Tips for building a Mk12-esque AR

[kjeff91]

I'm looking at building a mk12 ish type upper for some longer range fun out to 500-600 yards, maybe more if it can reach out that far. I have a few questions on this type of build.

1. Will the upper receiver quality impact the accuracy at all?
2. I'll be experimenting with 55gr, 62gr and 77gr bullets. What would be the best barrel length for these?
3. Any common misconceptions where people thing they need or don't need certain parts?

Thanks!

 

[longshot2000]

Well, as a Mk12 builder and reseller, and barrel and scope snob, what are you looking to do that an authentic Mk12 fails to deliver?

monty makes an improved Mk12 gun at Centurion, but not historically correct. I give him 100x kudo’s because he served a a Navy Seal and shot the real thing. I o my do research and engineering, so his gun is probably a better shooter than anything we build or resell. But we build the real deal.

1). Proper barrel length is 18”. There are 16” variants and I might stretch to 20” if I were designing something not historically correct, but a better shooter.

2). Extreme care is needed in the barrel profile if you indent to mount an AEM5 suppressor. Most commercial barrels will not properly support the suppressor.

3). Whatever the question, Black Hills 77 grain is always the answer.

4) go back to #2. Only use Allen brakes and collars.

5). Make sure your barrel is SPR contoured. We found 4 out of 12 that were done correctly. That means 8 of 12 failed.

so, what are you trying to do? I am roller et of Mk12-ish rifles, especially as it comes to optics, but what are you trying to achieve that the real deal does not deliver?

 

[The King]

I have the original Mod0. With a Leupold 2.5-8 optic.

It’s a neat piece of memorabilia and shockingly capable.

That said, I shoot a Radian M1 18” Proof carbon gun as my “MK12”.

Has a vortex 1-10 and that is an epic optic. Finally got me to sell my 2.5-10x32 NF.

The barrel fit in the upper needs to be tight. Shins and green locktite handle that well, or a Vltor upper.

The most accurate mk12 I have shot was on a San Tan tactical upper and a Lothar Walther
18”.

This is the current gun:

 

[kjeff91]

what are you looking to do that an authentic Mk12 fails to deliver?

Not that it doesn't do anything I want it to do, but some people get stupid butthurt if you call it mk12 mod whatever and one or two things are off. But one thing I might do is go with a 20" barrel.

 

[kjeff91]

The barrel fit in the upper needs to be tight. Shins and green locktite handle that well, or a Vltor upper.

Is the tightness of the upper to the barrel about the only part of the upper receiver that will affect the accuracy of the gun?

 

[schmi015]

just order this upper and throw it on a lower with a2 buttstock https://www.rockriverarms.com/index.cfm?fuseaction=category.display&category_ID=255

 

[The King]

If you mean upper wise, no.

However, upper fit is a first order issue.

Upper design is second order. AKA everything else must be pretty right for your upper design to do you any good.

For the upper, more stiffness is better.

First order issues seem to be, from my experience:

Barrel quality.
Bolt headspace and fit.
Gas system health.
No contact between rail and gas system even during firing.
Barrel fit to upper

Second order:
Perpindicularity of the front upper receiver ring
Design of upper

Of especial note for long range - I started smoothing my feed ramps with a cratex wheel and turning gas down to minimums to help with consistent low-damage loading. I also make sure my bolt carrier is traveling far enough that the buffer is contacting the bottom of the buffer tube.

Is the tightness of the upper to the barrel about the only part of the upper receiver that will affect the accuracy of the gun?

 

[kjeff91]

just order this upper and throw it on a lower with a2 buttstock https://www.rockriverarms.com/index.cfm?fuseaction=category.display&category_ID=255

I like building my own stuff, but thanks for the recommendation.

 

[kjeff91]

If you mean upper wise, no.

However, upper fit is a first order issue.

Upper design is second order. AKA everything else must be pretty right for your upper design to do you any good.

For the upper, more stiffness is better.

First order issues seem to be, from my experience:

Barrel quality.
Bolt headspace and fit.
Gas system health.
No contact between rail and gas system even during firing.
Barrel fit to upper

Second order:
Perpindicularity of the front upper receiver ring
Design of upper

Of especial note for long range - I started smoothing my feed ramps with a cratex wheel and turning gas down to minimums to help with consistent low-damage loading. I also make sure my bolt carrier is traveling far enough that the buffer is contacting the bottom of the buffer tube.

Great info, thanks! What do you mean by gas system health btw?

 

[S197]

I'm looking at building a mk12 ish type upper for some longer range fun out to 500-600 yards, maybe more if it can reach out that far. I have a few questions on this type of build.

1. Will the upper receiver quality impact the accuracy at all?
2. I'll be experimenting with 55gr, 62gr and 77gr bullets. What would be the best barrel length for these?
3. Any common misconceptions where people thing they need or don't need certain parts?

Thanks!

How much money are you wanting to spend? how close of a clone are you wanting? and don’t believe that all mil spec parts are the same. Buy quality parts from reputable companie.

without braking the bank PRI offers clone builds fairly priced and they where part of the original build.

 

[charnicus]

Building your own stuff is cool and all but I’d consider a Mod1 upper from High Caliber Sales. At least that way if you decide it’s not your thing you’ll be better off in the value department. You’ll also have after purchase support that may or may not pay dividends. You can barely build it for what they’re charging. Mk262 ammo may be good enough but after several hundred rounds I expect that one could do a lot better. Also agree with what has been said about the PRI stuff.

 

[kjeff91]

How much money are you wanting to spend? how close of a clone are you wanting? and don’t believe that all mil spec parts are the same. Buy quality parts from reputable companie.

without braking the bank PRI offers clone builds fairly priced and they where part of the original build.

Well I was looking to pick up a used upper for less than $700, doesn't need to be PRI or anything specific.

 

[charnicus]

Well I was looking to pick up a used upper for less than $700, doesn't need to be PRI or anything specific.

I mean no offense but that is going to be tough to impossible. Barrel and hand guard alone puts you over budget.

 

[kjeff91]

I mean no offense but that is going to be tough to impossible. Barrel and hand guard alone puts you over budget.

How so? BCM or Midwest Industries quad rail is sub $200, 18"-20" match barrel can be had sub $300. Those are the most expensive parts.

 

[RyanScott]

Ok so $500 for a flexible handguard and a barrel. Add a muzzle device, crush washer, gas tube, charging handle, upper receiver and tools or assembly and you have a $900 budget upper.

 

[kjeff91]

$60 for SiCo ASR brake
$75 for the upper
$80 for adjustable gas block
$10 for gas tube
Already have a nice bcg+ch

$700 is easy to do especially if going used.

 

[RyanScott]

Ok so we are at $725 not counting BCG, charging handle, shipping, tax, forward assist of you require one.

 

[Strykervet]

I think PRI sells complete uppers for ~$1000 IIRC and it looks like they use all the proper parts for the one that uses their CF handguard. You'd need to check but pretty sure. Worth looking at anyway.

IMO the money would be better spent on a KAC LPR or assembling a "recce" rifle on a 16" Krieger bbl. (I built one using Lilja's Navy barrel and it's damn fine but the Krieger is even better).

 

[tuckybill]

I have a DD mk12 tornado grey I'm thinking of letting go. If your interested PM me.

 

[kjeff91]

IMO the money would be better spent on a KAC LPR or assembling a "recce" rifle on a 16" Krieger bbl. (I built one using Lilja's Navy barrel and it's damn fine but the Krieger is even better).

What would make those better options?

 

[rpoL98]

How so? BCM or Midwest Industries quad rail is sub $200, 18"-20" match barrel can be had sub $300. Those are the most expensive parts.

$60 for SiCo ASR brake
$75 for the upper
$80 for adjustable gas block
$10 for gas tube
Already have a nice bcg+ch

$700 is easy to do especially if going used.

so, what about this build is MK12-ish? just the fact that the barrel is 18" long? just wondering...

 

[kjeff91]

so, what about this build is MK12-ish? just the fact that the barrel is 18" long? just wondering...

18" barrel, quad rail, fixed stock. Never said I was going for clone correct.

 

[drewthebrave]

How so? BCM or Midwest Industries quad rail is sub $200, 18"-20" match barrel can be had sub $300. Those are the most expensive parts.

The heart of the Mk12 system is the barrel. If you want to follow the "spirit" of the system, don't skimp out on the barrel. I'd look at barrels from White Oak Armament (best value, excellent accuracy), Compass Lake Engineering (clone correct, excellent accuracy), or Craddock Precision (heavier, excellent accuracy). If you're looking for an extremely precise rifle with the 77 gr Mk262 ammo, which is what the Mk12 set out to do, then these will be your best options. There are many other very good barrels available (Lilja, Bartlein, Krieger, Proof, etc) that can perform similarly, but you're probably looking at $400+ for something comparable.

If you just want to build a good "do-it-all" rifle, this might be better described as a "budget Recce" build. Which is fine. Build what you want and call it good. But when you mention the Mk12, you're going to get very specific answers.