80% C-5 Billet Lower


Manufacturer:  New Frontier Armory
Model:  80% C-5 Billet Lower Receiver
Caliber:  9mm
Magazine:  MP5

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SKU: 80-C5 Category:


80% C-5 Billet Lower Receiver — MP5 Mags


Manufacturer:  New Frontier Armory

Model:  80% C-5 Billet Lower Receiver

Caliber:  9mm

Magazine:  MP5

Weight:  1 pound

 Other Specifications and Features of the 80% C-5 Billet Lower Receiver:

  • Machined from 7075-T6 aircraft grade billet aluminum
  • Tumbled in ceramic media — Ready for blasting and your choice of firearm coating once you complete the machining
  • Precision machined magazine well with beveled feed lip for easy mag swaps
  • Compatible with most standard AR-15/9/45 components and most aftermarket parts
  • Take down pin spring and detent hole is threaded for a 6-32 set screw for easy capturing (hardware included)
  • Upper tension screw behind grip screw (threaded hole with hardware included) to adjust any “slop” out of vertical fit between upper and lower
  • Ambidextrous trigger finger rests with grooves (above and below mag release/mag catch)
  • Threaded bolt catch pin (Threaded hole and hardware)
  • Integrated large opening “winter” trigger guard
  • Proprietary magazine catch assembly and ejector is included and installed at the factory
  • Manufactured to MILSPEC (where applicable) on state of the art CNC equipment for consistent quality parts and exacting tolerances
  • Save money by building your own AR-9 rifle from the ground up exactly the way you want it configured and shoot for less!

You must use an upper that has widening cuts to accept the larger width MP5 Magazines — We sell three versions of C-5 Uppers that can be found HERE.
ETS magazines may require slight sanding to ensure proper fitment due to variations from factory H&K Magazines.  We have yet to find any fitment issues with HK, Zenith, or KCI mags.

AFTER FINISHING — Please be sure to use a drop of Loctite on both the ejector screw and the feed ramp screw when reinstalling them.

Completion Jigs & Fixtures can be found in the “Tool” category or by CLICKING HERE 

C-5 LPK is available by CLICKING HERE

Magazines can be found HERE



QUESTION:  What jigs can I use to complete this 80% lower?

ANSWER:  Because of our mag well design, any jig that attaches to the REAR of the mag well will work for completing this lower.  Other jigs can be modified to fit around our mag well and will also work.  We sell a jig that works with all other 80% lowers on the market which you can purchase by clicking HERE.


QUESTION:   What type of magazines will fit in this lower?

ANSWER:   The lower was designed to fit and drop free factory H&K MP5 9mm magazines.  It has also been thoroughly tested with many aftermarket magazines to fit and drop free.  Although it accepts and shoots other aftermarket magazines some slight modification may be needed to get them to drop free from this lower because of the varying width measurement we have seen when checking these magazines to H&K MP5 factory specifications.


QUESTION:   Do you offer a reliable last round / empty magazine bolt hold open (LRBHO) System for this lower?

ANSWER:  NO, like the original MP5 platform, there is no LRBHO function.  You must use either our C-5 Slick Side Upper or our C-5 Non-Reciprocating Side Charging upper as they are widened to accept the MP5 magazines.  Other manufacturers make uppers cut for MP5 mags as well, but we can’t guarantee their compatibility at this time.


QUESTION:   What bolt carrier group will work with a MP5 magazine fed AR-9?

ANSWER:  Both AR-9 BCGs that we sell (CLICK HERE for AR-9 BCGs) will work in both MP5 Mage AR-9s as well as traditional Colt style AR-9s and Glock Mag AR-9s.


QUESTION:   What LPK will work with this lower?

ANSWER:   Although we sell a specific LPK with just the parts needed to build the C-5 lower receiver HERE, any standard AR-15 LPK will work.  You will not need the magazine catch assembly, bolt catch roll pin, or trigger guard that are normally included with standard LPKs.


QUESTION:   Do I need to use a special buffer when I build my AR-9?

ANSWER:  Although it’s not technically required, it is advised that you either use a 9mm buffer (longer and heavier than carbine) or a spacer in your buffer tube before you place your spring in.  The reason for this is to limit the BCG travel to the rear so it doesn’t come back forward and smash the bolt catch hard enough to break it.  The 9mm BCG is shorter than a standard BCG.  You can use a carbine buffer with a spacer (WHICH YOU CAN PURCHASE BY CLICKING HERE).


The New Frontier Armory 80% C-5 Billet Lower Receiver is proudly MADE IN THE USA by hard-working American men and women.



**80% Lower Receiver Restrictions:  80% lower receivers are not legal for retail sales in all areas of the U.S.  Please be aware of your local laws prior to ordering.
Orders with prohibited items shipping to areas that have 80% lower receiver bans will be canceled and a 15% restocking fee will be deducted from refunded payments.

All GCA and NFA rules apply when you complete this project.  Please ensure you fully understand your local and federal laws before completing this 80% blank into a functioning receiver.  New Frontier Armory will not be held responsible once this blank is altered in any way.

Additional information

Weight 1.2 lbs
Dimensions 9.5 × 4.5 × 2.25 in

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