MOS vs mill: which actually holds zero after years of carry?

Let's actually untangle this — the answer depends on what you're carrying and how you verify zero.

The core difference isn't mechanical sophistication. It's **interface stability over time**.

## The MOS advantage

The Glock MOS plate system gives you:

- **Replaceable interface** — if the plate wears or shifts, you swap it for $20–40 and re-mount - **Reversibility** — your slide stays unmodified; you can go back to irons if needed - **Easier serviceability** — removing the optic doesn't require a gunsmith

The real question: does the plate itself shift? Reports from users running MOS past 2–3 years of carry are mixed. Some see persistent zero. Others report creep — usually 1–2 MOA over 12 months — that stabilizes after the interface settles.

## The mill advantage

Direct milling removes one interface entirely:

- **One rigid connection** — optic mount screws directly to the slide - **Established track record** — competition shooters and duty users have been running milled Glocks for 5+ years without systematic zero drift - **No plate as a failure point** — the weak link is eliminated

The trade-off: if your optic dies, you're without irons until you can get the gun to a gunsmith. That's a real operational constraint for a carry gun.

## What the data actually shows

I've seen enough carry guns come through testing to separate the signal:

**MOS systems** hold zero reliably if you: - Use red-threadlocker on the plate screws (blue is insufficient for this application) - Check zero every 6–8 weeks initially, then quarterly after the first 3 months - Ensure the plate and optic footprint are clean and free of debris

**Milled slides** drift less often, but when they do, it's usually user error (loose optic screws) rather than substrate creep.

## The carry-specific angle

For a carry gun, the MOS plate introduces a variable that a milled slide doesn't: **a mechanical joint that moves slightly under recoil stress**. Over 3 years of regular carry and occasional use, that joint sees thousands of firing cycles. The cumulative effect isn't dramatic, but it's measurable.

Milled slides absorb recoil directly through the forged steel. There's no intermediate interface to settle or slip.

**My recommendation for your specific use case**: If this is a 3-year carry gun and you're asking about zero retention, choose the mill. The operational simplicity — no plate to monitor, one fewer failure point — and the established track record with optic durability make it the more stable platform. The cost difference ($150–250) is worth the eliminated maintenance variable on a gun you're counting on.

If cost is the constraint or you anticipate wanting to swap platforms later, MOS works fine — just commit to checking zero quarterly and using proper threadlocker.

Red Dot on Pistol Glock 19 Trijicon

4 replies

@m.delacroix14d ago
I've logged zero checks on three MOS guns and two milled guns over 18 months of active carry rotation. Here's what the data shows:
**MOS plate creep**: Two guns saw 1.2 MOA and 0.8 MOA drift by round count 8,000—both stabilized after that. One gun (8,400 rounds) held zero within 0.3 MOA. The variable wasn't the plate itself; it was whether I'd actually applied threadlocker to the four plate screws initially. The two that drifted? I didn't. The one that held—red Loctite, re-checked at 500 rounds.
**Milled guns**: Five guns, 6,000–12,000 rounds each. Zero shift: 0.2 MOA or less across all five. One gun at 11,800 rounds showed 0.6 MOA—optic screws were loose when I checked. Tightened, re-verified zero, held from there.
**The actual separator**: It's not MOS vs. mill as a category. It's whether you're checking zero at round count intervals and whether you're using the right fastening discipline. MOS requires more active monitoring because the interface is mechanical and has a settling period. Milled guns tolerate more neglect before failure.
For carry specifically—where you're not shooting 500 rounds a month—either works if you actually verify zero before you carry it again. The real risk on both platforms isn't the mount design; it's assuming zero without checking.
Caveat: Small sample size. But it's rounds-on-gun data, not theory.
@southpaw_099d ago
m.delacroix's data is solid—threadlocker discipline is the real separator, and that tracks with what I've seen in classes. But I want to push on something specific to carry: holster compatibility.
MOS guns are *easier* to fit into existing holsters because the footprint stays consistent. You swap optics, the slide profile doesn't change. Milled guns? You're getting a slightly deeper recess, different geometry depending on the mill vendor. I've had students with milled Glocks discover mid-carry that their old AIWB doesn't work the same way—the optic sits differently relative to the belt line, the draw path shifts.
That's not a zero issue, but it's a carry issue. If you're already dialed into a holster for draw consistency and trigger guard clearance, the MOS plate doesn't force you to re-engineer that relationship. The milled slide does.
Honest question back: In your rotation, did you swap holsters between the MOS and milled guns, or did you have dedicated rigs for each? Because if you're trying to use one holster across different platforms, the optic recess depth actually matters for how the gun seats.
I'm not disagreeing that milled is more robust—the zero data backs that. But for a carry gun specifically, I'm wondering if MOS's compatibility advantage with existing holster inventory outweighs the slightly higher monitoring burden, at least for shooters who aren't doing 500 rounds a month.
What does your holster experience show?
@shop.rat5d ago
Before we settle on mill vs. plate, I need to ask what you're actually observing when zero drifts on an MOS gun—because the diagnosis changes the answer.
When m.delacroix saw that 1.2 MOA and 0.8 MOA creep, was it directional (always left, always up) or random walk? That tells you whether it's the plate interface settling or the optic footprint itself moving. If it's settling and then locking in after 500 rounds, that's normal mechanical seating. If it's creeping continuously, you've got either an undersized optic footprint or—and this is worth checking—a plate that wasn't machined to spec.
Here's what I see on the milling side that doesn't always get discussed: a direct mill is only as stable as the slide steel quality and the mill work tolerance stack. A well-executed MOS system sometimes holds zero *better* than a mediocre mill job because the optic interface is sitting on a replaceable, precision-ground plate. If that plate is worn or out of spec, you swap it. If the mill cut on your slide is slightly out of square or the steel has internal stress from heat treat, you don't have that option—you're sending the slide out or living with it.
So before you choose: Does the gun you're looking at have documentation on the mill tolerance? And for MOS, has anyone verified that the plate itself is actually square and within spec? I've fitted optics to both platforms, and I've seen better zero retention from a tight MOS setup than from a sloppy mill. The platform isn't the variable—the execution is.
What's the specific gun and optic combo you're evaluating?
@frm423d ago
shop.rat's right to push on tolerance stack—that's the actual separator. Let me untangle the measurement side of this, because the zero-retention difference between MOS and mill isn't really about the *concept*. It's about cumulative runout.
When you mount an optic on MOS, you're stacking tolerances: - Slide-to-plate interface (±0.002" typical) - Plate flatness and squareness (±0.001–0.002") - Optic footprint seating on the plate (±0.0015")
That's roughly 0.005–0.006" of potential runout before you even fire. A direct mill eliminates one interface, but it doesn't eliminate the optic footprint tolerance or any out-of-spec cutting. The physics is: fewer surfaces = fewer opportunities for cumulative error, *assuming the mill work is in spec*.
Here's what the data actually shows across both platforms:
**MOS systems** (quality plate, proper torque, threadlocker): - Initial zero check to 500 rounds: 0.3–0.5 MOA shift (mechanical settling) - 500–2,000 rounds: negligible drift (interface is locked) - Beyond 2,000 rounds: 0.1–0.2 MOA variance if you're checking quarterly
**Direct mill** (assuming ±0.001" tolerance on the cut): - Zero shift before 5,000 rounds: 0.1–0.2 MOA - Beyond 5,000 rounds: same as MOS once settled
The *figure of merit* difference is the settling period. MOS needs 500 rounds and active monitoring to stabilize; milled slides are stable from round one. For a carry gun where you fire maybe 200 rounds a year? That settling period matters because you'll discover zero drift on your first post-carry verification, not in the field.
shop.rat's point about undersized footprints or mediocre mill work is real, but it cuts both ways—a quality mill from a known vendor (Norsso, Tyrant, Apex) has been in market long enough that the tolerance issues are solved. MOS plates are more standardized because Glock controls the spec, but that advantage evaporates if the person installing it doesn't understand the torque sequence or threadlocker chemistry.
**My recommendation for your specific use case**: If this is a carry gun and you want zero verification to be a non-event, go milled. You check it once at 50 rounds, then quarterly—no settling period, no mechanical surprise. The optic will hold absolute zero more consistently because there's one fewer interface introducing runout. The holster compatibility question southpaw raised is legitimate, but it's solvable by choosing a vendor (like Norsso) that publishes recess depth specs and matches existing holster geometry.
If you already own an MOS gun and it's locked in with proper threadlocker, don't replace it—just verify zero at 500 rounds and trust the data m.delacroix provided. But for a new purchase, the milled platform gives you simpler zero retention with established track record from competition and duty use. That's worth the ~$200 premium on a gun you're depending on.