Enclosed vs Open Red Dot Sight: Which to Choose

By Accufire Editorial Team · June 7, 2026

You're outfitting a pistol for competition and the mounting plate question hits you: do you order the Holosun EPS — fully enclosed, sealed from the elements — or run an open reflex like the Accufire PCO on your RMR-cut slide? The choice is more nuanced than marketing language suggests. Open reflex sights are lighter, faster to acquire at close range, and perfectly suited for most pistol and carbine applications; enclosed designs add a sealed window that trades weight and bulk for added debris and weather protection that most shooters rarely need.

How open reflex sights work

An open reflex sight projects an LED-generated dot onto a curved, partially reflective lens tilted toward the shooter. The dot bounces back to the eye while the lens remains transparent to the target. Because the lens has only one exposed surface — the objective face — it gathers maximum ambient light and gives you the widest possible field of view for a given housing size. The Accufire PCO Reflex uses this principle with a 28×17.5 mm window, a meaningful amount of glass for a pistol-mounted optic. The trade-off is that the lens is open to the environment: dust, rain, and debris can contact the glass surface. In practice, a quality anti-reflective coating and lens cap habit manage that exposure for the vast majority of shooting applications. To understand the underlying optics in detail, the red dot sights complete guide covers reflex fundamentals from the LED emitter through parallax behavior.

How enclosed red dots differ

Enclosed designs — Aimpoint's ACRO line and Holosun's EPS series are the most widely recognized — place the emitter, lens, and sometimes a front glass pane inside a fully sealed housing. No part of the optical path is exposed to the environment. The result is genuine debris exclusion: mud, sand, and rain cannot reach the lens or the LED assembly. That protection comes at measurable cost. Enclosed housings are taller, heavier, and present a more complex occlusion profile than an open window. Window dimensions are often narrower than comparable-priced open reflex sights because the sealed tube constrains the glass area. For shooters working in extreme submersion, fine-particulate environments, or those who holster and draw thousands of times in varied weather, the sealed design earns its weight penalty. For a range shooter or even a duty carrier in most climates, a well-sealed open reflex optic — Accufire's QSO carries an IPX-7 rating — handles water exposure without requiring a fully enclosed housing.

Side-by-side comparison

Footprint and mounting: a practical constraint

Beyond sealed vs. open, the more immediate decision for most pistol shooters is footprint. Enclosed red dots typically require a dedicated slide cut or adapter plate that is specific to that design — an ACRO or EPS cut is not interchangeable with an RMR or RMSc cut without a new milling job or different plate. The Accufire PCO Reflex sits on the Trijicon RMR footprint, which is the most widely milled pattern in the industry for full-size pistols. The PCO Mini uses the RMSc footprint for slim and sub-compact slides. If your slide is already cut or your gun ships with a plate system, confirming footprint compatibility before ordering any optic — enclosed or open — saves a return trip. The best red dot sight for a pistol article walks through footprint matching in detail for common pistol families.

Honest tradeoffs and who should choose each

Open reflex sights carry one real limitation worth naming directly: the exposed objective lens. In a holster-heavy duty environment with frequent draws across varied terrain, fine grit can reach the lens glass. This is manageable — a quick lens wipe before a stage or a draw solves it — but it is a real maintenance variable that an enclosed design eliminates. If your application genuinely demands debris exclusion (extended field carry in dusty or sandy conditions, deep concealment with frequent draws in unpredictable weather), an enclosed optic is worth the weight and cost premium.

For most shooters — competition, home defense, range training, and general carry — an open reflex sight with solid IPX-rated sealing and quality glass delivers all the performance they need at lower weight and price. The PCO Reflex's 28×17.5 mm window is notably generous for a pistol optic in its class, and the 3 MOA dot keeps precision work achievable at distance without obscuring close targets. The QSO-S and PCO-S add solar backup for shooters who want power redundancy without moving to a heavier sealed housing. For a broader look at how dot size affects real-world aiming, the 2 MOA vs 6 MOA dot size guide covers the tradeoffs with concrete examples.

Frequently asked questions

What is the main difference between an enclosed and open red dot sight?

An open reflex sight has an exposed lens surface that the dot reflects off of, while an enclosed red dot seals the emitter and lens inside a fully closed housing. Enclosed designs exclude debris from the lens at the cost of added weight, a taller profile, and typically a smaller window. Open reflex sights are lighter and offer larger windows but require occasional lens cleaning in dusty environments.

Are open red dot sights waterproof?

Many open reflex sights carry water resistance ratings that cover rain and brief submersion. The lens surface remains exposed, but the internal electronics and emitter are sealed against water ingress. Accufire's QSO carries an IPX-7 rating, so rain, splashing, and brief submersion are handled without requiring a fully enclosed housing.

Does Accufire make an enclosed red dot sight?

No. All Accufire red dot and reflex sights — the QSO, QSO-S, PCO Reflex, PCO Mini, and PCO-S — are open reflex designs. If you specifically need a sealed enclosed optic, brands like Aimpoint (ACRO) and Holosun (EPS) offer those. Accufire's open reflex sights are suitable for the vast majority of pistol and carbine applications.

Which red dot design is better for a competition pistol?

Most competition shooters run open reflex sights because they are lighter, have larger windows, and cost less — factors that matter on a pistol you are reloading and drawing hundreds of times. Enclosed designs are rarely required in competition environments where weather and debris are controlled. An open reflex like the PCO Reflex on an RMR-cut slide is a practical competition choice.

Will an enclosed red dot footprint fit where an open reflex sits?

Not necessarily. Enclosed optics like the Aimpoint ACRO and Holosun EPS use proprietary mounting cuts that differ from the widely used RMR and RMSc footprints. If your slide is already milled for an RMR pattern, an open reflex on that footprint is a direct fit. Switching to an enclosed design with a different footprint requires a new slide milling job or a compatible adapter plate — confirm the cut before purchasing any optic.

Understanding the sealed vs. open distinction helps you make a confident purchase rather than chasing a spec you may not need. For shooters building out a full optics picture — co-witness height, zeroing procedure, or adding a magnifier behind a red dot — the co-witness height guide and how to zero a red dot sight are practical next steps.