DIY AR 15 Build–Inspecting the goods

Sign pointing to registration area at HPD
article top

By Rob Kay and RN Price

HPD sign points the way

The three lower receivers, along with their respective lower parts kits, finally arrived from the Mainland. We picked everything up in Kailua at “Ready on the Right” from Al Mongeon, our FFL.


After that it was off to the HPD firearms registry office on Beretania Street for the obligatory two hour “line up”. Why three lower receivers? The idea was to randomly select three different stripped lowers, compare them, and turn them into working AR 15 receivers. (Eventually we will take them to the range, mate them with a lower and see what they can do).

Although our selection was hardly scientific, we got an all American cross-section of products from the East Coast, the Midwest and West Coast. The manufacturers included:

Frm top to bottom: CMMG, Stag Arms, and Franklin Armory. Note sling mounts, oversized trigger guard and finger rest stippling on Franklin.
  • Stag Arms in New Britain, Connecticut 
  • CMMG in Fayette, Missouri 
  • Franklin Armory in Morgan Hill, California

The Inspection

At first glance, it’s easy to differentiate the Franklin Armory lower. Called the Libertas, ($250) I ordered it in desert tan, which distinguishes it from the black, anodized forged receivers from CMMG ($130) and Stag ($135.00). (Note that the prices are pre-election numbers).

Not only were the CMMG and Stag the same color, they were physically nearly identical. The Franklin Armory Libertas is a different color and a very different animal.

The Franklin receiver differed from the others because it’s manufactured in the billet process—that is, it’s machined out of a solid bar of metal. By definition, a billet receiver takes more time and energy to produce. The finished product is generally considered more aesthetically pleasing that the forgings. For these reasons, the billet receivers garner a premium in the marketplace and end up being roughly twice as expensive.

The Franklin receiver also has a few more bells and whistles than the others we reviewed.

For example, the front of the lower is “textured” which presumably helps if you use the magazine well as a grip on the gun.

The other feature that differs greatly is a prominently flared mag well. The theory is that the expanded well makes it easier for the shooter to pop a new magazine into the gun.

From left to right: CMMG, Stag and Franklin. Note progressively larger magwells and the stippled front on the Franklin.

Fair enough.

There are other differences. A cursory look at the Franklin lower reveals that the walls are thicker and in fact the whole receiver is more robust that the other two lowers we looked at.  (Randy Terbush, founder of Forgemark arms and an On Target consultant for this project commented that the Franklin Armory receiver was “a bit over-engineered.”)

The Franklin lower came with a (over-sized) trigger guard already “built in”. This differs from the standard “milspec” style trigger guard, which needs to be pinned to the lower receiver as part of the build-out process. The larger sized trigger guard works well if you have beefy digits or plan to use gloves.

“Rollmark” from Franklin Armory

The other conspicuous difference is the ambidextrous sling mounts Franklin incorporates on its lower. Everybody needs a sling, so it’s a nice touch.

The Franklin lower also has a tension screw to provide a snug fit to what otherwise may be a loose fitting upper receiver. This is not found on most lower receivers and certainly beats placing a wedge under the upper receiver lug to shore up a less than perfect fit.

Finally on the mag well, forward of the trigger is a textured finger rest, which serves as a spot to safely place your trigger finger if you’re not ready to fire.

There were a few minor aesthetic dissimilarities between the CMMG and the Stag but we couldn’t find anything monumentally different. For example the two manufacturers had some slight variations in their respective mag well bevels.

CMMG lower (rt) displays flat milling mark compared to the Stag model at left

Another minor disparity was a flat milling mark on the round transition on the web between the receiver extension and grip on the CMMG lower. The Stag’s was completely rounded.

The Stag lower had a few blemish marks. From my understanding some manufacturers will sell discounted lowers with aesthetic imperfections and I would presume that would explain our particular unit. BTW, to fix a blemish, all you need is a magic marker.  Bingo, blemishes gone).

There was also a very minor difference on the trigger guard ears between the CMMG and the Stag. The ears on the Stag were a bit more rounded than the CMMG.

We’re talking manini differences.

Randy Terbush, our consulting gunsmith, had a slightly different perspective.

“The CMMG,” said Terbush, “had very poor finish on the radius between the grip and buffer tube.” “However,” said Terbush,” this is less about machining and more about finish processes. CMMG’s machining of holes, with very nice chamfers, beat the Stag on machining quality. I also find the mag well comparisons between CMMG and Stag to show the CMMG to be much better work.

As we outlined in an earlier column, the differences between the comparative strength of the forged vs. billet lowers are negligible. (Perhaps buying a billet is more of a statement. A Hummer will get you to the grocery store as expeditiously as a Geo).

Lower Parts kit aka “LPK” from CMMG, a well regarded company

The lower parts kit or “LPK” in the vernacular, make up the viscera of the lower receiver.

These items include the trigger and hammer assemblies, take down pins and all manner of  detents, springs, screws, etc. They usually come in color-coded envelopes for ease of installation as with CMMG and Franklin or in the case of Stag Arms, are packed together in a single envelope. These items can include:

  • Grip
  • Grip screw
  • Grip screw washer
  • Buffer detent
  • Buffer detent spring
  • Forward takedown pin
  • Forward takedown pin detent
  • Forward takedown pin spring
  • Magazine catch
  • Magazine catch spring
  • Magazine catch button
  • Bolt catch
  • Bolt catch plunger
  • Bolt catch spring
  • Bolt catch roll pin
  • Trigger guard
  • Trigger guard roll pin
  • Hammer
  • Hammer spring
  • Hammer pin
  • Trigger
  • Trigger spring
  • Trigger pin
  • Disconnector
  • Disconnector spring
  • Safety selector
  • Safety detent
  • Safety detent spring
  • Rear takedown pin 
  • Rear takedown detent
  • Rear takedown spring

Nowadays the LPKs are tough to come by and their average prices have rocketed from $60-70 to $100 or more. We were lucky get them. Being a journalist helps. Is there a difference in quality between kits?

Eric Kiesler a veteran gun tech at Brownells, doesn’t think so. Brownells sells a number of different brands and he’s fine with any of them. Eric gets his dander up a bit when some manufacturers gratuitously use the term “Milspec” to describe their kits.

Up close and personal with a Stag Arms trigger assembly. Stag had a killer Milspec trigger in this kit.

“Unless you have a contract with the government to build milspec parts with military inspectors coming in to check out your operation”, says Kiesler, “you don’t have a true Milspec parts.” He reckons only Colt and FN have true Milspec LPKs”, says Kiesler. The difference in price is instructive. A Colt LPK, he points out, is $160 vs. a DPMS kit which sells for $60.

Do most people need a true Milspec LPK?

Eric didn’t think so. A kit from a reputable manufacturer is just fine.

On the topic of Milspec, Terbush said it was “the set of specifications that are delivered in an RFQ to a manufacturer that define manufacturing processes and tolerances that a manufactured product must hold in order to satisfy the contract. As we all know, government management and design of many such products leaves a bit to be desired in many cases. (Take toilet seas as an example). If you put a Milspec Colt and a nice custom AR in front of any soldier, I think you will find the soldier will happily pick the custom rifle over the ‘Milspec’ product. Any company in the business of designing and building custom rifles is generally far exceeding the requirements laid out by the ‘Milspec’. Pick a well finished product from companies that are passionate about firearms and you will likely not go wrong.”

In our next column we’ll describe how we do the build. Stay tuned.

 Questions?  Comments?  Contact us directly at

Rob Kay writes about firearms for Hawaii Reporter and is the author of How to Buy an AK-47.

Read more of Rob’s articles on