Comparators Explained — Relative, Not Absolute

What a Comparator Does

A comparator is an insert that fits between your calipers' jaws and contacts the ogive of the bullet — the curved section between the tip and the cylindrical body. Instead of measuring to the bullet tip (which varies), the comparator measures to a consistent geometric feature of the bullet.

Comparators come in different sizes to match different bullet diameters. A .264 insert (6.5mm) contacts 6.5mm bullets at the ogive. A .308 insert contacts .30 caliber bullets. The insert diameter is nominally matched to the bullet diameter, but the exact contact point varies between manufacturer, between insert lots, and even between individual inserts of the same nominal size.

Why 'Relative, Not Absolute' Is the Critical Concept

When you measure a loaded cartridge's CBTO with a comparator, you are not getting an absolute measurement of where the bullet sits relative to the lands. You are getting a measurement of where the bullet's ogive is relative to the base of the cartridge, as contacted by your specific comparator insert on your specific calipers.

Another shooter using a different comparator insert will get a different number for the identical cartridge. Neither measurement is wrong — they're just different coordinate systems. The number itself is meaningless in isolation. What matters is the change in that number over time, measured with the same tool in the same way.

The Common Mistake That Corrupts Data

The most frequent way shooters corrupt their CBTO data is by changing tools between sessions without establishing a new baseline. This happens in several ways:

• Buying a new caliper and not recalibrating the CBTO baseline
• Using a different insert size that still fits (e.g., a .277 insert on a 6.5mm bullet)
• Borrowing a friend's comparator for one session
• Dropping and damaging an insert, then replacing it with a nominally identical one
• Switching between comparator brands without documentation

In each case, the subsequent measurements are not comparable to the previous ones. If your recorded CBTO shows a jump of .008" between sessions but you changed comparators, you cannot know how much of that is real throat erosion and how much is tool change. Your erosion trend data is invalidated.

How to Use Comparators Correctly

Using comparators for reliable, long-term data requires a few firm habits:

• Designate one comparator insert and one caliper per barrel. Label them. Store them together.
• Record both with every measurement. Borely requires this for exactly this reason.
• Establish a baseline on a new barrel immediately. Measure at round zero (or as early as possible) so you have a reference point for all future readings.
• If you must change tools — measure with both the old and new tool on the same cartridge to establish a conversion factor, then document the transition explicitly in your notes.
• Measure multiple cartridges and average them. Three to five readings gives you confidence the measurement is stable.

Comparators vs. Datum Gauges — An Important Distinction

The terms "comparator" and "datum gauge" are often used interchangeably in reloading conversations, but they describe two different tools measuring two different things. Understanding the distinction matters both for selecting the right tool and for interpreting your data correctly.

A bullet comparator contacts the ogive of a bullet or loaded cartridge to measure seating depth — how far the bullet sits from the case base. This is the tool used for CBTO measurement and tracking bullet-to-lands distance over barrel life.

A datum gauge (also called a headspace comparator or shoulder comparator) contacts the shoulder of a brass case at the datum line — a specific diameter on the case shoulder defined by the cartridge's SAAMI specification. This is the tool used to measure headspace, track case shoulder bump during sizing, and ensure consistent brass prep.

Why does this matter? A datum gauge measurement is more nearly absolute than a bullet comparator measurement. The shoulder datum is defined by a specific diameter in the SAAMI specification — if two datum gauges are correctly made to spec, their readings should be directly comparable between shooters. A bullet comparator, by contrast, contacts the ogive at a point determined by the insert geometry, which varies between manufacturers — making those readings inherently relative, not absolute.

In practice: use a datum gauge to track case headspace and shoulder bump during brass prep. Use a bullet comparator to track seating depth and CBTO. Record the specific tools used for both, and never swap them mid-series.

Two Manufacturers Worth Knowing

Most precision reloaders start with Hornady comparator inserts — inexpensive, widely available, and functional. But serious competitors frequently upgrade to purpose-built systems. Two stand out:

Short Action Customs (SAC) makes both bullet comparator inserts and datum gauges through their Modular Headspace Comparator system, machined from 17-4 stainless steel. The key advantage of the SAC design is that each headspace insert is machined to match the specific shoulder angle of the cartridge — not just the datum diameter — giving a fuller contact surface rather than a single-point contact at the datum circle. Their inserts snap into the comparator body securely without set screws, eliminating a common source of measurement inconsistency. SAC explicitly distinguishes between their Modular Headspace Comparator (for relative measurements) and their separate Datum Gauges (for measurements based on reamer prints) — a meaningful distinction for shooters working with custom chambers.

Area 419 takes a different philosophy with their ZERO Headspace and Ogive Measurement Kit. Their headspace gauges are traditional datum-style gauges cut with a reamer, in an effort to produce measurements that are more than just relative — so that two shooters using the same kit can meaningfully compare their numbers. Their ogive inserts go further: rather than a standard datum measurement, Area 419 uses custom tooling that replicates the lead area of a standard 1.5° chamber, measuring seating depth and jump relative to the chamber rather than to an arbitrary contact point on the bullet's ogive. The full kit includes all headspace gauge sizes, all bullet ogive gauges, and is designed specifically for Mitutoyo calipers. A heavy steel magnetic height stand is available separately. All components are machined from American steel on DMG Mori lathes and nitrided for durability.

Both represent a meaningful step up from Hornady aluminum inserts in repeatability and material consistency. SAC suits reloaders working multiple calibers who want a modular system; Area 419 suits those who want the most comprehensive unified kit and the ability to compare data with other shooters using the same tools.

Choosing a Comparator

From entry-level to competition-grade, the main systems:

• Hornady Comparator Inserts — the most common starting point. Aluminum, inexpensive, functional. Adequate for reloaders who work a single caliber consistently.
• Sinclair / Brownells Hex Inserts — slightly better than Hornady with a tapered hole rather than a sharp edge, more consistent contact geometry.
• Whidden Gunworks — precision-machined inserts popular with benchrest competitors. A solid mid-tier option between aluminum systems and full stainless kits.
• Short Action Customs Modular System — 17-4 stainless, cartridge-specific shoulder angle matching, modular body. Makes both bullet and headspace inserts. Best step-up option for multi-caliber reloaders.
• Area 419 ZERO Kit — the most comprehensive unified kit available. Datum-style headspace gauges plus chamber-profile ogive inserts, designed around Mitutoyo calipers with a magnetic height stand option. Best for reloaders who want near-absolute measurements.

The brand matters less than consistency. Pick one system, document it, and stay with it for the life of the barrel.