Weight-conscious 308 “Battle Rifle” setup

[JediGuy]

Weight Conscious 308 “Battle Rifle”

My initial thought was to set this up as a lightweight Battle Rifle or something a rural patrol officer might keep on hand but not usually grab, with a weight goal of somewhere in the range of an AR-15 and the Portuguese’ AR-10 without using any special alloy components. The weight had ticked up by the time the Portuguese contract was filled, though I couldn’t find my reference weight once I actually started typing this out. I’ll add it if I find again, or feel free to mention if you have the reference.

Accumulation of parts:



Assembled:





I’ll mention a couple missteps and “don’t fit” parts that others may find useful.

The project actually started with the Troy handguard purchase. Carbon fiber M-LOK on a 308 sounded cool as a weight saving measure and stiff for a fixed front sight, and Troy had them on a solid discount. This would also be the sole exception to using components from nonstandard material. Honestly, I love the handguard now that it is on. And I hate the mounting system/installation. The barrel it simply did not want to line up with the gas tube hole at any reasonable torque value. So, I got some shims from Bison and made it happen.
Weight w/ all hardware: 10.83 oz



The Barrel. I wanted as lightweight as possible without a Gucci price. Brownell’s BRN-10 308 Retro 20” Lightweight Barrel https://www.brownells.com/rifle-part...rod117135.aspx was chosen due to having a front sight base that I could cut down, which was a preferred option keeping with the battle rifle theme. My original intent was to cut the FSB down myself, and then also have the barrel itself cut down from 20” to 16.” However, once I actually opened the packaging up and looked at it, I realized that, as best I can tell, the barrel profile would be too narrow to re-thread. I absolutely wanted a muzzle device, and I also don’t have any desire to clone a KLM survival rifle, as unique as that might be. So, I revised my expectations a bit and have been satisfied with the 20” barrel; ultimately, this is probably a win via whatever velocity increase comes with the extra 4.”
I shaved down the front sight to make a pretty streamlined, double pinned, low profile gas block. Cut with a Dremel, ground smooth with a Harbor Freight grinder, polished a bit with both the grinder and Dremel, then blued. Very pleased with the result, particularly as I had never used either a Dremel or a grinder before. Or bluing creme (G96 Gun Bluing Creme https://www.amazon.com/G96-1064-Gun-.../dp/B0001AV6CK), so thanks to a couple members here who provided tips. Nor had I ever removed a front sight, which I found required utilizing both a freezer and propane torch as well as a new set of punches. Credit to SOTAR or their FB group for pointing toward cup tip punches to make something like this manageable. Also...using a flat tip punch to drive a coil pin close to contact, then switching to a roll pin punch. Simple, but makes one’s life easier to have someone say this if you aren’t used to working with a lot of this stuff.
Overall, at least through assembly and dry fire, it matches my goals.
Weight of stripped barrel: 33.37 oz
Weight of modified FSB and pins: 1.69 oz





The bolt carrier group is all chrome. That’s all I got. Hadn’t seen a captured firing pin retaining pin before, but it works. Purchased this from PeaceGeek when I first started collecting parts. Probably should have gotten a known brand on Black Friday, but didn’t. By comparing to other 308 bolt carriers, I’m 95% sure this is a Toolcraft carrier.

The Aero Precision M5 receivers were purchased separately from each other and fit well, as expected based on other experience with Aero’s AR-15 uppers and lowers. The screws for the bolt catch and forward assist are nice but unnecessary if one has basic tools. I say that, but the last time I drove out a forward assist roll pin from my BCM upper, it ended up looking a little nasty. So...the screws are nice. I used blue Loctite on the forward assist pin, but not the bolt catch pin, for reasons.
Side note: Realistically, I think that the “intermediate frame” 308 AR’s (for example: Colt CM762) are the ideal. The SR-25/whatever pattern is larger than it needs to be, with a massive BCG. I’m not an engineer, so I’ll step away from that conversation, but multiple companies have made this system work without being inordinately large. But, as a build, the Aero M5 set is the right price and accessible for anyone who has the “need” for a large frame AR. It is unfortunate that the oversized receivers have become the standard.

Aero M5 lower and upper receiver parts kits were used. For all small parts. Originally, I had intended to mix in some Sionics or Forward Controls Design lower parts, as Aero’s parts kits are not known for being high quality. But, I ended up going ahead and rolling with them.
I found no problems with the Aero parts kits during installation. I like the movement of the takedown and pivot pins. The takedown pin spring and detent are installed from underneath the pistol grip, versus in front of the receiver extension. I used a knife to just barely dig into the grip to mark this; this is entirely unnecessary. Just know it is installed from underneath, and your pistol grip won’t have a hole for it.
Based on reading around the internet, the Aero bolt catch has a tendency to break. Question for anyone: Does the LMT 308 bolt catch work in the Aero M5 lower? I want to be prepared.
I did use the grinder again to remove the lip from the forward assist plunger. Blued afterward. Words great, and nothing to catch a finger on if operating ambi charging handle with right hand. The picture below is not flattering due to lighting, but it actually looks clean. The lighting suggests a nick on the receiver, as well, which does not exist.



Important (in)compatibility note: I originally installed both a Forward Controls Design EMR-A and EMR-HC. However, no matter how I adjusted, the Magpul 25 rd windowed PMAG would not lock in. D&H did, and the Magpul 10 rd did. All of mags are brand new. I mixed parts with the Aero mag catch and mag release to try to get it to work, including varying the depth of screwing in. Neither the EMR-A nor the EMR-HC will work in my Aero M5 lower receiver at a proper depth below the shelf. I’m going to assume this will be the same for other Aero M5 lowers, but certainly this is just a sample of one. A casualty of the lack of standardization. I now understand why Roger seems to be getting away from 308 AR parts. Definitely disappointed.

Let’s do an aside from my actual build and comment on magazines, since it seems to fit here. Both the 10- and 25-round PMAGs drop free just fine. All three 20-round D&H mags will not drop free and require effort to remove them.

Let’s do a second aside. Pro tip I got from Chad at School of the American Rifle on YouTube: a little grease on the end of a roll pin or small spring will hold it in place in either a roll pin starter punch or a pistol grip. My live was made easier.

Safety is Noveske STS. Took it off my SBR. Sanded down the edges a bit because I’m a pansy and am tired of grinding my knuckles against it. Zero effect on function, 100% improvement in feel. I actually like this safety a lot. It has a very positive “snap” into both Safe and Fire. Also, though this is not why I installed this particular safety, the M5 lower receiver has the Safe/Fire markings spread such that a 45 degree safety lines up appropriately when set to Fire. This can be seen in the image showing the forward assist above.

Hiperfire’s EDT2 Heavy Gunner is super smooth, and fits here. I did a ton of dry fire in a LaRue TAC before installing this. I’m not a trigger expert. I just know that the pull is a little longer than some other single stage triggers, but it is very smooth along the way. Not too heavy, not too light. For a multipurpose rifle, this seems pretty darn good. Pic prior to assembly, still in the TAC.



Pistol grip is Ergo Suregrip aggressive texture. The same or close to what LMT uses. Figure they know what they are doing. At the moment, it seems about perfect for something that is going to kick a bit more than a 5.56. I did not have high expectations based on a fairly strong affinity for the K2, but the overall package of angle, grooves, and texture is working.

Radian Raptor LT 308. Bigger version of what I have on all my 5.56 uppers. It works.

My plan all along was to use the Magpul UBR Gen 2 stock, which is what i have on there now. It is not light, but it is solid. Mine was purchased used, which explains the ugly markings you can see in the picture.
I have reconsidered this stock. I got a MWS receiver extension kit from LMT, which included a QD end plate, castle nut, rifle spring, H3 carbine buffer, and AR-10/A5 length receiver extension. Though not currently being used, the receiver extension is the absolute nicest I’ve ever seen or held. Incredibly smooth compared to VLTOR’s A5 or BCM/Colt/PSA/Aero’s carbine. Just slick. The spring and buffer are in use with the UBR2 already. To jump to the end a bit, the rifle is a little heavier than it needs to be, and some of those ounces are concentrated in the stock. At the same time, the rifle “moves” well with the extra weight in the rear. I expect curiosity, and a desire to use the LMT receiver extension, is going to lead to moving to a Magpul SL or SL-S stock. For now, the UBR2 works well.
I also installed the Enhanced SL butt pad to offset some of the expected kick. I’m sure there is no need, but it’s something I want to try out with the 308. It’s an easy switch if I want to save some more fractions of ounces or find it serves no purpose.

Sights are Daniel Defense fixed front and rear. Sling is padded VCAS. Muzzle device is a LaRue A2.

Used the Midwest Industries 308 upper receiver tool to assemble.

As mentioned above, this is essentially set up as a battle rifle without frills. 20” should maximize velocity. And frankly, I’m finding that it is completely manageable within a house. Probably not fun getting in/out of vehicles, but if on foot, it works.

Weight of rifle with sights and sling: 8 lbs 13.4 oz





Considering my 12.5” BCM Kino on a standard lower with A5/A5H2 setup, folding rear sight, MRO on ADM cowitness QD mount, SL-S stock, standard VCAS with loop in front and QD in back, Surefire Warcomp, and CavArms CAR-style midlength handguard is 8 lbs 2.4 oz, I’m not displeased. But, I would like to get the 308 under 8 lbs w/o sights or sling. Changing from the UBR2 to an SL would do this.

My next step is to test function at a local short, indoor range, and get a rudimentary accuracy assessment.

Then, I have a couple things set up to do in the following order:
1. Test headspace. Though this should happen before I test fire...I’m not going to wait, as the gauges are on backorder. However, the rifle will set after the single range trip until gauging can happen.
2. Lap the receiver. I received the tool from PTG last week.
3. Bed barrel with shim stock and possibly Loctite 609.
4. Install Surefire Warcomp. Have in hand, but didn’t want to have to take off to lap/bed. The grand plan includes getting a Surefire 762 mini.


There is a slight identity crisis with this rifle. I’ve got a 20” but lightweight barrel. Then a UBR2 stock for something that isn’t set up for precision work. But I also have a Leupold VX-R 3-9x and Insight SQFS that could be installed, along with a Magpul bipod. Or an extra red dot. Or a 1-4x/1-6x.

At the end of the day, though, the intent of keeping weight down will remain. I think of this as the “hike half a day, set up, take 1-5 distance shots, sneak away” gun that could still be used in close as needed.

Hopefully some of the info is useful to those considering or building currently. Feel free to ask questions, I’ll do what I can to answer or get answers.