Headspace Comparator Bushing Size Chart: Which Bushing Fits Your Cartridge (2026)

If you have ever opened a reloading kit, seen six anodized rings stamped A.330, B.350, C.375, D.400, E.420 and F.188, and had no idea which one belongs on your .308 brass, you are not alone. One of our own customers put it perfectly in a five-star review: "The code numbers on the outside of gauges are meaningless to me." That is exactly the gap this headspace comparator bushing size chart closes. Below you will find the full bushing-to-cartridge mapping, the arithmetic that covers every case not on the list, the five-step shoulder bump procedure, and a blunt explanation of the one misconception that causes more frustration than any hardware fault.

What a Headspace Comparator Bushing Size Chart Actually Tells You

A bushing letter is not a caliber code — it is a hole diameter, and several unrelated cartridges legitimately share the same one. That single fact removes most of the confusion around headspace bushing selection.

Headspace, as Ultimate Reloader defines it, is "the mechanical interface whereby a cartridge stops moving forward when chambered." On a bottleneck rifle case, that interface is the shoulder cone, and the measurement runs from the case head to a reference circle on that cone — the datum. A comparator bushing is simply a hole of known diameter. Slide it down the case neck and it stops where the expanding shoulder reaches that diameter. The caliper then reads case head to datum.

Because the bushing rides on a cone, its diameter determines where on the shoulder it stops. A .375" hole rests higher up a shallow shoulder than a .400" hole does. Both are valid reference points. Neither one is "the" headspace of your chamber. This is why the same .375" bushing serves cartridges as different as .30-30 Winchester and 6.5 Creedmoor — they simply happen to have shoulders that pass through .375" at a useful spot.

The Complete Headspace Bushing Chart: A.330 Through E.420

Five bushing diameters cover essentially the entire commercial bottleneck lineup, from .17 Remington to the .375 H&H belted magnums. The table below reproduces the mapping published in the official Hornady Lock-N-Load headspace bushing instructions, which is the reference standard the industry has settled on. The lettering and diameters follow the same convention across brands, so this chart applies to any comparator body using that convention.

Two practical notes the chart itself does not spell out. First, Ackley Improved chambers use the same bushing as the parent case — a .30-06 AI still reads on the C.375. Second, if your cartridge appears nowhere on this list, that is not a dead end; the next section covers it.

How to Choose a Headspace Bushing When Your Cartridge Is Not Listed

There is a one-line formula that settles every wildcat, obsolete military round and proprietary case in existence. Hornady states it directly in the instruction sheet linked above:

"If you add the neck diameter and the shoulder diameter and divide that number by two, you will know the proper bushing diameter to use."

Then round to the nearest available bushing. Worked example on .308 Winchester: neck diameter .3433", shoulder diameter .4539". Their average is .3986", which rounds to the D.400 bushing — exactly what the chart says. Run the same arithmetic on a 6mm Creedmoor, a .22 Nosler or a 6.5-284 wildcat and you will land on a bushing every time, without needing anyone's chart.

You can read neck and shoulder diameters straight off a SAAMI or CIP cartridge drawing, or measure a fired case from your own chamber with calipers. The second method is arguably better, because it reflects the chamber you are actually loading for.

The 6.5 Creedmoor Question: C.375 or D.400?

Both bushings work on a 6.5 Creedmoor, and the choice matters far less than the consistency of the choice. This is the single most-argued question about headspace bushings, and it deserves a straight answer.

The disagreement is real. On the Shooters' Forum thread on exactly this topic, one member opened with "Some say use the C.375 and some say use the D.400. There is nothing on the packages that say 6.5 Creedmoor, so which one are people actually using???" The replies split immediately: the official chart lists 6.5 Creedmoor under C.375, while another member pointed out that "according to SAAMI case drawing the datum line is at .400."

The member who actually tested it reported the outcome that matters: "I tried both today, and measured 100 cases that I have already bumped and sized. Other than where it's landing on the shoulder, there was hardly any noticeable difference between the 2 gauges when zero'd out." The thread's best answer followed: "It doesn't matter, it's a comparative measurement, not absolute, just don't mix it up — choose one and stick with it."

That is the correct takeaway. Pick C.375 if you want to match the published chart, pick D.400 if you want to sit closer to the SAAMI datum, then write your choice on a label and never switch mid-load-development. Switching bushings halfway through a sizing session is what actually ruins data.

Why a Headspace Comparator Is Not a GO/NO-GO Gauge

A comparator measures change in your brass; a GO/NO-GO gauge certifies a chamber against a specification. They answer different questions and are not interchangeable.

This confusion has real consequences. In our own customer feedback, one reloader returned a kit with the note "I measured a 2 different go gauge, and it is not accurate." The tool was working correctly. A comparator reading taken on a GO gauge will not reproduce the gauge's certified headspace number, because the bushing contacts an arbitrary diameter on the cone rather than the certified datum circle, and because the caliper has no absolute zero at that point.

The manufacturer documentation is unambiguous on this: the comparator "measures variations in your brass before and after firing, or resizing" and "allows comparison between fire-formed brass and your resized brass." Every word in that sentence is relative.

If you need to verify that a rifle's chamber is within specification, buy a GO/NO-GO set. If you need to know how far your die is pushing the shoulder back, you need a comparator. Most precision reloaders eventually own both.

How to Measure Shoulder Bump in Five Steps

Shoulder bump is the difference between a fired case and the same case after sizing, and one to two thousandths of an inch is the widely accepted target for full-length sizing.

1. Mount the body. Thread the brass thumb screw into the comparator body, seat the body on the caliper's fixed jaw, close the jaws snugly to square everything up, then tighten both screws.
2. Fit the correct bushing. Select from the chart above, or run the neck-plus-shoulder-divided-by-two formula. The bushing must slide freely over the neck and stop on the shoulder.
3. Zero on fired brass. Close the caliper on a fired, un-sized case from your own rifle and zero the display. This is your chamber's reference length, and it is the step most often skipped.
4. Measure a sized case. Center the shoulder in the bushing, bring the jaw to rest on the case head, rotate the case as you take up slack, and read. The difference from zero is your bump.
5. Adjust in small increments. Target .001" to .002" of setback. Move the die a few degrees at a time and re-measure — never a full turn.

Two cautions worth internalizing. Different lots of brass measure differently because of alloy and temper variation, so re-zero when you change lots. Lubrication technique also shifts sized dimensions measurably — a comparator will detect a change in how heavily you lube, which is a feature, not a fault.

If your readings still wander after following these steps, the diagnostic path is the same one we documented for ogive measurements in our guide to inconsistent comparator readings.

Three Mistakes That Send Comparator Kits Back

Reviewing our own returns notes, the overwhelming majority of "it doesn't work" reports trace to three specific misuses rather than to a defective tool.

Mistake 1 — Using a headspace bushing on a bullet. One customer wrote: "The space where the bullet goes in seems to be measured WAY too big. The bullet just slides in and out. Either I'm doing something really wrong or the openings are very large." They were using a headspace bushing, which is bored for a case shoulder, on a bullet. Bullet ogive work needs the stepped inserts, not the lettered bushings — the difference is covered in our step-by-step CBTO measurement guide.

Mistake 2 — Choosing the wrong bushing diameter. Another note read: "the .330 bushing hole is too large. It does not fit the case properly." An A.330 on a large-shouldered case will simply pass over the shoulder without contacting it correctly. Run the formula.

Mistake 3 — Expecting it to work on pistol cartridges. A return marked "Not for pistols" says it all. Rimless pistol cases such as 9mm and .45 ACP headspace on the case mouth, not a shoulder. There is no shoulder datum to measure, so a headspace comparator has nothing to contact. This tool is for bottleneck rifle cases only.

What the F.188 Bushing Is Actually For

F.188 is the sixth position in the standard lineup, sitting below A.330 for cases too small for the five common sizes — and on most kits it is a separate purchase rather than an in-the-box item.

Here is where kit contents diverge, and it is worth checking before you buy. Hornady's published lineup defines F-.188" as a bushing "blank" with a .188" hole, intended to be drilled and reamed for sizes not listed, and marks both it and the G-.240" (.17 Hornet) bushing as sold separately — the kit itself ships with five bushings, A through E. Any cartridge outside those five therefore means a second order.

That detail matters if you shoot anything unusual: a .219 Donaldson Wasp, a 6mm Dasher, an obsolete Mauser chambering. One of our own reviewers hit exactly this wall, calling the kit an "excelente herramienta" while noting it lacked coverage for his 7.65×53 Argentine Mauser.

The AltitudeCraft Bullet and Headspace Gauge Comparator Kit includes six bushings in the box — A.330, B.350, C.375, D.400, E.420 and F.188 — plus fourteen bullet comparator inserts covering .172" through .458", in a foam-lined tin. If you only need shoulder measurement, the six-bushing headspace-only kit covers the same range without the inserts. If you only work on seating depth, the fourteen-insert bullet comparator kit is the narrower tool.

Two honest limitations, whichever brand you buy. No six-bushing set covers every wildcat and obsolete chambering ever cut — for those, opening a blank to size is a lathe-and-reamer job, not a drop-in part. And as covered above, if what you actually need is certified chamber verification rather than brass comparison, no comparator kit substitutes for a proper GO/NO-GO set.

Frequently Asked Questions About Headspace Comparator Bushings

Which headspace bushing do I use for .308 Winchester?

The D.400 bushing. It also serves .243 Winchester, .260 Remington, 7mm-08 Remington, .270 WSM and .300 Savage. The formula confirms it: .308 Winchester averages roughly .399" between neck and shoulder diameter, which rounds to .400".

Which headspace bushing do I use for 6.5 Creedmoor?

The published chart lists C.375, but D.400 is also widely used because the SAAMI datum sits at .400". Either produces usable bump data. Choose one, label it, and do not switch between them within a load workup.

Can I use a headspace comparator to check my rifle's chamber?

Not to specification. A comparator gives comparative numbers on your brass, not a certified chamber dimension. You can infer that a chamber is generous if fired brass consistently measures long, but confirming a chamber against spec requires GO and NO-GO gauges.

Does a headspace comparator work on .223 Remington and 5.56 NATO?

Yes — both use the A.330 bushing. This is a common application, since open-tip match bullets make overall-length measurement unreliable and push reloaders toward datum-based measurement in the first place.

How much shoulder bump should I set?

One to two thousandths of an inch back from your fired-case dimension is the standard recommendation for full-length sizing. That minimizes case stretching and work hardening while still chambering reliably. Semi-automatic rifles are often given slightly more setback for feeding margin.

What is the difference between headspace bushings and bullet comparator inserts?

Bushings are lettered by bore diameter and contact the case shoulder to measure base-to-datum length. Inserts are numbered by caliber and contact the bullet ogive to measure cartridge base to ogive. They are not interchangeable. Our comparator kit comparison covers both sides of a combined kit in more detail.

Do Ackley Improved cartridges need a different bushing?

No. Ackley Improved chambers use the same bushing size as the parent case, so a .30-06 AI still reads on the C.375 bushing.

Related reading: Complete guide to die bushings for precision reloading — note that neck-sizing die bushings are a different component entirely from the headspace comparator bushings covered here.