Fitment Issue & Tolerance Shift: A3 Industries HDA3 Stock on 2026 Springfield Kuna (Caliper & CAD Breakdown)

[u/TommySpeaks]

Hey everyone,

I wanted to share a detailed fitment issue and get input from other Springfield Kuna owners, particularly those with 2026 production models.

I recently purchased the A3 Industries HDA3 Hybrid Polymer Stock for my factory-standard Springfield Kuna (2026 production date). Unfortunately, the rear takedown pin cannot pass through the upper receiver when the A3 adapter is seated. The pin hits the wall of the upper receiver by roughly 1 mm, preventing assembly.

What I’ve verified so far:

• Disassembled the firearm and attempted to seat the A3 adapter directly onto the upper receiver alone — the ~1 mm downward pin hole offset persists.
• Compared the A3 interface with the factory stock: the pin hole on the A3 adapter is positioned slightly too low relative to the indexing surfaces.
• Measured critical dimensions using digital calipers:
  • Top internal recess to top of pin: 52.25 – 52.30 mm
  • Middle shelf to top of pin: 26.74 mm
  • Receiver bottom to pin hole bottom: 4.28 – 4.33 mm
  • Pin diameter: 5.65 mm

I contacted A3 support, provided photo evidence, caliper measurements, and serial number info. They tested 20 adapters on an initial US release Kuna and reported no issues. A3 provided a snippet of their CAD drawing (showing a 49.20 mm height from top shoulder to pin center), but the drawing omitted the lateral mating shelf dimensions where the adapter actually indexes.

A3’s official stance is that my rifle is the only report of this issue they have received, and they suspect either an EU-market variance or a tolerance shift by Springfield/HS Produkt in newer batches. Because they claim it's an isolated case and all current stock is machined to the same legacy drawing, they declined a replacement or CAD update.

Questions for the community:

1. Has anyone with a newer (2026) batch Kuna experienced similar binding or pin misalignment with A3 Industries or Haga Defense rear adapters?
2. Did Springfield/HS Produkt alter internal receiver tolerances/shelf heights after the initial US release?

Imgur album with caliper measurements and CAD comparison: https://imgur.com/a/LbdAp0Y

Just wanted to post this as a heads-up for anyone buying newer Kuna batches, and to see if the community has noticed any receiver tolerance shifts on recent imports.

[u/BifferUSMC]

yes, I have the HBindustries kuna stock and its a common issue that the pin needs to be tapped in with a hammer on those as well.

[u/TommySpeaks → u/BifferUSMC]

Thanks for the confirmation. Did you have to modify it, or just a brute force was enough?

[u/BifferUSMC → u/TommySpeaks]

No confirmation... it looked like it should go but just really tight. So it also requires a drift to punch the pin out to break it down.