Retention Pins: Solid Pins, Roll Pins, and Spring Pins
TL;DR: Article Summary
Solid, slotted (split), and coiled pins each serve distinct roles in AR-15 assembly. Solid pins offer strength with no flex, ideal for staked retention like extractor pins. Slotted spring/roll pins are cost-effective but apply pressure unevenly and risk deformation under repeated stress. Coiled spring/roll pins distribute force uniformly, offer superior fatigue resistance, and are often preferred in critical applications like ejectors — though not called for in Mil-Spec. Choose the right pin type based on load, shear requirements, and long-term durability.
Introduction
Retention pins play a critical role in the mechanical integrity of the AR-15 and other firearms. From securing the extractor in the bolt to holding the trigger guard in place, these small components are responsible for maintaining alignment, resisting vibration, and withstanding repeated cycling forces. While often referred to generically as “roll pins,” retention pins include multiple types — each with distinct construction, performance characteristics, and intended use cases.
This article defines the three primary categories of retention pins — coiled spring pins, slotted spring pins, and solid pins — and explains why each is chosen for specific functions in the AR platform.
Types of Retention Pins
Retention pins are a foundational element in firearm and mechanical design. In the AR-15 platform, they are used to secure everything from bolt internals to trigger guards. While they may appear similar, retention pins fall into three primary categories — each with distinct material properties, retention methods, and use cases.
🔵 Solid Pins
Solid pins are cylindrical rods made from hardened steel or other alloys, with no flex or compression. These pins are press-fit or retained mechanically through staking, shoulders, or tension. Solid pins offer superior shear strength and maintain strict alignment, making them ideal for pivoting or high-load parts that must not move under pressure.
- Construction: One-piece, non-compressible steel or stainless
- Fit: Dependent on precise hole tolerances; not self-retaining
- Common Use: Extractor pin, trigger/hammer pins, pinned FSB
- Advantages: High strength, does not deform
- Limitations: No expansion
©️ Slotted Spring Pins (Split Roll Pins)
Slotted spring pins are formed from a single piece of steel rolled into a “C” shape with a longitudinal gap. These pins compress along the split during installation, creating friction against the hole walls.
- Construction: Single wrap of spring steel with visible gap
- Fit: Applies pressure at two opposing contact points (90°/270° from the slot)
- Common Use: Gas tube roll pin, forward assist roll pin, trigger guard roll pin, bolt catch roll pin, M4 carbine stock latch roll pin
- Advantages: Low cost, easy to install
- Limitations: Uneven force distribution, stress concentrated opposite the slot, can flatten or drift under vibration
🌀 Coiled Spring Pins (Spiral Roll Pins)
Coiled spring pins are made from flat strip steel wound into a multi-layered spiral. When compressed into a hole, the coils apply uniform outward radial force across the pin’s full diameter. Their multi-layered structure makes them highly resistant to fatigue and ideal for cyclic or spring-loaded applications.
- Construction: Multi-turn spiral of spring steel
- Fit: Expands uniformly along the circumference
- Common Use: Upgraded pins and premium aftermarket components, pinned gas block
- Advantages: Superior fatigue resistance, consistent pressure, shock-absorbing
- Limitations: Higher cost, not Mil-Spec standard
Materials and Finishes
Materials
- Spring Steel (Tempered Carbon Steel): The primary material for coiled and slotted spring pins. These pins are made from high-carbon steel grades (such as 1060–1095) that are heat-treated to a spring temper, giving them high elasticity and fatigue resistance. Commonly labeled as “carbon steel” in military documents, but functionally spring steel.
- Stainless Steel (302, 420, 17-7, 18-8): Used where corrosion resistance or non-magnetic properties are important. Slightly lower in shear strength than hardened carbon steel, but suitable for marine or outdoor environments.
- Alloy Steel (Hardened): Typical for solid pins. Provides high shear strength and excellent dimensional stability under load.
Finishes
- Manganese Phosphate: Standard Mil-Spec finish for carbon steel roll pins (e.g., MS16562). Provides corrosion resistance, oil retention, and a matte black or gray appearance. Commonly used for bolt catch, trigger guard, and ejector pins.
- Passivated (Stainless): Stainless steel pins are typically supplied passivated per ASTM A967 or MIL-DTL-121. This removes surface iron contamination and enhances corrosion resistance without affecting dimensions.
Function and Purpose: Spring Pins vs. Solid Pins
Spring pins (also called roll pins) and solid pins are both used to retain components in assemblies, but their application depends on how the pin is expected to behave under stress, during installation, and throughout the part’s service life.
In the AR-15 platform, a great example of this distinction is the difference between the ejector pin (spring pin) and the extractor pin (solid pin). Below we break down the rationale behind each.
Why Use a Solid Pin?
Solid pins offer maximum rigidity and are ideal for fixed pivot points or components that cannot tolerate deflection or compression over time.
| Design Requirement | Benefit of Solid Pin |
|---|---|
| Precise alignment | No flex or movement under load |
| Fixed positioning | Won’t compress or shift over time |
| High shear strength | Full cross-section resists mechanical load |
| Permanent or semi-permanent install | Sometimes staked or press-fit |
Why Use a Spring Pin?
Spring pins are self-locking and designed to flex under compression, making them ideal for retaining parts subjected to vibration, cyclic loads, or spring tension.
| Design Requirement | Benefit of Spring Pin |
|---|---|
| Retains with friction | Expands in the hole, holds via radial preload |
| Accommodates minor hole variations | Flexible under compression |
| Installs blind without deformation | No threads or staking needed |
| Withstands shock and vibration | Coiled structure absorbs impact and resists fatigue |
| Serviceability | Can be removed/reinstalled without damage |
- Radial expansion keeps the pin retained
- Easy serviceability with basic tools and no deformation
- Tolerance forgiveness allows use with slightly variable hole sizes
- Lightweight and flexible under shock loading
- Ideal for spring-loaded components under axial or lateral stress
AR-15 Spring Pin / Roll Pin Sizes
Replacement spring pins — both slotted/split and coiled/spiral — can be purchased from several online retailers, including McMaster-Carr, Grainger, and MSC Direct.
| AR-15 Roll Pin Size Reference | |||
|---|---|---|---|
| Component | Diameter | Length | Part Number |
| ComponentBolt Catch | Diameter3/32" | Length1/2" | Part NumberMS16562-119 |
| ComponentTrigger Guard | Diameter1/8" | Length5/8" | Part NumberMS16562-129 |
| ComponentCharging Handle Latch | Diameter1/16" | Length17/64" | Part Number8448521-2 |
| ComponentForward Assist | Diameter3/32" | Length5/8" | Part NumberMS16562-121 |
| ComponentForward Assist Pawl | Diameter1/16" | Length17/64" | Part Number8448521-2 |
| ComponentGas Tube | Diameter5/64" | Length5/16" | Part NumberMS16562-106 |
| ComponentEjector | Diameter1/16" | Length3/8" | Part NumberMS16562-98 |
| ComponentRear Sight Elevation Post | Diameter3/32" | Length5/8" | Part NumberMS16562-121 |
| ComponentRear Sight Windage Knob | Diameter1/16" | Length11/16" | Part NumberMS16562-103 |
| ComponentM4 Front Sling Swivel | Diameter1/8" | Length11/16" | Part NumberMS39086-93 |
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Frequently Asked Questions
What’s the difference between coiled and split roll pins for the AR-15?
Slotted spring pins (also called split pins) are made from a single piece of rolled steel with a longitudinal slit. They exert pressure at two opposing points inside the hole and are widely used in Mil-Spec AR-15s due to their simplicity and low cost.
Coiled spring pins are rolled in a spiral with multiple layers, distributing radial force more evenly around the bore and offering better fatigue resistance, making them ideal for critical components like the ejector, bolt catch, or trigger guard.
While military Technical Manuals (TMs) and NSNs typically specify slotted pins (e.g., MS16562 series), seasoned builders prefer coiled pins in critical applications due to their superior strength and easier installation.
Both types can be used effectively when properly sized and installed, but they differ in how they handle stress and distribute load within the host component.
What size roll pin is used for the AR-15 bolt catch?
The AR-15 bolt catch uses a 3/32″ × 1/2″ slotted spring pin, matching Mil-Spec part MS16562‑119. This size provides a secure friction fit while minimizing the risk of receiver ear damage during installation.
We recommend upgrading this pin to a coiled spring pin.
What is the correct spring pin size for the AR-15 trigger guard?
The trigger guard is retained by a 1/8″ × 5/8″ slotted spring pin, part MS16562‑129. This pin must be installed carefully to avoid cracking the lower receiver trigger guard ears — support with a proper block or install a winter trigger guard with threaded fasteners.
We recommend upgrading this pin to a coiled spring pin.
What size roll pin retains the charging handle latch?
The charging handle latch uses a 1/16″ × 17/64″ slotted spring pin, specified by part number 8448521‑2. This same pin is also used in the forward assist pawl. Due to its small size, use a roll pin starter punch during install.
What is the roll pin size for the forward assist body?
The forward assist body is secured with a 3/32″ × 5/8″ slotted spring pin, part MS16562‑121. This pin is installed through the upper receiver while depressing the forward assist slightly.
We recommend upgrading this pin to a coiled spring pin.
Some upper receivers (e.g., Aero Precision M4E1) use a threaded pin to secure the forward assist.
What pin is used for the forward assist pawl?
The forward assist pawl uses the same 1/16″ × 17/64″ slotted spring pin as the charging handle latch — 8448521‑2. It retains the pawl inside the assist housing and is often installed pre-assembled by OEMs.
What is the correct gas tube roll pin size for an AR-15?
The AR-15 gas tube is retained by a 5/64″ × 5/16″ slotted spring pin, part MS16562‑106. This pin goes through the gas block or front sight base and the gas tube collar, preventing movement during firing.
We recommend upgrading this pin to a coiled spring pin.
What pin is used for the bolt ejector?
The ejector in the bolt is retained by a 1/16″ × 3/8″ slotted spring pin, part MS16562‑98. Because of the strong spring pressure behind the ejector, use a proper bolt fixture to compress the spring before installing or removing the pin.
We recommend upgrading this pin to a coiled spring pin.
What pin sizes are used in the AR-15 rear sight assembly?
The A2-style rear sight uses two different slotted spring pins:
- Elevation post: 3/32″ × 5/8″ — MS16562‑121
- Windage knob: 1/16″ × 11/16″ — MS16562‑103
These pins retain the detent and adjustment components inside the sight base.
What roll pin is used for the M4 front sling swivel?
The M4 uses a clamp-on sling swivel mounted below the barrel, independent of the front sight base. This swivel is secured with two 1/8″ × 11/16″ slotted spring pins, part number MS39086‑93.
This setup differs from A2-style sling loop, which is mounted to the front sight block with a rivet.
Should I use stainless or phosphate roll pins in my AR build?
Mil-Spec AR-15 roll pins are typically made from carbon spring steel with a phosphate finish for corrosion resistance and oil retention. Passivated 420 stainless steel roll pins are a common upgrade for builders prioritizing long-term corrosion protection — especially in humid or maritime environments. Both are dimensionally interchangeable if matched properly.
Final Thoughts: The Smallest Part has a Huge Impact
Roll pins may be small, but they play a critical role in the reliability and service life of your AR-15. Whether you’re securing a bolt catch, trigger guard, or gas tube, using the correct diameter, length, and pin type ensures proper function without damaging surrounding components.
While Mil-Spec builds typically use slotted spring pins for simplicity and standardization, coiled spring pins offer performance advantages in critical areas, especially for builders prioritizing longevity and ease of installation. Solid pins have their place in high stress applications, such as extractor pivots or trigger/disconnector pins.
If you’re maintaining or building a duty-grade rifle, clone-correct upper, or competition platform, take the time to verify your pin specs, select appropriate materials and finishes, and install with the correct tools. These details help prevent costly wear, sheared pins, and frustrating malfunctions down the line.
As with any small part, attention to detail makes the difference between a build that lasts and one that lets you down. Choose wisely.
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