The Special Purpose Rifle

[Dan]

The Mark 12 Mod 0/1 is a rifle (or just the upper in some cases) put to use by the Special Operations Command starting back in the late 90s.  Used by both the Army, Navy, and Marine Corps, it is a heavily modified light sniper / designated marksman variation of the AR-15 / M-16 line of infantry weapons and is chambered for NATO standard 5.56x45mm.

While in it's military form you may only see a few different variants of the SPR, in the civilian world there are only a few guidelines to classify a rifle an SPR.  The 18" barrel in 1:7 twist, an optic, and the ammo.  Everything else strapped to the rifle are tools for the shooter.  The heart of the SPR comes from the Mk262 77-grain Open-Tipped Match/Hollow-Point Boat-Tail cartridge.  The Mod 0 features a Sierra Matchking bullet, while the Mod 1 features either Nosler or Sierra bullets.  Those unfamiliar with 5.56/.223 cartridges, that's pretty heavy compared to the average 55g or 62g rounds in use.   While decent accuracy can be obtained with M855 62g green tip, the rifle really needs to be fed Mk262 77g to obtain the range and accuracy it was designed for.

I built a Mod 0 (the first type to enter service) recently, and found it wasn't impossible to source the parts but it was difficult.

Sorry for the craptastic resolution, It looked so much better on my phone screen. (I've got to stop using it for pictures)

Everything is correct for a military Mod 0, minus the optic.  I used an SWFA fixed 10, while the Army generally had Leupolds (im cheap).

This is the OPS Inc brake and collar used with their suppressor.  It's an excellent suppressor and mount, but the downside is it really only looks at home on an SPR.  When mounted to another 5.56 gun, it can look goofy.  The collar is used to make sure the suppressor is mounted perfectly straight, and the suppressor threads onto the external brake threads.  



The MOD 1 is more popular to build, especially with the movie Lone Survivor really showing it off.  Parts as of today are extremely hard to come by, especially the Knights RAS.  Good quality MK12 profile barrels are increasingly hard to come by too.


The MOD 0 and MOD 1 share the same heart, but the PRI rail and ARMS top rail being replaced for a Knights free float RAS.  Some don't believe this change was for the best, as it limited the mounting of the scope.  Although this did lead to many of the popular offset mounts you see today, such as the Larues.


Mk12 Mod 0 on the top, Mod 1 on the bottom.  Note they share the same barrels, and Ops Inc Comp/collar.  I found this picture in google images, and can tell you mounting a scope between the upper and a handguard is a TERRIBLE IDEA.  NEVER mount rings in that way, your zero is guaranteed to shift when that handguard eventually gets bumped or shifted. This goes for any AR build (really any rifle where this could be possible), stick to mounts that offset the scope for proper eye relief such as:


While these are the Military SPRs, options to build your own are vast.  Even though barrels are hard to come by right now, I'm sure they will start trickling in again when the makers get caught up from the gun craze taking up all their machine time away from specialty barrels.  There are also other options for 18" barrels; it's the MK12 profiled barrels that are sold out everywhere. (They are profiled in the front to accept the Ops Inc collar)  Many have built their SPRs using Larue handguards, Midwest industries, Troy, or pretty much anything they thought would serve them the best.  Your average civilian shooter has no need for quad rails and the ability to mount Night Vision or IR Lasers.  Heavy rails and gizmos aren't going to help your round impact their target through the power of "coolness".  There are also other (better?) options for muzzle devices and suppressor attachment, especially if you want to use it with other rifles.  

I think thats a pretty basic covering of the SPR, if I missed something or if you have questions let me know.

[john]

Dan,
I really appreciate you taking the time for the above write up, as you know The SPR bug has bit me hard and I've already made the decision to go all in. I'm not as worried about getting an exact MK12 clone done as I am about just building the best SPR I can with available components. My end state is to have an extremely accurized 18" free floated barrel ar15 with at least a 10x piece of glass on top, held in place by a Larue mount.
Other than that I'm widely open to recommendations. I like the functionality of the Magpul PRS stock, but the SOP-MOD and a few others look tempting as well. Do you have any experience or recommendations with the PRS?
My #1 concern right now is this flood of 1/8 twist SPR barrels out there. What gives? As you stated, the original MK12 is a 1/7, so it can stabilize the wonderful MK262 round. (I'm a huge proponent of this round and is what I recommend for any serious work, including SBRs.)

Dan, do you have any knowledge or experience with these 1/8 twist barrels? I do not, and frankly I'm concerned at the idea of compromising something this critical when it comes to long range shooting. The only 5.56 I plan in shooting with this SPR is MK262, and while it would be "nice" to occasionally throw some cheap 55 grain down range, my two other 1/7 twist ARs have no problems doing so accurately. So I can't for the life of me figure out why multiple manufacturers are jumping on the 1/8 bandwagon.

In doing my research I've found multiple higher end manufacturers using 1/8 for their 18" SPRs, such as Larue Tactical, Bravo Company, Rainier Arms, etc.

Can these 1/8s keep a MK262 stabilized to 800 yds?

Better question: is an SPR/MK12 going to get me to 800 yds if I do my part? I realize .308 or bigger is more practical for this distance, there is a specific purpose for me staying AR15.
I'm imagining its well within the realm of reality, a few of us here (myself included) qualify out to 500yds each year with beat up, rack grade M4's, non free floated 14.5 barrel, stock triggers, no bipod and fixed 4x ACOG. I would imagine a match grade free floated 18" barrel, JP trigger, bipod and either a Leupold, NXS or U.S.O optic riding in a Larue mount would get me the extra 300yds....

I also plan on taking every long range precision shooting class available with this rig.

So, any recommendations? Should I avoid the 1/8s that have flooded the market?

[Dan]

DAMNIT I pulled a Frankie.  Spent a half hour typing a lengthy link and information filled post to have it go poof on me.  I'm going to go kick a puppy and go to bed, I'll try again tomorrow.

[Dan]

Alright take two, although this is going to be a bit more condensed.

First, the PRS stock is the gold standard of adjustable AR stocks. There is another made by CAA that has a monopod built in.  We have one mounted to an AR10 at the range, along with a PRS on another rifle if you'd like to compare them.  Another option is the Magpul CTR stock with a cheek riser and monopod adaptor.  I believe the earlier OBRs had this configuration.

Onto the barrels, 1:8 is fine.  I don't have a figure but I believe most 1:7 are actually 1:7.7.  So you can see that's pretty close to 1:8 anyways.  My SPR has a 1:8, and handles Mk262 just fine. There are a lot of discussions out there about it, like this one at ARFCOM.

The last point that I wrote a lengthy post about was shooting SPRs past 600.  It can be done, and there are plenty of guys who tell stories of hits past 800m in Afghanistan, but what it comes down to for me is fun.  When your laying on a line with other shooters watching them ring the snot out of the steel and you're listening for a ping because your round isn't moving it and you can't tell if it hit....well that makes me reach for a .308 gun quickly.   By no means do I think you shouldn't build one, i'm just saying it won't take long for you to want some more ass behind your rounds.  I think of mine as an at most 600m gun, it's just a little wild after that.  I do love shooting it because it's quick and forgiving to shoot with, but I don't think i've ever gone out without throwing some .308 after i've had my fun with the 5.56.  

[Sigma05]

I had a Mod1. I still do, only I transformed it into a USPSA rifle. Basically switched off the brake for a Battlecomp, and got rid of the suppressor rest. Switched the handguard for a longer Troy one. I run a Bushnell 1-6.5 with Insight MRDS offset. I love the accuracy on it, and the grouping with the 77grs is outstanding.

[Dan]

There is another option for a stock that just came out called the MBA, it looks like a stripped PRS but it's also 1/2 the price.


Perfect for your average shooter (and great price) but I can already hear guys saying it's going to get full of crud and lock up.  Who adjusts after initial setup anyways?  That cheek rest could be longer too, some guys still like to put their noses up on the charging handle, even with optics.

They've also got this guy for around $50


Nice to see some new stuff, the AR needs some new options.

[john]

AAAnd I'm back!

My apologies for the leave of absence, some stuff came up. Dan, much appreciated on all the info provided and validating the 1/8 twist. That really opens up my options as to which barreled upper I'm going with. I'm also good with a 600 yd AR15, it was my understanding that realistically this was about it's threshold for effective terminal ballistics as well.
As of now I've got my name on a waiting list for a Rainier Arms UltraMatch 18.5 complete upper, made with a Shilen barrel blank and ratched rifled. I'm also on the waiting list for a Noveske 18.5 SPR barrelled upper, this one is poly rifled. From all my research either one will provide sub MOA accuracy and dependability. Now it's just a waiting game for which one comes up available first....

If the waiting game gets too long I may consider going with a BCM SPR upper, although for some reason I've really been drooling over the Rainier Arms and Noveske over others.
The MBA stock looks like a great light weight option as well... But the PRS is pure sexiness. Since it's a build off of a complete lower I'll hold off on upgrading the buttstock until the end, though.

Any recommendations on triggers?

[Dan]

If you get your hands on either of those barrels (or a Douglas) you'll be happy. Since you're going top dollar, consider the 5.56 Larue OBRs too. They aren't terribly priced for what you get.


A few weeks ago Frankie and I took a handful of guns outside to compare triggers.  In the mix were a Geissele, Knights SR-25, Black Rain Ordinance, CMC, and one of the new Hiperfire trigger setups. 

They each have their own tiny betters over each other (very tiny), but really almost every one of them performed about the same except for the black rain.  That trigger is extremely light, crisp,and has very little reset travel.  The Hiperfire is a pretty radical redesign and has the benefit of an adjustable trigger shoe (if you like that sort of thing), the knights is adjustable and we use them in a good majority of our AR based (556 and 762) long guns but they are expensive and hard to get. You really can't go wrong with a Geissele, but if you see me or Frankie at the shop ask us to try the Black Rain trigger.  Heck try them all, that's what we're here for.

[john]

Dan,

Thanks for the link! I had priced the 7.62 OBR (over $3,000) and assumed a similar price tag for the 5.56... At $2,250 thats not a far stretch from the upper only prices only at all. In doing my pricing the high end barrelled uppers are floating around the $1,500 price tag, but if I can get an all matching gun with the infamous LaRue roll stamp for 700 more... But therein lies the slippery slope.

I'll definitely have to take a look at the black rain trigger, I had honestly never considered it until now.

[john]

Dan,

Thanks for the link! I had priced the 7.62 OBR (over $3,000) and assumed a similar price tag for the 5.56... At $2,250 thats not a far stretch from the upper only prices only at all. In doing my pricing the high end barrelled uppers are floating around the $1,500 price tag, but if I can get an all matching gun with the infamous LaRue roll stamp for 700 more... But therein lies the slippery slope.

I'll definitely have to take a look at the black rain trigger, I had honestly never considered it until now.