Today, I started piecing together my 12.5" .308 AR-10. Before anyone asks, no - it doesn't have any practical application that an AK or .300 BLK couldn't do better or for less $$$. This is purely a range toy, and an I'm-bored-and-want-something-different build. It'll start life as a pistol, so that I can "try before I buy". No sense in paying the $200 tax if I don't like it, right? If I do like it, I'll finish the Form 1 draft I started and make it a proper rifle. Maybe I'll even have Dana (dreadi) do a cool cerakote job on it - black multicam looks pretty cool. Anyway, on to the build...

I knew there were going to be some compatibility issues somewhere, because this is not a "standard" AR-10 (as if there was one for the AR-10!). I had Tom (tpryder) mill out my Aero M5 lower to accept the PDQ lever, so I picked that up from him today:



I installed the lower parts first. Aero should really ship their M5 LPKs with one takedown/pivot pin spring that is shorter than the other, so you can actually USE the threaded allen screw they provide to capture the rear takedown spring & detent. I thought about just tossing the allen screw aside, but instead I snipped about 3-4 coils off the spring, and that was just the ticket.

Next, I decided to see which short throw safety lever I liked better. A couple weeks ago, I ordered a Seekins and a Radian Talon. The Seekins was almost half the cost, but it felt a lot more solid. The Radian has a little wobble to it, but it actually feels better on my thumb than the Seekins, so the Seekins went in the parts drawer and the Radian stayed in. I tested it with the CMC 3.5# trigger, and it's solid. Man, I love this trigger.

Next, I tossed the Aero BCG in my Gibbz G10 side-charging upper and then attached it to the lower. I wanted to test the PDQ lever to see if it would catch the bolt. Whaddya know, it wouldn't. The PDQ lever was hitting the inside lip of the upper receiver before the bolt catch face could get high enough to actually do its job. Rather than take it back to Tom, I just pulled out my trusty dremel and gently took material off the upper. It took 3 rounds of dremel work and test fitting again to finally get it, but now it works like charm.

The last step was to mock up the gun to get a feel for how it'll look proportionally:





The handguard I want (CMT UPHR 11.9 Mod 2) is still out of stock, so I may be waiting a while to finish this build, but that'll give me time to figure out which optic I want. Anyway, I put a Seekins adjustable gas block on it and checked the fitment of the Spikes midlength gas tube, since I had read somewhere that some AR-10s need a different gas tube in order to work (I actually ordered a LBE .308 midlength tube, just in case). Well, that's nonsense, because this AR-15 gas tube works just fine. The plan was to also put a Seekins flash hider on it, but when I opened the package (which said 5/8"-24), it was actually a 1/2"-28 inside. Looks like I'm sending that back! Maybe I'll put a can on it, maybe I won't; I don't want to make it any heavier than it already is, and that would kinda defeat the purpose of making it short.

Anyway, I think it's going to be a fun range toy...definitely loud and bright. And if I decide I don't like it, I'll just rebuild it into something else, right?

IIRC Armalites use a slightly diff length gas tube, but its been serveral years since I built a 308.

What barrel did you use?

Maybe that's what it is. The LBE .308 midlength tube I ordered is much too long for use on this build (probably a good 1/2" to 5/8" longer than a standard midlength tube), and it has a more pronounced dog-leg in it - maybe to clear a larger barrel nut...? Anyway, not necessary...glad I didn't pay much for it.


I'm using a CMMG barrel with 1/10" twist. I was looking at the Ballistic Advantage, but then a deal came up on the CMMG ($120) that I couldn't pass up.


The handguard is proving to be a difficult last piece to this puzzle for a few reasons. I really want the CMT UHPR 11.9 Mod 2, but I don't know when CMT is going to get around to making more of them. I considered the Aero 12" M5 ATLAS R-ONE, but it's actually a bit too long on a 12.5" barrel; the muzzle device is slightly recessed, and I want it slightly exposed. The SLR 308L Ion 11.5" is sexy, but not $300 sexy. I guess I just need to keep hunting around.

Cool pistol. For the record whenever some guy says,"AR's are adult legos!" My first thought is that they haven't built one. Yes, any mall ninja can take a complete upper from Palmetto state and put it on his complete lower and say,"look what I built!" But this is an example of a custom AR build complete with "some gunsmithing required"!
Long lead times, wrong gas tube lengths, gas blocks too big to fit inside the hand guard, upper receiver to tight, take down pin holes to small: the list goes on and on!

I have a 12.5" Sig 716 barrel in my parts box, needs a custom gas tube from WOA and a small spacer to keep the gas block in place. Need to cut a slot in the forend to access the gasblock once its installed etc.

Not everything is "legos simple" as you pointed out

That looks like it's going to be fun!

Yeah, I'd finish up the draft.

Well, my CMT handguard finally came in, so I put it all together today:





I cycled a few rounds by hand, and everything fed and ejected as it should. Just need to get the adjustable gas block dialed in, then sight in the scope.

This handguard is PERFECT in the length department. The end literally stops right at the barrel shoulder just behind the muzzle threads. It's not so perfect in the height department; it's about 3/32" taller than the Gibbz upper. Whatever...I'm probably not going to put iron sights on it anyway. My OCD may get the best of me later, though...


I'm going to attempt to test-fire it on Friday; if I like it, I'll finish the eForm 1 draft Friday evening.

I personally would leave it as is, and spend the $200 towards a suppressor, its going to need one you know.
Good looking pistol

That side charging upper looks great. Good to see that it actually fits the Aero M5 lower so well.

Here here. #cansnotbans

Thanks, guys! I hope it's a shooter.

I do have a .30-cal silencer, but I don't have any plans to use it on this gun. I feel like it would kinda defeat the purpose of building a compact .308, only to then throw another 8" in length and close to a pound in weight. It would still be loud as hell with the can anyway. I think I'll just leave it a fire-breather! If it was intended to be a defensive gun (home defense, truck gun, etc.), then I would probably reconsider the use of a can, but since I intend to use it purely as a range toy, I don't really care enough to try to tame it.

Needs video

So, once again, this project has taught me something new that I admit I had heard of before embarking upon the build, but never actually paid attention to until now: the 2 different DPMS 308 upper receiver tang/rail heights. I think I knew there was a Low and a High, but what I had read on the internet (there's my first mistake!) is that most parts manufacturers are following the High profile, so you don't encounter Low too often. For those following along, here's the difference:



When I started the build, I was intent on building a side-charger. I have experience with Gibbz Arms uppers, so when one came up for sale on this site, I jumped on it. What I failed to research was which pattern they use, and wouldn't you know it, it's the Low:



And, as luck would have it, both of the handguards I bought (Aero M5 ATLAS R-ONE 12" and CMT UHPR-11.9 Mod 2-HP) use the High:





Adding insult to injury was the fact that the Aero M5 ATLAS R-ONE 12" was not actually 12" long...that would make too much sense! Unlike the CMT UHPR 11.9" handguard, which actually is 11.9" - perfect for a 12.5" barrel, and the primary reason I was intent on getting it - the Aero handguard is actually 12.56", which makes it just a hair too long for a 12.5" barrel:



Adding to the confusion is the fact that Aero's same handguard in the 15" flavor is not actually 15", either...but rather than being longer, it's actually shorter than its namesake!

So, the moral of the story is, if you set out to build a one-of-a-kind AR-10 that mixes and matches brands, don't expect all the parts to play nice together. And, if you're OCD like me, don't expect it to look seamless, either...it won't be.

Coulda been worse, you could've not remembered that, while the buffer tubes can be the same, the buffers have to be shorter. That's a rough mistake to make. Throw an optic mount forward enough and it'll cover up fine. Iron sights will be a challenge if you plan to use them.

I wonder if the height different is close to the difference between an absolute co-witness optic and a lower 1/3 optic. You could put a front sight on the gun and use a tubular red dot with a lower 1/3 mount, and use the powered-down red dot as a rear aperture if you needed to.

Sweet build, though. Ballistics shmallistics, shorty .308's are fucking cool. A DSA SA58 OSW is still on my short list.

My very very first AR build from the ground up, one where I picked out every piece from receivers to pins, was a .308. I avoided the handguard/receiver height kerfuffle by getting an Aero upper/lower/handguard set and building on that.

I applaud you for going for it on your first custom build! Starting out with .308 is a challenge.


If only Aero would get on the side charging upper bandwagon!