DIY AR 15 build–Range Ready

Test firing the Franklin lower was a real joy. It's mounted with a Sig upper, a Trijicon scope and a Blackhawk sling.
article top

by Rob Kay and RN Price

Now that the build-out of the three lowers has been completed, it’s time to assess the job. Call us superficial, but the first, and perhaps the most obvious question is “how do the the three lowers compare aesthetically?”

Finally ready for the range. This Franklin lower is attached to Magpul CTR and topped with a Sig upper with Trijicon glass

Naturally that’s a subjective call.

With the exception of a few slightly dissimilar milling marks, the two forged lowers, from CMMG and Stag Arms, are identical.

The Stag had a few tiny blemishes, but that was probably why we got it at a discount. Many times manufacturers will sell aesthetically challenged lowers for less, and that’s a good deal if you can find them. (You can always apply a little magic marker to address that issue).

The billet lower from Franklin, which was nearly double the price of the forged units, wore a desert tan finish as opposed the anodized black of the other two. Structurally, it was much more robust than the forged models from CMMG and Stag. (We went over that in detail in an earlier column we called “Inspecting the Goods”).

The finish on the Franklin was noticeably better—but for the price, you’d expect that to be the case. It’s a very different animal than the forged lowers.

The three sisters, from top to bottom, CMMG (with Rogers magwell grip and Rogers Super Stoc); Franklin Armory (with Magpul CTR buttstock) and Stag (with Vltor buttstock)

Assembling the Lowers

With the exception of one tiny safety detent on the Franklin lower, all the LPK parts on all the units fit perfectly. Franklin provided a couple of replacements and it’s fortunate they did. Only one actually fit. It’s hard to know whether this was the case of the detent’s diameter being too large or the bore too small? The good news is that we got it working.

The upshot? If you buy an LPK, chances are pretty good all the parts will fit. To test this theory, we switched LPKs. The CMMG set ended up in the Stag lower and vice versa.

Compared to the standard safety lever (at left) the Blackhawk version (at right) has a larger flange which in theory makes it easier to manipulate. We added it to the Franklin lower (illustrated above this photo)

In the case of the Franklin lower, a couple of non-standard parts were added.

We acquired a Blackhawk safety lever that sports a much wider flange that makes moving it with your thumb theoretically easier. We can see it doesn’t hurt to have more area for the thumb to manipulate the lever. In practice it takes a bit getting used to if you’re accustomed to the stock version. The fit on the Franklin lower was pretty tight and will need to be broken in.

There were a few other non-standard or non Milspec LPK parts on our build out, namely the Franklin Armory trigger group. That trigger assembly comes stock with the Franklin lower and is much closer to “match” than milspec. The hammer and trigger springs are lighter and we were told the trigger and the sear have been polished. Normally, Franklin installs these parts for you rather than selling them as a kit.

There’s a good reason for this, namely it takes some gunsmithing to get the trigger pull just right.

Here’s how it works: The pistol grip screw on the Franklin is tapped and there’s another screw inside that’s designed to take up any slack in the trigger. It takes some doing, but you can adjust the trigger yourself if you have a long 4” long 1/16 Allen wrench or ¼ inch hex bit socket (see photo at right). The wrench or the hex bit has to fit inside the pistol grip. Another option is to purchase a socket kit from Wheeler Engineering which provides a handle with a 4″ shaft and a 1/16 Allen head bit that will do the job.

You’ll need one of these babies to adjust the Franklin trigger properly.

The point is, unless you have the right tool, it’s a pain in the butt and better to let the factory do the job. If you want to swap out triggers there are plenty of modular units available. However, I suspect you’ll like the Franklin trigger.

That begs the question…how did the triggers differ on these three lowers?

For starters, we took them down to famed Hawaii gunsmith, Ed Masaki’s shop. Ed measured each trigger and came with these numbers:

  • CMMG:  6.5 lbs
  • Stag:  6 lbs
  • Franklin: 4.5 lbs

In short, the triggers on the CMMG and the Stag were more or less “Milspec”. The Stag’s trigger (which was actually inside the CMMG lower) was the better of the two, with a nice crisp release. The CMMG trigger (inside the Stag) had a bit of creep and broke at about a half a pound more.

The triggers were evaluated by three of us, including our friend Shannon, an active duty airman with 25 years of experience shooting government issued M-16/M-4s. It was his opinion, the CMMG trigger came “close to standard but with less creep and a crisper finish.” He rated the Stag trigger better than standard milspec, with “less creep and a crisp finish.” We all agreed the Franklin Armory trigger was a completely different animal. As mentioned above, it comes standard with the Franklin but is installed at the factory for the reasons stated above. It had a beautiful clean break at 4.5 lbs.

Any of the lowers we tested would make a great platform. The stock (milspec triggers) however were mezzo-mezzo.If you’re going to do precision shooting you’ll have to drop in a modular assembly from an outfit such as Timney (easiest) or a custom kit from another manufacturer.

Said Shannon, the hammer on the Franklin “fell in such a way that you no longer even think about the trigger and just are left to concentrate on the follow up shot…truly the way a trigger should be, but is not the way it is on any standard M-16/M-4 that I have shot in my military career. Should you ever be in a tactical or life threatening situation, trigger pull should be the last thing on your mind. The Franklin lower will allow for faster follow up shots on target to assure the threat is neutralized.”

Assembling the other parts

To complete a lower build, you’ll need to attach a buttstock and buffer tube to finish the job. We acquired stocks from Magpul, Prezine (the Rogers “Super Stoc”) and Vltor. The Vltor came with a buffer tube as part of the kit and we acquired separate buffer tubes from Stag and Palmetto State Armory.

(Note that in order to install or swap out a buffer tube assembly you’re going to need a wrench for the castle nut. We liked the Wheeler Engineering combo tool for this process. You can get it separate or with a torque wrench–which can come in very handy).

All stocks and tubes were milspec. All fit perfectly. With parts in short supply these days, we were happy to get good quality products.

(You’ll find the Vltor, Rogers and Magpul buttstocks reviewed in this column).

Mating Ritual

If you build your own lower, Harvey Gerwig, a gunsmith and President of the Hawaii Rifle Association told me it’s best to get an upper from the same manufacturer. “That way”, said Gerwig, “you’re guaranteed to have a good fit.”

Successful mate: You’ll want the Goldilocks fit–snug but not too tight. Loose enough so that you can pop out the pivot pins with your fingers but not so loose so that that the fit is sloppy. The CMMG lower mated perfectly with this Sig

So what exactly is the right fit?

If your upper and lower fit snugly and, you can pop out the pivot pins with your fingers, you’ve got exactly the right fit. Not too tight nor not too lose. Thus what you want is the Goldilocks experience.  Tight enough to keep the upper and lower snug and stationary but just loose enough to pop the pins out with your fingers.

We tested our three lowers with a variety of uppers and the results varied. These included one from Olympic Arms; a Sig 516; a Colt M16A4; a PWS Mk 1 Mod 0 (300 AAC Blackout); and a Rock River Arms (458 SOCOM).

The three lowers all mated perfectly with the Sig. All were snug fits while allowing us to easily dislodge the pivot pins with our fingers. This was not the case with an older Olympic A2 variant. With the Olympic, the fit was only correct with the Stag.

The other uppers fit all of the lowers remarkably well, with little discernible play. The only exception was the Colt M16A4 upper, which fit tight enough on the Franklin lower that we had to use a nylon drift and a small mallet to get the rear pin started for removal. A light tap was all it took, so we would not call it a failure, but it’s certainly a combination you would think twice about for, say, a duty weapon.

The rule of thumb was, the better quality the upper, the better odds that it would pass the Goldilocks test.


Building a lower receiver is not all that difficult. It’s an excellent way to familiarize yourself with the inner workings of your AR 15 and you’ll be in a much better position to upgrade your rifle, if and when you decide to do so. Yes, it does take a bit of time and you do need the right tools. We both believe the investment in time and tools is well worth it.

As for the lower receivers we put together…any of them would be good candidates as a platform for your future gun.

Aside from aesthetics such as the roll mark and color, the trigger is really the key component. The good news is that if you’re not happy with what the stock trigger that comes with the LPK, you can swap out your milspec version for a very fine trigger from Timney or another manufacturer for around $200 or less. (Stay tuned for a future column where we drop in a custom trigger from Timney).

Many thanks to CMMG, Franklin Armory, Stag Arms, Windham Weaponry, Blackhawk, PrezineGrace USA and Wheeler Engineering for supporting this project.

All photos by On-Target Hawaii Staff

Questions?  Comments?  Contact us 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





  1. Our teachers can ask us to write an essay on a specific topic. We should do some research on that topic and start writing the essay. We shall do well if we use our knowledge and education.

Comments are closed.