Several weeks ago I was walking around my favorite gun store, BMC Tactical in Albuquerque, New Mexico, when I saw a fancy looking Glock lower frame sitting in a plastic jig.
“What is that?” I inquired.
The clerk working the gun counter pulled the assembly out of the case and explained that it was Polymer 80’s PF940C Compact frame for a compact-sized Glock pistol.
“Sold. Do you have a lower parts kit?” I asked.
Beginning the Build
I walked out of BMC Tactical, drove home, and went straight to my armorer bench. Placing the Polymer 80 frame in the plastic jig, I squeezed the jig assembly in a typical work vise under my vertical drill press.
After attaching the supplied end mill in my drill press chuck, I turned the drill press on and started to remove polymer. The end mill sliced through the polymer like butter. After slowly and methodically removing all necessary polymer along the top of the frame, an act which took a whopping 10 minutes, I cut the slot in the frame that interfaces with the guide rod. After milling off as much plastic as possible with the end mill, I used a standard hand drill to bore holes for the pins that hold the internal parts. Polymer 80 provides two small drill bits for these holes. Note that the manufacturer does not recommend the use of a drill press for this step.
Assembling the Lower Parts Kit
After drilling out the holes, I used a set of Ramrodz Diamond Tipped Tweezers to smooth out the edges of the milled plastic. I assembled the lower frame kit and attached the slide assembly.
Initially, cycling the slide felt gritty. I disassembled the slide from the frame, lubed the pistol, re-assembled, and cycled it 100 times. After 100 cycles, I cleaned the pistol, relubricated it, and cycled 100 more times. I repeated this process until I had cycled the pistol a total of 500 times. After 500 cycles, the pistol felt smoother but was still difficult to rack. The slide was particularly tight when I held the trigger to the rear. I did an armorer level inspection, determined that the Glock Safe Action system would work properly, and drove out to my local range to test the pistol.
Since it was dark, I attached a Streamlight TLR-1 pistol light and hung an Action Target gong at 15 yards. I fired off 100 rounds of Remington 115-grain 9mm ammunition. The pistol required an aggressive grip to function and would fail to eject if my grip was loose. Time to hand fit.
Hand Fitting a Glock Pistol
When I held the trigger to the rear, the slide was damn near impossible to rack. This problem was the first I wanted to address through hand fitting. When a Glock trigger is in the rear position, the part of the trigger bar that interfaces with the firing pin safety is at its highest point. This position means that it makes contact with the bottom of the slide. Interestingly, the first time I attached the slide to the lower frame, the slide got stuck. The trigger bar got stuck distal to the firing pin safety. Removing the spacer sleeve and firing pin assembly allowed me to separate the slide from the lower frame. With a diamond file, I removed metal from the trigger bar until the bar and firing pin safety could interface correctly. This area was then sanded with 2000 grit sandpaper and Brasso.
Rear Rail Module
Another area that I wanted to improve during the hand fitting process was the Rear Rail Module, proprietary to the Polymer 80 frame. On a factory Glock pistol, this part is molded into the plastic. On a Polymer 80, it is a stand-alone piece that houses the Trigger Mechanism and is pinned in place along with the Trigger Mechanism. The rails were sharp and rough. After going over them several times with a diamond file, I sanded them with 2000 grit sandpaper, then polished them with Brasso. I also polished the Trigger bar and Connector.
Removing a little bit of metal off the trigger bar and polishing the rails on the Rear Rail Module worked wonders! After reassembling the slide and lower frame, I cycled the pistol, and it felt like a factory Glock. I cleaned and lubricated the pistol then conducted a pencil test. I cycled the pistol, then dropped a pencil down the barrel. Pointing the gun in the air, I squeezed the trigger. I was pleased to see the pencil jump several inches. This test checks to make sure that the Glock Safe Action System is working and that the pistol will fire.
More Range time
Heading back out to the range, I fired 100 rounds of PMC Bronze, Remington UMC, and Tula. The pistol performed flawlessly.
The final round of testing took place during a pistol course taught by QPro Defense. Former Recon Marine Oscar Sanchez led the course, which was excellent: the instruction was top notch, the drills were relevant, and the coursework was current. I ran the Polymer 80 pistol for about half the course, and it performed 100%.
So why did my Polymer 80 need the fine-tuning? My best guess is that my Rear Rail Module sat a little bit higher than average. This difference caused my trigger bar to put extra tension on the bottom of the slide. I have conferred with others who have also completed the kit, none of whom experienced the problems I discussed here.
All in all, finishing the Polymer 80 Compact lower frame kit was a lot of fun, and took surprisingly little time. The Frame itself has some nice additions, such as an enlarged back strap to mitigate slide bite, and comes with beautiful stippling. The ergonomics on the frame are superb, and this pistol is a joy to shoot.
Build one! The Polymer 80 Compact kit sells for around $150. If you have questions, feel free to comment below.
For more information about BMC Tactical, click here.
To purchase a Glock pistol on GunsAmerica, click here.
For more detail about the Polymer 80 Frame, click here.