Retro Arms CNC-machined M4 ULTRA hop-up chamber

2025-10-29T11:10:11+02:00

Price: 99.90

RetroArms ULTRA is designed for players who want complete control and tunability in their hop-up system. ULTRA is based on RetroArms’ proven design, but introduces major improvements: an adjustable nozzle axis, a fine-adjustable feed tube, and a redesigned inner barrel locking mechanism.

The chamber is CNC-machined from aircraft-grade aluminum and anodized for wear and corrosion resistance. Precise machining and a rigid structure ensure excellent accuracy and air seal, even in tuned or long-range builds.

The key difference of the ULTRA compared to previous versions is the adjustable nozzle axis, which allows precise fine adjustment of the hop-up chamber’s longitudinal position using the included 0.2 mm shims. This enables a perfect fit with different gearboxes, nozzles, and magazines. Note that this adjustment can affect the optimal nozzle length; in some cases, a different-length nozzle may be required.

The inner barrel lock has been completely redesigned: the solution uses a POM plastic C-clip and a union nut tightened over it. This combination centers the inner barrel and practically eliminates both longitudinal and rotational movement compared to a traditional C-clip-only attachment, where the inner barrel can both shift and rotate. The union nut’s outer diameter is up to 12.95 mm, so ensure compatibility with your outer barrel’s inner dimensions. Tighten the nut by hand only, do not use tools.

The chamber also features a modified feed tube, improving feeding reliability and compatibility with different magazines. The hop-up rubber seat is made as a flat area where the bucking is installed. The included filler rings compensate for the missing chamber material, and especially with traditional buckings the filler rings should be used. If needed, the filler rings can be adjusted for a better fit by sanding or shaping them, allowing the chamber to suit different bucking profiles.

Features

CNC-machined from aircraft-grade aluminum, anodized finish.

Adjustable nozzle axis, adjustable in 0.2 mm increments with the included shims.

Adjustable feed tube improves feeding reliability and compatibility.

Improved inner barrel lock: POM C-clip + union nut, eliminating inner barrel movement and rotation.

Flattened hop-up bucking seat and filler rings for different bucking types (e.g., ROBUST).

R-hop compatible design.

Weight approx. 16 g.

Who this is for

RetroArms ULTRA is designed for enthusiasts and builders who want maximum accuracy and tunability. It is especially suitable for:

DMR and precision builds where inner barrel movement must be fully eliminated.

Tuned guns where you want to perfect the compatibility between the hop-up and the nozzle.

Installation and tuning tips

Before installation, check that the union nut’s outer diameter (max 12.95 mm) fits inside your outer barrel.

Adjust the nozzle axis to the correct position using the included 0.2 mm shims. Start from the middle position and fine-tune based on feeding and air seal.

Tighten the union nut by hand; avoid over-tightening.

Ensure the nozzle moves freely in the gearbox and that the contact between the nozzle and the front edge of the chamber is airtight.

When using R-hop or a ROBUST bucking, adjust the feed tube and inner barrel position step by step and test-fire to confirm proper feeding and accuracy.

Technical specifications

Material: aircraft-grade aluminum (CNC-machined, anodized)

Weight: approx. 16 g

Compatibility: AR-15/M4-type AEG rifles

Inner barrel: compatible with barrels with 8.4–8.5 mm outer diameter

Nozzle adjustment capability: via the nozzle axis, enabling optimization for different nozzle lengths

Type: ULTRA – adjustable tuning component, R-hop compatible