Mk12 Mod 1 Build - Knight's Armament SR-15 stripped lower

The KAC SR-15 stripped lower I ordered back in January from One Shot Tactical finally showed up.  It’s not OST’s fault as KAC sold a bunch of these in the states and it turns out some of them were blemished.  As I understand it, KAC held the Canadian shipment back until additional QC could be done to ensure there were no blemished ones in the batch coming up here.

I’ve always known KAC to have had an excellent reputation, but I’d never gotten around to trying out any of their products and have always stuck with Colt or other brands.  It also didn’t help that some of their gear was prohibitively expensive for what you get.  For example, $465 for the KAC Free-Float RAS (Mk12 mod 1) which is an old school, free float cheese-grater handguard, or a $146 for the M4QD flash hider that really isn’t much more than a standard A2 birdcage.  $200-300 for a set of used, non-free-floating RIS handguards for carbine length, an at least $300 for a set of used non-free-float rails for rifle length.

These sort of prices turned me off their products in the past, however in recent times, I got into clone building and my mind got so twisted, I was able to justify to myself these prices were acceptable.  Then One Shot Tactical announced stripped lowers for $260 and though that was just a bit more than a stripped Colt Canada lower, I figured why not, let’s get something with a different roll mark.  These ended up being $274, but I was fine with that.  Anyway, ordered in January, it finally arrived this week.

The box seemed a bit fancier than the previous KAC products I’ve purchased, and it definitely appears that they had the consumer in mind.  That being said, it was a simple cardboard sheath with the KAC logo and markings, and a simple cardboard box housing the stripped lower.

Inside the box, the lower was sealed in a plastic bag and nothing else.

The lower wasn’t soaking in oil or grease, so I was able to get an idea of the finish right away.  It came in a nice dark black finish (as opposed to the light gray of my Colt Canada stripped lowers).  I’d say the KAC lower is a bit darker than my Colt LE6920s, and though it’s basically a matt finish, there appears to be a bit more gloss to it than the Colts.  

I scrutinized the lower for blemishes but came up totally blank and didn’t find any.  The finish on this lower is definitely one of the best I’ve had.  I haven’t owned many other ARs, but I would say the typical consumer would probably be more satisfied with the finish on the KAC over say a Colt USA or Colt Canada.  I owned a PWS lower that came close, but at the end of the day, we all know I just paid an extra $50 for the roll marks, lol!  The consumer in me really likes it, but if I saw one of these on the rack at work, I wouldn’t care if it had KAC roll marks or Colt Canada/Diemaco.

Anyway, I dug out some parts out of my spare parts box that I had set aside for this build, and realized I was short an A1 grip because a week ago, I’d purchased an LE6920 lower that I wasn’t expecting to.  It was a good thing I had a spare Ergo from who knows what build, so I decided to use it (pretty sure these are clone correct for Mk12 Mod 1 clones).  I also decided to go with the A2 stock (pretty sure these are kosher as well) because the A1 stock I have is in pretty rough shape.

I found a Norgon ambi-mag release in my box of parts, and I had these nightmarish flashbacks of going on a shopping spree at Nordic Marksman.

I’ve done step-by-step lower builds before, but I learn something new every time I do it, so I WILL bore you will tons of pictures and if necessary, attempt to explain what’s going on in the pictures, lol!

So, I found the easiest way to install the mag release is to insert the mag catch on one side, then go to the other side and drop in the spring and thread on the mag release button a few threads to keep everything locked in place.  Then find something like a pen or punch and depress the button side, pushing the catch out of the receiver and spin it until it’s at the right setting. 

Once that’s complete, it’s best to function test it with a magazine and make sure it catches the mag and drops free when the button is depressed.

The trigger guard is one of the things I hated about building a lower.  I’ve always had trouble with the trigger guard pivot roll pin and they’ve always been a colossal pain in the dick to install.  I’ve heard nightmares of guys breaking off the ears of the lower in an attempt to get this roll pin in, but that’s mostly due to failing to support the ears.  The worst I’ve ever experienced is peening the pin and having it mushroom on one side.  In the past, I’ve used a Mickey Mouse technique of clamping down with a pair of pliers, like a vice, and squeezing the roll pin in.  It worked sometimes, but if I got a really stubborn pin, sometimes the pliers would lose grip and the pin would fly off into space, never to be seen again.

This time around, I got the Wheeler AR-15 Bench Block, which made a world of difference, and I’ll get to why when it comes.  I installed the trigger guard from the detent side first, and swung the guard into place to support those ears that break so easily.

The other tool that made a world of difference is a roll pin starter punch set.  I think I bought this kit off amazon for $15 and I wish I had it before for all my previous builds.

One last trick I had up my sleeve was something I picked up from my buddy Hermes, and that was a drop of CLP to lube up the roll pin.  Fat, skinny, thick or thin, CLP will get it in.  I put a little dap on the end and the roll pin slid right in.  Once I maxed out the roll pin starter punch, I took out a brass punch and finished it off.  No mess, no peening, no sweat, no blood, no tears, and no swearing, lol!  This was probably the cleanest trigger guard install I’ve ever done.  Even my best effort resulted in some minor chipping in the finish because the roll pin was going in dry.

Next up was the bolt catch, which can equally a pain in in the buttocks because it involves…..yes, another roll pin.  This time, it’s on a funny angle, and without the Wheeler AR-15 Bench Block, I’ve always had to do this sideways, trying to secure the lower, while tapping in the roll pin.  Every try to hammer something into a hole that didn’t want to go into said hole, sideways, and not scratch up your lower receiver?  Not to mention trying to hold the stupid roll pin in place with either your fingers, a pair of tweezers or pliers when you’re trying to tap the stupid thing in.  I’m sure many have, but I’ve always taped up my lowers when doing this part because a slip of the hammer or brass punch and you’ve got that chipped look on your AR.  With the bench block, you are able to stand your lower on end, thus eliminating having to do this sideways.

So, first step, drop in the bolt catch spring and plunger and plop the bolt catch on top.

Once again, a little dab of CLP on the tip of the pin which is sitting in that wonderful roll pin starter punch. 

With gravity on my side, I didn’t even really need tape, just a cloth would have been fine.  Anyway, the roll pin went in with very little effort…..it was almost too easy, lol!  For anyone thinking about building your first AR, I STRONGLY recommend you get an AR bench block, and a roll pin starter punch set.  I know what you’re thinking, I’m only ever going to build one AR, why would I want to get all this equipment for one build?  Let me tell you that it will never be just one build.

Next up the front take down pin.  There was no setting on the bench block for the lower to sit upright this way, but I managed to find part of the block was just the right height to stand my lower up on end, and again using gravity to help me.  

Drop in the pivot pin detent spring, then the detent.

I use an old technique which I found on youtube which seems to work, though you are taking a chance with this method because if you slip, your detent with shoot up in the air and land in your shag carpet, never to be seen again.  Anyway, if you feel like rolling the dice and don’t know a better way, here’s how.  Depress the detent and spring down and cover it with an X-acto knife.  Slowly slip the front take-down pin in and once it’s covering the spot where your detent would normally come flying out, slowly remove the knife.  A little dab of CLP around the take down pin helps as well, not to mention some in the channel of the take down pin so the detent doesn’t bind up in it.

Next up, for me at least, is the Armalite 2 Stage Tactical Trigger.  I had to dig around for this information because the instructions tell you that it’s adjustable, that there are three weight settings, but doesn’t tell you the weight.  The “tactical” setting is the middle one, and the lightest weight is the highest most setting.  I like my triggers light, so I pulled the spring leg off its perch and adjusted it accordingly.

It took a bit of effort to line up this hole for the trigger pin, but I somehow managed.

It helps to wiggle this part of the trigger (where my index finger is pointing) until the hole lines up.

The hammer was a little easier to install.

To install the selector switch, it helps to have the hammer down.

Pretty self-explanatory.

Almost there.  Since I didn’t have any more A1 grips (not fond of the A2 grip), I threw on my Ergo grip.  Grips are grips, and are pretty much the same all across the board when it comes to installation.  Drop the safety detent into the receiver, throw the safety detent spring into the grip and bring the grip up to the receiver.  The Ergo grip’s tactile nature had enough friction between the rubber of the grip and the metal of the receiver that it stayed in place without any effort and then it was just a matter of screwing in the grip screw.

For A2 stocks, the procedure might be slightly different than with the Carbine collapsible stocks in that there is no end plate and you can install the rear takedown pin a bit after installing your receiver extension.

First drop in the buffer retainer spring.

Then drop in the buffer retainer.

Then spin on the receiver extension and crank it on.  Unlike the collapsible butt stock receiver extension, you don’t need to time it, just spin it on and you can even crank down on it at the end with a wrench.  For A2 fixed butt stocks, there’s a spacer.  

Drop in the rear takedown pin (you can add a drop of CLP into the channel if you want).

Drop in the take down pin detent, then drop in the spring.  

After adding the receiver extension spacer, slip your A2 stock onto the receiver extension and slowly bring it towards the end of the lower receiver, capturing the rear takedown pin spring.  Spin on the butt plate screw and that’s basically it.  Throw your buffer spring and buffer into the tube and bob’s your uncle.  Actually, I like to lube the spring with CLP before putting it in, but some use motor oil or some sort of grease. CLP is good enough for me.

I got this stock a few months ago, and test fit it to one of my LE6920 lowers, so it’s not that new to me, but it’s been a while since I handled it, so it’s kind of nice to see it on a lower again, and a nice lower at that.  I don’t have any fixed stock ARs, so it’s kind of nice to have some variety in the safe.  I did have a PRS stock at one point in time, and actually quite liked that stock, but in the name of clone building, it had to go and I ended up selling it to a buddy.  

Aaaaaaaand that’s my KAC SR-25 stripped lower build, lol!  It was the easiest lower build I’ve done so far, and it might have had something to do with the excellent quality lower, but I also have to give credit to the bench block, the roll pin starter punches, and more experience.  My buddy Vincent got his stripped lower a day before I did and he was texting me every few minutes, cursing the Gods about those roll pins from hell.  For me, I could hardly believe how easy and fun this latest experience was. 

The lower’s finish was excellent, and because I haven’t had much experience in the past with KAC, it’s kind of all fresh for me and coming up roses.  It looks good, it feels good, and it was easy to put together.  Although I’m quite happy with the results, I try not to think about the cost of all that KAC gear, hahahaha!