Building a Large Frame AR

I have mentioned elsewhere that I am not a huge fan of large frame ARs. They are a little harder to shoot and there is a fair amount of mass cycling back and forth, so you kinda have to “manhandle” more so that I am sued to with small frame ARs.

Still, I have built a few and since a friend of mine has asked me how I would go about selecting components for one, I figured I should make a post out of this. First a little about nomenclature: I have only messed with building DPMS-pattern guns, so I will use the term “LR-308” throughout to mean a large frame AR.

My LR-308 is a somewhat specialized set-up since after many changes I settled on a heavy barreled 243Win gun that I use for testing scopes and will also use as a heavy varminter. Here is what it looks like in its latest iteration with a Dracos barrel and an inexpensive, but surprisingly decent Guntec handguard:

One thing to keep in mind is that I was not looking to save weight with this gun. In the past, I ran it for a bit with an 18″ 308 barrel and had I stayed with that configuration I would definitely go for a somewhat lighter build. In this particular case, I was asked how I would configure such a gun for a use case where it would be utilized for home defense and for occasional hunting. First of all, I will freely admit, that for home defense I would be more likely to use a smaller AR platform with a smaller cartridge. 308Win is a bit of an overkill for home defense. However, if we extend this to “estate defense” and with hunting thrown in for good measure a large frame AR makes reasonable sense (although I have an AR-15 chambered for 6.5 Grendel for this purpose and do not feel undergunned).

Anyway, here are some component considerations:

1. Make sure you get matched upper and lower receivers. There is no mil-spec standard for large frame ARs and I have seen some variations that do not fit each other. On top of that, I have also seen some fit issue with handguards and upper receivers from different makers. If I were starting a build now, I would probably get a matched kit from Grey Ghost that has a matched upper and lower receivers together with their handguard. Rainier seems to have it on sale right now and I happened to like the camo patterns they have. This pretty much guarantees that the pieces will fit together.
2. If you decide to not get a fully matched up set above, you can get matched upper and lower receivers with Grey Ghost being a good option again, although Rainier’s own set is quite good too.
3. Keep in mind that there are multiple DPMS standards out there, so if you buy a handguard separately make sure you pay attention to whether it is a high rail or low rail standard. I prefer to go with “Low rail” everything, but it does not matter too much as long as you stay consistent. Handguard aesthetics are in the eye of the beholder, so choose what works for you. I have reasonable mileage with different makers so this is where you kinda have to decide what fits your build. If you are looking for making the gun as light as possible, you may have to spend some money on a Brigand handguard. On the other hand, if weight is less of a concern, options really open up. Since I prefer to have the handguard go over the gas block, I would stick with something around 15″ in length. That all having been said, if I were doing a new build for myself, I would either go light with a Brigand or a little heavier with a Blklbl handguard that has an integrated bipod (or to minimize compatibility issues, see the Grey Ghost links above).
4. Barrel is also a personal choice, but an important one. There are many good barrels out there and for a general purpose rifle, I would lean toward some light to mid-weight design with 18″ length. I have a Fulton armory 18.5″ barrel that is built on a Criterion blank in a light-ish profile. It is chrome-lined which makes it almost impervious to elements and pretty unlikely to ever be shot out with any normal use. Given the application, I would lean heavily toward either QPQ or chrome-lines barrel, so for a lightweight QPQ option, I think this pencil weight Faxon is one of the better “bang-for-the-buck options out there right now. For hunting/home defense gun, one of these would be my choice.
5. Gas block: many people opt for an adjustable gas block, but for someone doing it for the first time, I would probably go for a standard low profile gas block, preferably of the clamp-on variety, but a well fitted set screw gas block works just fine. For something a little more dedicated for precision or for a gun that will have a suppressor on it, I might go for an adjustable gas block. However, since we are talking about something that has to be California legal (no suppressors) and used for home defense and hunting, standard gas block is fine.
6. Furniture: in CA, you can’t have a collapsible stock or regular pistol grip, so this is the grip you have to use (for right handers). With buttstocks, I am a firm believer in using a rifle spring and buffer if you can’t have a collapsible stock. The recoil is going to be a little softer with a rifle buffer. There are a few buttstock options out there that will work fine, but I would probably just get Magpul’s MOE rifle stock and be done with it.
7. Extension, spring and buffer. The extension is the same as on the AR-15, but buffer and spring are different, so make sure you pick the right ones. Standard weight rifle buffer from anyone reputable (like this one from Brownells) will work fine. With springs, I would go with JP’s tuned and polished spring (part number JPSOSR308 ).
8. Last, but not least, you’ll need a trigger and lower parts kit. A couple of parts are different between large and small frame ARs, so make sure you get the right one, like this one from DPMS. Technically, this lower parts kit has everything you need, but in practical terms, you should really get a better trigger and an ambidextrous safety (ambi safety is key for California since you can’t have a proper pistol grip). There are many ambidextrous safeties out there and most work just fine. I am partial to the Radian Talon ambi safety, personally. Finally, we get to triggers. I tend to use Geisselle SSA-E in a lot of my builds and I have yet to regret it. I think it is the best general purpose trigger available for the AR Platform right now. It is not cheap, so for the budget conscious, ALG’s ACT trigger is a good option. It is, essentially, a GI trigger that is tuned and adjusted to be about as good as a GI trigger can be.

That takes care of the rifle, so we can spend a minute on optics. Home defense means red-dot or a scope that goes down to 1x. Hunting means low light and large objective. The two requirements are essentially mutually exclusive. Also, for any gun that is intended for defensive purposes I really like to have two independent sighting system.

If you want one scope that goes down to 1x, I think it is wise to start with Steiner P4Xi 1-4×24. It is exceptionally quick on 1x and quite decent on 4x. The reticle is intended for 5.56, but inside of 500 yards, the drops are almost the same.

If you want a little more reach, consider Hawke Frontier 1-6×24. Its reticle is mrad based, so it is not cartridge specific.

If you are willing to consider a dual sighting system set-up, I would suggest something like Meotpa Meostar R1 1.5-6×42 with a micr red dot like Burris FastFire II set-up on an offset mount. Mestar is a great general purpose scope for hunting and all sorts of other use that happens to be quite fast on 1.5x. With an Fastfire mounted on an offset Daniel Defense mount, you ahve a red dot that can be in front of your eye by rotating the rifle just a bit.