I assembled this with a bunch of spare parts I had laying around. I’m unsure which direction to take it. Thinking either an 18” SPR in 5.56 with a good barrel and then toss some optics on it, or a 16” .300 BLK upper and a Primary arms Prism optic (I don’t have a .300 nor have I tried their prism scopes). I have a .30cal can so that’s tempting, but 16” .300 BLK seems dumb to me. Is it?  I also don’t have a 5.56 geared towards precision shooting.  Looking for suggestions…

The rail is a 15” KAC URX4 Keymod. I have a brand new Geissele gas block sitting in a bin somewhere as well as some BUIS. Really just need a barrel and optic. I have an AAC 51T brake in 1/2x28 and a an AAC flash hider in 5/8x24 so I can suppress either option.

Attached File

6mm Arc?

https://www.sixfiveoutfitters.com/product/rexus-6mm-arc-ar15-18-barrel/

They have bolts and gas blocks etc.

Best mags can be had from ASC and Geissele.

That's my vote as well.  A 16-18" 6ARC or 6.5 Grendel is a hell of a fun time blasting steel at 600-800 yards, and Rexus is great bang for your buck.

URX4 in parts bin? Must be nice.

For a precision build you will want a good trigger. For cheap, a Geissele G2S-E or Larue MBT2.

A 16" 300 BLK gets you to 2400+ FPS with Barnes 110 black tip. Hits as hard at 100 yards as a 9" from the muzzle. So no, not silly, but more a dedicated pig hunter for 150y and under.

If you want to have longer range, consider a 6.5 Grendel 18"over an 18" 5.56 which is just dying on terminal performance past 400 yards. 6mm ARC is either a bunch of cases of expensive Hornady ammo or handloading. Not really a parts bin proposition.

I'm a guy on the internet, so my opinion is important!

Since you explicitly mentioned the option of a .300 Blackout and a Primary Arms prism scope, I vote to go that way!  Me, I'm a fan of the .300 Blackout . . . as a supersonic load almost exclusively.  As a functional equivalent of the 7.92x33.  Not that there's anything wrong with the subsonic loads, it's just that I've never fallen down the suppressor rabbit hole.  (I am terminally unhip and uncool.)  One of the real advantages I see to the .300 Blackout (as compared with other niche cartridges) is the logistical support that is in place for it now.  I see AAC offering 110 grain supers for like pretty decent prices regularly.  The Grendel?  It's not dead but it feels like it's on life support . . . and that keeps me from getting serious about it.  Which is a damn shame, since I'm a fool for the 6.5mm class of cartridges in general, going back to "The Short Happy Life of Francis Macomber" and some Swedish Mausers back in the day!

I'm also a fool for prism optics, particularly the modern mini-prisms.  I bought my first one, a PA Cyclops 1x, and since then, dang, I've just gotten silly.  Burris RT3, 2 of the Bushnell Lil'P 1xs, 2 of the 2d generation PA 1x.  Everything in this world is a tradeoff, but I'm happy with .300 Blackout and prisms.

What setups do you have already? And yes a 16" 300 blk is stupid, don't do that.

Various Colts…6920 SOCOM, 6920, 6920, 6720, an 11.5” 5.56 built with high end parts, KAC SR-15 Mod2…a few budget builds.

I had a 12.5” 6.5 Grendel but with Wolf disappearing from the market I sold it. Right now all the small frame AR’s are 5.56.  I don’t like the idea of a 16” .300 BLK so I think the SPR route is a better bet after considering it a bit.

The Keymod rail came off and I replaced it with a URX 3.1 when I bought the rifle. Couldn’t find an MLOK at the time.


Attached File

Yea might as well do an spr, you have everything else covered.

Still deciding on either 16” or 18” 5.56 barrel…

Attached File

Neither. 6.8 SPC II 18" do something you haven’t done before.

Man I do love the parts-box builds, already have a couple, mulling over what to do next as I have almost enough spare parts for another build or two myself.

I think i'd try the 18" build OP, use heavier 5.56 stuff and just see how far out there you can get good performance.

Yep. A 1-7" 18" barrel.

Ended up with an 18” SPR barrel. Should be here in a few days.

You litterally get another 20 yards of terminal performance going from 16" to 18" with most ammo in 5.56 loads.  You use a tenth of a mil less elevation at 400 yards. You use less than 0.1 mil less windage out to 600 if wind is 10 mph and  just .1 mil more at 20 mph.

16 300 blk would be a good rifle for hunting.

No worries. I have several lightweight AR’s. Don’t want to add any calibers unless it’s .300 BLK but I’ll likely go short and build an upper for one of my SBR’ed lowers when I do that. I don’t have a “precision” 5.56 and with the heavy barrel and brake it should shoot really soft. I have an Arken SH4 on one of my .22’s so worst case I toss that on top, although it’s a bit overkill.

Test fit…will toss it together this weekend.

Attached File


Attached File


Attached File


It’s still light enough I may just leave the 1-4x on it. Should be a smooth shooter and accurate too.

Almost done…just needs a better trigger…probably an MBT. This build reminded me why I hate URX 4 rails. Finally together though.

Final specs:

Aero receiver set
Colt BCG
Colt FCG (to be replaced with a MBT)
Ballistic Advantage 1/7 5.56 18” SPR barrel
AAC brake
Aero buffer tube/H2 buffer
Springco white spring
ADM Recon mount
Arken SH4 4-14x50 mil optic
KAC URX Keymod 15” rail
Troy BUIS
Harris bipod

Attached File


Attached File

Zeroed today. Printed several 3/4” five shot groups and a few 1/2” groups at 100yds in the rain off a wobbly plastic folding table. I can’t wait to see what this thing will do once I get a better trigger in it. Impressed with this BA barrel so far.

I was going to suggest this BA barrel but you made a good choice.
https://www.ballisticadvantage.com/18-223-wylde-spr-fluted-stainless-steel-rifle-length-ar-15-barrel-premium-series.html

I’m not disappointed. Zeroed with M855 and it impressed. Just bought 200 rnds of AAC 77gr OTM to try.