Words and Pics: Killen Custom


Since I was first aware of them, the photos that have been an influence for my Mk12 builds are the ones of Navy SEALs Matt Axelson and James Suh (RIP).

Through the years I’ve built several NSWC Crane inspired builds with a heavy focus on getting every little part right; like the Mk18 mod 0, Mk11 mod 0 and Mk46 mod 0. I’m working on a Mk13 mod 5 now.

In 2008/2009 I actually built my first PTW Mk12 mod 1. Back then correctly profiled Mk12 barrels didn’t exist so I went with a G&P 18” steel barrel with G&P 12th model muzzle brake and collar. I remember those parts were pretty difficult to source. That first Mk12 build was also using a real Knight’s Armament 12” free float RAS, real Colt M16A1 buttstock and real Harris bipod. Needless to say I now regret selling those!

Well, fast forward to the end of 2019 and I’d just gotten back into the scene after an 8 year hiatus. What sparked the interest again was moving back home after several years abroad, and it started off by 3D designing and making parts for an airsoft Mk48 mod 1 conversion kit. This was the stepping stone to take up the gun tinkering again.

The Plan

The decision to build on the PTW platform was a no brainer. It takes most real parts, is really easy to maintain, and it works in cold winters where I live! Simply put, it’s my M16 platform of choice.

The goal was to build a fairly clone correct “as issued” Mk12 mod 1, but on a budget. I wanted all the essential Mk12 parts right, like a lower with M16A1 trades, M16A1 buttstock, long 12th model collar, gas buster charging handle, ergo grip, and A.R.M.S #22 high scope rings. As for the scope, being on a budget didn’t allow for getting the issued Leupold TS-30 2.5-8×36 mm scope for my build, nor the civilian version of the scope. I therefore settled for a Leupold European 30 2-7×33 mm scope I got second hand for cheap.

The reason why I didn’t just go with a regular M3 40 mm airsoft scope is that I wanted to maintain the slimmer profile of the smaller scope issued on the Mk12. A bonus with using a genuine scope is that you’re actually able to see through it!

With all that said, the catalyst for the entire endeavour was the lower…

Real Steel M16A1 Lower Conversion for PTW

One day I was browsing around and found real Colt M16A1 export lower receivers for dirt cheap (less than the old Prime CNC M16A1 PTW lower). I found them on a German website called Zib Militaria and I was able to obtain one, as they’re completely legal to own where I live. However the uppers require a license to purchase.

Having already modified a WE416 GBBR lower to accommodate PTW internals back in 2010, I felt fairly confident I would be able to modify a real one as well. As I have no access to a machine shop, I did all the modifications using a Dremel, hand files and various other hand tools. It took an effort but it’s definitely worth it. Guess I spent around 12 hours on the conversion, and several hours more for the finer details. It’s not really difficult to convert, it just takes a lot of measuring and time. Obviously I wouldn’t recommend someone who’s not tinkered before to have a go at it!

Zparts Front Set

The Zparts front set really impressed me. Upon opening the box I kept thinking, “This rail is exactly like my old KAC rail!” The quality of material, machining and finish is really spot on. Broken edges just like the real one – very different from your regular airsoft replica. The surface finish has got that slightly chalky matt finish just like the real one. The markings seem to be spot on. It’s just a stunning piece!

Moving on to the Zparts barrel; it’s got the correct heavy barrel profile like the Mk12 Douglas barrels, with a nice phosphated finish. It’s also got the step for the 12th model collar alignment. From what I can see it’s a 100% clone of the real barrel.

The muzzle brake, collar and thread protector are made to the same great quality like the rest of the kit. A real plus, which many other parts and kits have done wrong in the past, is that the collar is the long version; which is the correct one for Mk12 mod 1.

I don’t work for Zparts, I promise, but they are releasing some excellent kit!

The kit also includes a couple of crush washers in different sizes for proper muzzle brake alignment. I had some real KAC rail panels laying around and they fit nicely, although a bit looser than they would on a real rail. I also attached a real KAC vertical foregrip.


The only thing I could have done differently in hindsight is to have the lower properly machined at a shop. However, this was intended as a clone build on a budget. So any money saved counted.

With this build I was aiming to get the essential details right; those that so many forget when building Mk12s – not to mention all the generic off the shelf “Mk12s” out there.

Parts List

  • Systema PTW
  • Zparts Mk12 mod 1 kit
  • RS Knight’s Armament VFG and rail panels
  • RS Colt M16A1 70’s/80’s full auto export lower
  • RS Brownells M16A1 type D buttstock
  • RS DPMS M16 receiver extension
  • RS DPMS (?) ejection port cover (had it laying around from previous build)
  • RS PRI M84 gas buster charging handle
  • RS ERGO rigid ambi pistol grip converted for PTW
  • RS A.R.M.S #22 QD scope rings, high
  • RS Leupold European 30 2-7×33 scope
  • RS Butler Creek scope covers
  • No name “Harris” bipod (cut to the right length)
  • No name “Docter” sight
  • No name “KAC” front sling mount
  • King Arms “12th model” suppressor
  • Pantac “Eagle Industries” 3 point sling

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