good looking piece which has a modified M1913 flattop rail, M4 ramps, shell deflector and forward assist. Examining the upper you will notice that an Allen screw rather than a roll pin retains the forward assist. This is a nice feature if you are buying a stripped upper to assemble as it simplifies the process. Riding inside is a black nitride treated bolt carrier assembly. This was easily retracted via an over-size Aero Precision ambidextrous charging handle.

The upper receiver is fitted with a 14.5-inch long barrel machined from chrome moly vanadium steel and treated with a QPQ corrosion resistant finish both inside and out. Rifling twist is one turn in seven inches and it features a 5.56mm NATO chamber and M4 barrel extension. The barrel also undergoes high-pressure testing and magnetic particle inspection. Threaded to the muzzle is a Gamma SL muzzle brake. This is a three baffle design with five oval slots on the top. An effective looking piece, it extends the barrel length out beyond the required 16 inches. This is pinned in place and then the pin is covered with weld. To the rear of the brake is a low profile steel gas block which is retained by two Allen screws. This model features a mid-length gas system although a carbine length is also offered.

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Surrounding the barrel is an Aero Precision Atlas S-One handguard. This is a good looking piece which has had a significant amount of unneeded material removed to reduce its weight. Rather than a full length M1913 rail running along its top, it has a short section at the front for mounting a back-up iron sight or other accessory, and a section at the rear. M-LOK slots run at 3, 6 and 9 o’clock for easy mounting of accessories such as a white light. Quick-detach-sling sockets are provided at the rear of the handguard at 3, 6 and 9 o’clock. The design has a robust locking system which secures it to the barrel nut. My initial impression of the Atlas S-One was very favorable both for its looks and features. This is a design I would consider buying for one of my own home builds.

With the upper receiver assembly picked out, I needed something to put it on. Searching their website I came across a complete lower receiver assembly which seemed to be just the ticket. This was built on an Aero Precision M4E1 lower receiver and fitted with a Magpul MOE SL grip and collapsible SL carbine stock. The lower receiver is machined from 7075-T6 aluminum and sports a few nice upgrades. These include threaded bolt catch pin, integrated trigger guard, upper-tension screw, threaded take-down pin-detent recess and a magazine with a bit more flare.

The threaded bolt catch pin, threaded take-down pin detent recess and integrated trigger guard make for very easy assembly by the home builder. You do not have to worry about driving pins or possibly breaking the trigger guard ears on the receiver. As such a stripped Aero Precision lower receiver would be a good option for the novice home builder, anyone who hates driving roll pins, or someone looking for a step-up from a G.I. style receiver. I chose a lower outfitted with standard controls, standard weight buffer but added a Rise Armament RA-535 single-stage 3.5-pound drop-in trigger. The RA-535 trigger is light in weight, breaks very crisply and has a very short reset. After playing with it for a bit, I decided this would be a good trigger on a competition gun or recreational rifle.

Both receiver assemblies looked good out of the box with no flaws or issues. Aero Precision offers uppers and lowers in a few colors besides basic black, and so I chose flat dark earth. Popping the upper onto the lower revealed a compact and handy AR-15 carbine just as I hoped. My initial impressions were very positive. The controls all operated cleanly, a variety of makes of magazines all ejected with the push of a button and the bolt carrier moved smoothly inside the upper receiver. Overall length with the stock collapsed came in at just 31.8 inches.

Seeing as this is a lightweight and handy 14.5-inch carbine I wanted to top it with a suitably small magnified optic. Nightforce’s NX8 1-8x24mm F1 is a highly versatile scope so I selected it for testing. This is a very compact optic which zooms from 1x to 8x and features a front focal plane daylight illuminated Mil reticle. On 1x you can utilize it like a red dot and on higher magnifications you have marks for hold-over and windage/lead corrections.

Very short in overall length, the NX8 measures just 8.75 inches long and weighs in at 17 ounces. The scope features is available with either Mil or MOA adjustments. The one I used for testing features .5 MOA adjustments with 50 MOA of adjustment per full turret rotation. The windage turret is capped, which I prefer and there are 10 illumination intensity settings with an off position between each one. All optical sights are composed of a number of compromises, and that is certainly true here. However, this small scope brings a lot to the table working well at both near and far distances. I attached the optic using a Nightforce one-piece 30mm mount.

Three different loads ranging in weight from 55 to 77 grains were selected for testing. These were Wolf Performance Ammunition’s brass cased 55-grain FMJ from their Gold line, Hornady’s 75-grain OTM and Black Hills Ammunition’s 77-grain OTM Match. Four five-shot groups were fired with each from a rest at 100 yards, with velocity readings recorded with a LabRadar Doppler chronograph. The Aero Precision carbine proved very comfortable to shoot from the bench. The trigger is quite good and broke crisply, the muzzle brake is effective and the gas system well-tuned out of the box. Magazines inserted and ejected cleanly, the bolt cycled smoothly, rounds fed and chambered without issue and ejection was positive. Empty cases ejected to 3 o’clock. The fun factor was high, and I enjoyed my bench time with the 14.5-inch gun with the only negative being a bit of blast coming off the brake.

Accuracy was quite acceptable for such a lightweight semi-auto. My best groups were shot using Black Hills’ 77-grain OTM Match load which averaged 1.2 inches. This load clocked 2,685 fps. Hornady’s 75-grain OTM load averaged 1.5 inches at 2,514 fps. Wolf’s brass cased 55-grain FMJ load averaged two inches at 2,903 fps. Groups were consistent with no issues encountered.

Following the bench testing, I decided to see how the Aero Precision/Nightforce combo would do at distance. So, I moved from the bench to shooting prone. Conditions were bright and sunny but a bit of wind running eight to 12 mph. Only Black Hills’ 77-grain OTM Match load was used for this portion of testing. I started by putting five shots onto a steel silhouette at 280 yards. These clustered nicely in the upper chest. So, the Aero Precision carbine has no issues reaching out to the distances most would use a 14.5-inch carbine at.  Next, I increased the distance to 500 yards. Here the Nightforce NX8 1-8x24mm F1 scope proved a real asset. The resolution and image quality are very good for the size of the optic facilitating easy target identification and engagement at this distance. I fired at an 11x20 inch LaRue and then switched over to hanging plates of various sizes. The Aero Precision/Nightforce combination performed well and I scored frequent hits despite the wind.

How To Win a Defensive Rifle Fight

I’d also like to mention that I didn’t have a spotter helping for any of this. I had to fire, spot my shot through the NX8 1-8x24mm, and make my correction. The recoil on the Aero Precision is mild enough and the optical performance of the Nightforce good enough to facilitate this. I made elevation corrections using the reticle and simply held for wind. I had zero problems with Nightforce’s NX8 1-8x24mm F1 and thought it performed very well.

Next, I popped the Nightforce off and replaced it with Meprolight’s New Foresight Red Dot. This is a very interesting design with a lot of innovative features thanks to liberal use of modern technology. You’ll find more information on the Foresight in the included sidebar so I will not dwell on it here. With  the optic mounted I moved to running the Aero Precision/Foresight combination through a variety of drills from seven to 100 yards. Here the lightweight carbine really shined. The muzzle brake is very effective and helps to keep muzzle movement to a minimum facilitating rapid follow-up shots. Handling is very good, and the carbine is quick to the shoulder and carries well.

If you are looking to build an AR rifle, carbine or pistol Aero Precision is a company you should be aware of. They have a diverse line of parts and 
accessories specifically geared towards the home builder or the person interested in customizing their firearm. Complete upper and lower assemblies, as seen here, vary in price depending upon options. The lower receiver assembly seen here starts at $269 while the upper receiver assembly starts at $554. Adding in options will of course increase the price. All in all I was pleased with the quality and performance the Aero Precision exhibited.

Aero Precision M4E1 Custom Build Specs

• Action: Rotating bolt via direct gas
• Caliber: 5.56x45mm
• Barrel: 14.5 in. with QPQ anti-corrosion finish
• Rifling Twist: 1 turn in 7 in. 
• Overall Length: 31.8 in. (Collapsed), 34.4 in. (extended)
• Feed: 30-rd. detachable box magazine
• Sights: None
• Optics Rail: Yes
• Stock: Magbul SL
• Weight: 6.2 lbs. w/out mag
• Finish: Flat Dark Earth
• MSRP: Price varies depending upon options
• Contact: Aero Precision

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About the Author

David M. Fortier has been covering firearms, ammunition and optics since 1998. He is a recipient of the Carl Zeiss Outdoor Writer of the Year award, and his writing has been recognized by the Civil Rights organization JPFO. In 2007 he covered the war in Iraq as an embedded journalist.

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