6.5 Grendel build inspired by the military SPR Mk12

[spr mk12 mod 0]

Hello, I’m new to the forum and to the 6.5 Grendel cartridge. I’m looking forward to my first AR build in this caliber and I’m hoping I can share my project here with other Grendel enthusiasts.

My idea for this build is from the 1999 military requirement calling for a compact, semi-automatic sniper system, the Mark 12 Special Purpose Rifle (Mk12 SPR) but with current day updates.

Below is my 5.56 “clone” of the first generation SPR Mk12, the Mod O:



Here’s a link to a good writeup of the original SPR:

Small Arms Defense Journal – The Mk12 SPR (Special Purpose Rifle)

Some of the key features of my build are:

Match grade but not excessively heavy 18” barrel
Rifle length gas system
Free float handguard
Match grade trigger
Modern optic
Most importantly, the 6.5 Grendel cartridge replacing the M262 77gr 5.56x45mm NATO round

I’ve finally got all the parts gathered up. They are either components I’ve used before and like or something new I want to try:



I’m using a matched forged GI style receiver set from JP Enterprises. I lightened up the pictures a bit so you see the markings that come standard. You can choose a variety of different “Lasermarks” from them too:





Here a link these JP receivers:

JP-15 Forged Upper and Lower Receiver Set

More on the "matched" part of these receivers coming.........

[spr mk12 mod 0]

JP threads the lower receiver take down pin detent hole for a 2-56 X 3/16" long set screw. That holds the detent and spring in the receiver without the butt stock on. They shorten the spring 3/16" too. The stock length spring is shown on the right:



JP also has a line of oversize pivot and take-down pins that they match up to each receiver set for a tight fit between the upper and lower. The nominal pin size is .248". JP also offers them in .249" and .250". This is a great way to tighten up a rifle without shims, rubber wedges or other gizmos:



Here's a link to them on the JP site:

JP MicroFit Takedown Pins

[biodsl]

What a sweet rifle this will be. Congratulations.

[spr mk12 mod 0]

Thanks Paul

I did something on this build that I've always wanted to try. Here's the picture from MAD Custom Coatings that opened my wallet:



I sent them my receiver set and hand guard to be Cerakoted in that camo pattern. Here's my receiver set:



Here's a link to their camo gallery:

MAD Custom Coating MADLAND CAMO

[biodsl]

Now you're just showing off! This is going to be a VERY sweet build!

[VASCAR2]

Looks great, I’ll be waiting for pictures when your done and a range report!

[Mustangfreek]

In for pics of rifle together.

Along with Welcome to the forum!!

[spr mk12 mod 0]

Thank you!

I'm hoping this 18" Proof carbon fiber barrel will ease the weight of my finished rifle.......we'll see:



I'm using a JP bolt:



I stripped the bolt and checked the head space which was OK:



I lapped the face of the receiver before I sent it off to Cerakote but I decided to lap it again to get to the bare metal:



The lapping tool and head space gauges both came from Pacific Tool and Gauge. Here's a link to them:

Pacific Tool and Gauge

[spr mk12 mod 0]

Before the barrel goes on I installed the forward assist and, of course, the dust cover:

The all steel GI forward assist (3/4 oz.) on the left and the lightweight V Seven (1/4 oz.) on the right:



The dust cover is from Custom Dust Cover in FDE with the smaller of the available "6.5 GRENDEL" logos:



The logo is on both sides of the dust cover:



Ready for the barrel:

[spr mk12 mod 0]

Getting the barrel on will really start making this look like a rifle.

The barrel wasn't a light press fit into three different uppers like my last Proof barrel. I measured things to see how much "slopski" I'm dealing with. I have a .999" barrel and a 1.001" receiver. So there is .001" clearance around the barrel extension.



I've read tons about bedding a barrel with various compounds. Some say it helps, some say it doesn't but I don't recall anyone saying it hurts accuracy. So I'm giving it a try. The Loctite 620 is made as a, "retaining compound for cylindrical fitting parts with up to .008" clearance" so I'm trying it.

There are heavier duty barrel installation tools but my Geissele reaction rod seems to work fine for torquing a barrel to 40 foot pounds:



Here's my madness in progress. Reaction rod, receiver, barrel in the padded vise and angled downward for the excess Loctite to drip away from the receiver threads. The barrel nut has its threads and thrust face coated with Aeroshell 33MS grease. It's held at the ready with tape:



I always get nervous installing a barrel but once again it was simple. Apply Loctite, slide in barrel, wipe up excess (none of which went into the threads), hand tighten barrel nut and torque......done:



With the Geissele handguard I'm using there is no "timing" of the barrel nut to align any handguard mounting holes. Just torque it on and you're done.

[spr mk12 mod 0]

I'm starting out with a non adjustable gas bock. I'll switch to an adjustable if necessary. I compared a few different gas blocks and went with the light weight titanium SLR Rifleworks model in the middle:





I tried 4 different manufactures gas tubes. All of them contacted the front of the barrel nut:



I'm a stickler for the gas tube "floating" in the center of the receiver hole and gliding smoothly into the carrier key with minimum resistance. It makes the action so smooth.

I spent lots of time reshaping the gas tube to clear the barrel nut. Man that gas tube was tough to bend! It's also a SLR Rifleworks black nitride coated piece:

[spr mk12 mod 0]

Using just the bolt carrier without the bolt I tweaked and tweaked the gas tube and now you can barely feel it making any contact:



Ready for the hand guard..........

[spr mk12 mod 0]

The hand guard I'm using is a 15" Geissele Mk 14:



Geissele's stuff is very precisely made. The hand guard slips over the barrel nut with only .001" clearance. That fit means no cut in hand guard is necessary to allow it to be squeezed tight against the barrel nut with the cross bolts:



I added a low profile Arisaka 3-slot pic rail for an Atlas bipod:



M-loc mounting screws had to be shortened to clear the gas block:

[biodsl]

I spent lots of time reshaping the gas tube to clear the barrel nut. Man that gas tube was tough to bend! It's also a SLR Rifleworks black nitride coated piece:

What's your process for doing this?

[Klem]

SP,

Have you thought about getting an SLR handguard? For the same price their Solo 15" is made out of stronger 7075 aluminium (Geissele is 6061) and incorporates a forward rail section for the bipod. You wouldn't need that Arisaka rail and all the fiddling to clear the gas block, or the risk it comes loose. No need to bend the gas tube either.

Are you going to mount the scope's front ring on the handguard like your 5.56?

[spr mk12 mod 0]

Paul it was pretty basic. I made a mark around the middle of the bends with a sharpie. Then placed one side of the gas tube in the V-cut of a drill press vise jaw, the other side against a delrin rod and clamped it lightly in place. Applied pressure to affect the bend, check progress and repeat. Like I said it was pretty basic but I'm not doing it for a living, just a fun project for myself.

Klem, I like SLR products and they sure have lots to choose from. I see 6 different styles in the Solo series alone. I'll check them out when I build my 6mm ARC . I'm not sure their barrel nut would make a difference with the rifle length gas tubes I tried. It seemed like all of them were not bent to allow the tube to enter the gas key parallel with the barrel. They all angled slightly upward.

The 5.56 SPR rifle's scope rings mount on a one piece rail running the full length of the receiver to the end of the hand guard. It's an old 90's design, it's heavy but it's part of the charm of the rifle that I like. And it does allow you to put a ring anywhere you want.

[Klem]

Sp,

"all of them" - how many did you buy? What did SLR say about this fault? Most unusual.

[danm]

Oh man looking GREAT! I've got a Proof 18" carbon on order for my 6ARC build, can't wait to hear how it shoots with 6.5G.

[spr mk12 mod 0]

Klem, I had 4 different brands not just SLR. I chose to work with theirs because I think they make good products. That tube was quality metal as it was a bear to bend. I imagine the tubes would fit much better with a short GI style barrel nut. Also if the "nut" building this rifle wasn't so anal.

Thanks Danm, I can't wait to shoot it and let you know.

[spr mk12 mod 0]

Just a few more components to finish off the upper (less optics). I know this is old school and there are many updated charging handles but this is still my favorite:



I guess the same goes for the BUIS too:



For the muzzle device I using the APA gen 2 Little Bastard self timing muzzle brake. I have one on my .308 Win Ruger RPR and it really seems to work:



Finally some Magpul M-LOK rail covers: