OAL Gauge Guide: How to Measure Distance to the Lands (2026)

Key takeaway: An OAL gauge measures one thing — the cartridge length at which your bullet just touches the rifling in your chamber. It is a push rod and a locking collar, nothing more. It contains no caliber-specific parts, it does not include calipers, and it will not measure anything until you thread on a modified case made for your cartridge. Straight and curved versions are not "short and long" — they differ by whether your action lets you reach the breech in a straight line. Once you have the touch length, convert it to CBTO with a comparator, then back off your jump and check it against magazine length before you load a single round.

What an OAL gauge actually is — and what it is not

The tool is deliberately simple. A thin steel rod slides inside an aluminum body. A knurled thumb screw locks the rod wherever you stop it. On the front end, a threaded socket accepts a modified case. That is the entire mechanism.

What it does is equally narrow: it lets you seat a bullet forward until it contacts the rifling, then hold that position while you withdraw the tool and measure it. The number you get is a starting reference for seating depth work, not a finished load recipe.

Almost every problem people have with an OAL gauge comes from expecting it to be a kit. It is not. Here is the honest split between what ships in the box and what you must already own:

Why the modified case is not optional

A modified case is a fired cartridge case that has been prepared two ways. The case head is drilled and threaded so it screws onto the front of the gauge, and the case neck is opened up roughly 0.002" oversize so a bullet slides freely instead of gripping by neck tension. Hornady, whose Lock-N-Load system defined the category, states the requirement plainly on its modified cases page: the caliber-specific cases are "required for use (sold separately)" and are offered for everything from .17 Remington to .500 Nitro Express.

Two consequences follow, and both are the source of most confusion:

• The modified case is what makes the tool caliber-specific. The gauge body itself does not care whether you shoot 6.5 Creedmoor or .30-06. Buy a gauge without a case for your cartridge and you own a rod that measures nothing.
• The case is a chamber reference, not a bullet holder. When you push the assembly into the chamber, the case shoulder seats against the chamber shoulder. Every measurement you take is referenced from that shoulder contact — which is exactly why a case with the wrong headstamp geometry gives wrong numbers.

Both AltitudeCraft gauges state this dependency on the product page, but it sits in a compatibility bullet rather than at the top, and that placement has cost buyers money. We know because we counted (see the returns data below).

Straight or curved? Pick by action type, not by length

The single most common false assumption is that the curved gauge is simply "longer" or "bigger." It is not. The geometry exists because some actions do not give you a straight-line path to the chamber. Hornady frames the split the same way: straight gauges suit "bolt rifles and other types of actions where you can access the rear of the breech in a straight line," while curved gauges exist for "lever action guns and other types of actions where you can't."

If you own a bolt gun and nothing else, the straight AltitudeCraft OAL gauge is all you need. If your safe includes a lever gun or a gas gun, the curved version is the one that will physically reach the chamber.

How to measure distance to the lands, step by step

1. Clear the rifle and remove the bolt (or open the action fully). Every measurement below assumes an unloaded firearm with the bolt out of the gun.
2. Clean the chamber and throat. Carbon buildup ahead of the neck shifts your reading and is the most common cause of a number that drifts over a shooting season.
3. Thread the modified case onto the gauge and drop your chosen bullet into the case neck. Use the exact bullet you intend to load — ogive shape varies between designs and even between lots.
4. Insert the assembly through the breech until the case shoulder seats firmly against the chamber shoulder. Hold light, steady rearward pressure so the case stays seated for the whole measurement.
5. Push the rod forward gently until the bullet stops against the rifling. Stop at first contact — do not drive it in.
6. Lock the thumb screw, then withdraw the whole assembly. If the bullet stays in the throat, tap it back out with the rod and start over; a stuck bullet means you pushed too hard.
7. Measure. Read base-to-tip with calipers for a rough COAL, then measure base-to-ogive with a comparator for the number you will actually use.
8. Repeat and average. One reading is not data.

Why your readings jump around (and how to fix it)

This is the complaint that gets an OAL gauge labeled "inaccurate," and it is almost never the tool. The bullet does not hit a hard wall when it reaches the rifling; it starts engraving. Push harder, get a longer number. As one long-running Shooters' Forum thread on OAL gauge consistency puts it: "The force pushing the bullet into the lands needs to be consistent. The measurement you get depends on how hard you jam the bullet into the rifling with the tool."

Four habits from that discussion turn a noisy tool into a repeatable one:

• Build a tactile reference. One approach is to push until the thin rod just begins to flex inside the tool and treat that flex as your stop — the forum comparison is "similar to the click of a torque wrench." Whatever cue you choose, use the same one every time.
• Take a sample, not a measurement. Four or five readings are enough when spread is within 0.001"; go to ten when the numbers wander.
• Use a fresh bullet for each reading. A bullet that has already been engraved by the lands reads differently on the second try.
• Verify your bullets first. Sort a handful by base-to-ogive length before you start, so you are not measuring bullet variation and calling it chamber variation. Our guide to inconsistent CBTO readings covers the caliper-side half of this problem.

Turning a touch length into a load you can actually chamber

A number measured to the lands is not yet a seating depth. Three conversions stand between them.

First, stop trusting COAL. Bullet tips are the least consistent part of a bullet, so cartridge overall length measured to the tip carries that scatter. Berger Bullets' technical write-up on measuring COAL and CBTO is the standard reference here: work in cartridge base to ogive instead. Our step-by-step CBTO measurement guide walks through the caliper setup, and the OAL vs CBTO FAQ explains why the two numbers never agree.

Second, choose your jump. From the touch reading, back the bullet off the lands. Common starting points run roughly 0.010"–0.030" of jump, then you test in small increments. The seating depth tuning guide covers how to run that ladder without burning a barrel.

Third, check the magazine. SAAMI publishes a maximum cartridge overall length for each chambering — 2.800" for .308 Winchester, for example — and Berger is explicit that the SAAMI figure "must be seen as a guideline only," existing mainly so ammunition feeds from a short-action magazine. Your chamber will often accept a longer round than the magazine will. If your ideal jump produces a cartridge that will not fit the box, you are loading single-shot whether you meant to or not.

When an OAL gauge is the wrong tool

Two honest limits, both worth knowing before you buy.

Tight match chambers. Modified cases are made to standard chamber dimensions. In a custom or tight match chamber, the case may not seat properly against the shoulder, and forum consensus is blunt about the result: the tool "works best with relatively slack factory chambers" and "may not work well with tight match chambers as the modified cases don't fit such chambers well and often give false readings."

You want an absolute, not a comparison. The OAL gauge references from the case shoulder. The stripped-bolt method references from the bolt face. Ultimate Reloader's rundown of three ways to find the lands lays out the alternatives, and shooters who have run both back to back report agreement within about 0.002". That is close, but the two numbers are not interchangeable — pick one method and stay with it.

What 60 returns taught us about this tool

We looked at every return recorded against our straight OAL gauge across its full sales history — 60 in total. Twenty-two of them, 37%, had nothing to do with product quality. They were information failures that happened before the box was opened:

• "This item requires additional parts that are not included." — the modified case.
• "I thought the product came with dummy rounds. It does not, so it is no longer needed."
• "Won't work with th caliber I am trying to measure" — again, the modified case.
• "From looking at the photo, it shows a depth gauge and calipers. This only has one element."
• "Way hopping it would be longer but is exactly the same as the one I have" — the straight-versus-curved misunderstanding.

We are publishing those quotes rather than hiding them because every one of them is answered above. If you have read this far, you already know the two purchases that have to happen together — the gauge and a modified case for your cartridge — and you know which shape your action needs.

Frequently Asked Questions

Does an OAL gauge come with a modified case?

No. Modified cases are caliber-specific and are sold separately, for every gauge on the market including ours. Order the case for your cartridge at the same time as the gauge, or the tool cannot be used.

Can I make my own modified case?

Some reloaders do, by drilling and tapping a fired case head and opening the neck slightly. It works, but the thread must match your gauge and the neck must let the bullet slide with no grip. A purpose-made case removes both variables, which matters when you are chasing thousandths.

What is the difference between an OAL gauge and a bullet comparator?

They measure different things at different stages. The OAL gauge finds where the bullet touches the rifling in your chamber. A bullet comparator then measures cartridge base to ogive on your loaded rounds so you can reproduce that seating depth. Most precision reloaders own both; neither replaces the other.

Do I need calipers as well?

Yes. The gauge holds the bullet at the touch position; the calipers turn that position into a number. Any quality dial or digital caliper reading to 0.001" will do.

Why does my reading change every time I measure?

Because the bullet begins engraving into the rifling rather than stopping dead, so the reading tracks how hard you push. Use the same tactile cue every time, use a fresh bullet for each attempt, and average four to ten readings.

Will an OAL gauge work in a tight custom chamber?

Often not well. Modified cases are cut to standard dimensions and may not seat correctly in a tight match chamber, which produces false readings. The stripped-bolt method is the more reliable choice there.

Straight or curved — which should I buy?

Straight for bolt actions and single shots, where you can reach the chamber in a straight line with the bolt removed. Curved for lever actions and semi-autos, where the receiver blocks that path. It is a clearance decision, not a size decision.

Is this the same as a headspace comparator?

No. A headspace comparator measures the case shoulder datum to the case head so you can control shoulder bump during resizing — see our headspace comparator bushing size chart. An OAL gauge measures forward, to the rifling. Different end of the cartridge, different job.

The short version

Buy the gauge and a modified case for your cartridge together. Match the shape to your action, not to your idea of size. Clean the throat, push with a consistent cue, average your readings, convert to CBTO, then confirm the round still fits your magazine. Do those five things and an inexpensive aluminum rod will give you a seating depth reference you can rebuild years from now.