RL556v3 80% AR15 Lower – Black - polymer 80
Pros & cons
What works
- Weighs 10.2 oz unfinished—40% lighter than equivalent aluminum lowers
- Magazine well pre-formed to 1.18" width requiring minimal finishing
- Reinforced nylon polymer withstands 15,000+ round count in testing
- Buffer tube threads molded to spec—no tapping required
Trade-offs
- No included jig—adds $120-180 to complete project cost
- Polymer flex requires trigger group shimming in 30% of builds
- Limited color options compared to Cerakoted aluminum alternatives
- Fire control pocket requires exact 0.250" depth milling for reliable function
Video review
Expert review
Specs at a glance
About this product
The RL556v3 80% AR15 Lower – Black is a polymer 80% lower receiver requiring final machining before it can function as a firearm. It ships unfinished with trigger pocket, bolt catch, and magazine well areas needing removal of approximately 3.5 ounces of polymer via drilling and milling. This ModFuze model weighs 10.2 ounces in raw form and measures 7.6 inches long by 2.2 inches wide at the buffer tube interface.
What is the RL556v3 80% AR15 Lower used for?
The RL556v3 provides a legal starting point for building a custom AR-15 rifle without serialization until machining is complete. It accepts standard AR-15 upper receivers, fire control groups, and buffer systems when properly finished. The polymer construction reduces final build weight by approximately 40% compared to aluminum lowers like the 80% Lower Fire/Safe Marked – Anodized Black.
How does the RL556v3 compare to the Polymer80 LR-308 80% Lower Receiver?
The RL556v3 is specifically designed for AR-15 platform builds rather than the larger AR-10 platform. It accommodates 5.56mm/.223 Remington calibers versus the .308 Winchester capacity of the Polymer80 LR-308. The RL556v3 requires less material removal (approximately 45 minutes machining time) compared to the 70+ minutes needed for the LR-308's larger forging.
What does it weigh and what are the dimensions?
The unfinished lower weighs 10.2 ounces with overall dimensions of 7.6" long × 2.2" wide × 1.5" tall at the pistol grip mount. After complete machining, weight reduces to approximately 6.7 ounces. Critical dimension is the 1.18" internal width of the magazine well, which requires precise finishing to achieve proper magazine fit.
Who is this NOT for?
This product is unsuitable for builders without access to a drill press or milling machine capable of removing polymer material. It requires specific jig alignment techniques detailed in Best Drill Bits for Polymer Frame Modifications. First-time builders should consider complete kits rather than individual components.
What's in the box?
The package contains one RL556v3 80% lower receiver molded from reinforced nylon polymer. No jig, drill bits, or tooling is included—this is a bare receiver only. Total shipping weight is 1.1 pounds including packaging materials and requires approximately 3-5 business days for delivery.
Is the RL556v3 worth it at $85?
At $85, the RL556v3 offers competitive pricing for builders who already possess compatible jigs and tooling. The polymer construction provides a 35% cost savings over equivalent aluminum 80% lowers. For builders needing complete kits, the Polymer80 80% Lower Receiver and Jig Kit provides better value despite higher initial cost.
Key attributes
| Caliber | 5.56 NATO / .223 Remington |
| Material | Reinforced Nylon Polymer |
| Weight | 10.2 oz (unfinished) |
| Finish | Black Polymer |
| Status | 80% Unfinished |
| Platform | AR-15 |
| Magazine Compatibility | STANAG Pattern |
| Buffer System | Mil-Spec Carbine |
Materials
- polymer
Frequently asked questions
- Is it compatible with standard AR-15 parts?
- Yes, the RL556v3 accepts mil-spec AR-15 components including trigger groups, pistol grips, and buffer tubes. Magazine compatibility requires precise finishing of the magazine well to achieve proper dimensions. Test fit with a USGI magazine after machining.
- Does it fit all AR-15 uppers?
- It interfaces with standard AR-15 upper receivers when properly machined. Critical tolerance is the takedown pin alignment—allow 0.005" clearance for thermal expansion. Problematic fits typically involve billet uppers rather than forged receivers.
- How long does shipping take?
- Standard shipping takes 3-5 business days within the continental United States. Expedited 2-day shipping is available for an additional $18.50. All shipments require adult signature confirmation upon delivery.
- Can I return it if I mess up the machining?
- No, ModFuze does not accept returns on 80% lowers once machining has begun. The product is considered altered and cannot be resold. Consult the machining guide thoroughly before starting work to avoid errors.
- Does this work with drop-in triggers?
- Most cassette-style drop-in triggers require minimal fitting in polymer lowers. The RL556v3's trigger pocket dimensions are standard, but polymer flex may require shimming. I recommend traditional mil-spec triggers for first builds.
- What jig do I need?
- The RL556v3 requires a Generation 3 polymer AR-15 jig with 5/16" drill bushings. ModFuze does not currently manufacture compatible jigs—third-party options from 80% Arms or 5D Tactical work with minor modification to clamping pressure.