Technique
Rim Thickness Sorting: The Benchrest Trick That Actually Works
Sorting rimfire ammunition by the thickness of its rim is one of the few accuracy rituals in shooting that survives honest testing — but not for the reason most people give, and not on every rifle.
The short version
- The rim is the primer. In a rimfire cartridge the priming compound is spun into a hollow rim, so rim thickness controls both how the round headspaces and how hard the firing pin crushes the compound.
- Sorting reduces one variable, not all of them. Consistent rims mean more consistent ignition and more consistent seating depth, which shows up as reduced vertical dispersion — not as a magically smaller group in every direction.
- It is measurable, and it is small. Expect a real but modest improvement on a rifle capable of showing it. On a sporter shooting bulk ammunition, the rifle and the ammunition are already contributing far more error than the rims are.
- Match ammunition is already partly sorted. Premium rimfire has tighter manufacturing tolerances, which is a large part of what you are paying for. Sorting bulk ammunition delivers the biggest relative gain and the worst absolute result.
Why the rim matters in the first place
Every centerfire cartridge has a separate primer pressed into a pocket in the case head. It is a discrete component, manufactured to its own tolerances, and it does one job: ignite when struck.
A rimfire cartridge has no primer. The priming compound is a wet paste spun into the hollow interior of the case rim during manufacture and then dried in place. That one design decision, inherited from the nineteenth century, produces every characteristic that makes rimfire ammunition behave the way it does — including why it cannot be reloaded, and why the rim is simultaneously the most important and the least controllable dimension on the cartridge.
The rim is doing three separate jobs at once:
- It is the headspace surface. The rim stops against the barrel face, so rim thickness alone determines how far the cartridge sits into the chamber.
- It holds the priming compound. How much compound, and how evenly distributed, depends on how well that rim was formed and filled.
- It is what the firing pin crushes. The pin strikes the rim against the chamber edge, crushing the compound between two surfaces. How much material is between them changes the character of that impact.
What sorting actually controls
The common explanation is that sorting produces more consistent ignition. That is true, but it is only half of it, and the half people usually leave out is arguably more important.
Ignition consistency. A thicker rim means the firing pin crushes more material and encounters more resistance, which changes both the timing and the intensity of ignition. Small differences in how the compound ignites propagate into small differences in how the powder burns, which becomes a small difference in muzzle velocity.
Seating consistency. Because the rim is the headspace surface, a thicker rim holds the entire cartridge slightly further out of the chamber. The bullet therefore starts slightly further from the rifling. That distance — the jump to the lands — is one of the things precision shooters in every discipline work hardest to control, and in rimfire it varies purely as a function of a dimension the factory did not tightly control.
Both effects show up on paper primarily as vertical dispersion. Velocity variation and jump variation are both vertical phenomena. If your groups are noticeably taller than they are wide and you have already eliminated the obvious causes, that is the signature sorting addresses.
How to sort properly
The process is simple and tedious, in that order.
- Work from one lot. Sorting across lots mixes in powder and bullet variation you cannot see, which defeats the purpose. One box or one brick at a time.
- Measure the rim, not the case. The measurement is taken across the rim thickness at the case head. The tool needs to index consistently on the same part of the rim every time or the readings are noise.
- Take several readings per round if you are being rigorous. Rims are not perfectly uniform around their circumference, and the variation within one rim is itself a quality indicator. In practice most people take one consistent reading and accept the approximation.
- Bin, do not rank. Sort into groups by measurement band rather than trying to order every round. Three to five bins is plenty; more bins means more handling and smaller batches.
- Shoot each bin separately and label everything. The whole point is comparing bins against each other, which requires knowing which is which weeks later.
- Keep the outliers. Rounds that fall far outside the distribution are not useless — they are your fouling shots, your zero confirmation and your plinking ammunition.
The priming compound sits in the rim, which means a hard strike on the rim is exactly the thing that fires the round. Do not drop rounds, do not let them tumble against each other in a container, and do not apply force to the rim with any tool that could impact rather than press it. Sort over a soft surface, work with a small number of rounds at a time, and wear eye protection. This is the one accuracy ritual that involves repeatedly manipulating the most sensitive part of a live cartridge.
Tools and what they cost you
| Method | Speed | Consistency | Notes |
|---|---|---|---|
| Dedicated rim gauge | Fast | High | Purpose-built to index the rim identically every time. The right tool if you are going to do this repeatedly. |
| Dial or digital indicator on a stand | Moderate | High if set up carefully | Common home-built approach; the fixture matters more than the indicator. |
| Calipers | Slow | Low | Very hard to index consistently on a small rim by hand. Fine for a rough look, poor for real sorting. |
| Sorting by weight instead | Moderate | Different variable entirely | Measures total mass, which correlates loosely with charge and bullet consistency but not with rim thickness. |
Whatever you use, the fixture is what determines whether your data means anything. An indicator that touches a slightly different point on the rim each time produces a spread of readings that reflects your technique rather than the ammunition. Before sorting a single round, measure the same round ten times and see how much your own readings vary. If that spread is comparable to the spread across the ammunition, your tool is not resolving what you are trying to measure.
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What to expect, honestly
Sorting is real, and it is smaller than the internet suggests.
The improvement appears mainly as reduced vertical spread, and it appears mainly in rifles and ammunition already good enough for that spread to be the limiting factor. Stack the sources of error in a rimfire group in rough order of magnitude and you get something like: the shooter, the ammunition's overall quality, the rifle and its bedding, the wind, the optic and mount, and then rim consistency. Sorting attacks the bottom of that list.
The correct way to find out what it does for you is to test it rather than believe it:
- Shoot five or more groups from a properly supported bench with unsorted ammunition from one lot.
- Sort the remainder of the same lot into bins.
- Shoot the same number of groups from a middle bin, on the same day, in similar conditions.
- Compare vertical dispersion specifically, not just overall group size.
Do this from a front rest and rear bag, not from a bipod, and shoot enough groups that one flyer does not decide the answer. A three-shot group proves nothing about anything.
When it is not worth doing
Be honest about the situation before spending an evening at a kitchen table with a dial indicator.
- If you are shooting a sporter with a factory trigger, the rifle is contributing far more dispersion than the rims are. Bedding, a trigger job and better ammunition all return more.
- If you are shooting bulk-pack ammunition, the round-to-round variation in powder charge and bullet dimensions swamps the rim contribution. Sorting bulk ammunition gives the largest relative improvement and still lands well short of unsorted match ammunition.
- If you are shooting a speed discipline, it is irrelevant. Steel Challenge is decided by transitions, not by vertical dispersion at fifty yards.
- If you are hunting, also irrelevant. Field positions and range estimation dominate by orders of magnitude.
- If you have not yet found which ammunition your rifle likes, do that first. The difference between a load your barrel prefers and one it dislikes is far larger than anything sorting will recover.
Sorting belongs to benchrest, to serious smallbore, and to anyone who has already exhausted the larger variables and genuinely wants the last increment.
Sorting by weight and by inspection
Two adjacent practices worth knowing about.
Weight sorting measures the whole cartridge, so it captures variation in powder charge, bullet mass and case mass together, without telling you which. That is both its strength and its weakness: it catches gross outliers that a rim gauge would pass, and it cannot isolate anything. Some shooters do both, sorting by weight first to remove outliers and then by rim within the remainder.
Visual inspection is the cheapest and most neglected step, and for bulk ammunition it is probably the highest-value one. Look at each round for deformed rims, bulged cases, visibly out-of-round bullets, damaged noses and cases with inconsistent lubricant coverage. A round with a dented rim is not a precision round and may not be a reliable one. This costs nothing, requires no tools, and removes the worst rounds in any bulk lot.
If you only adopt one habit from this page, make it that one. Then decide whether the rim gauge is worth your evening.
Frequently asked questions
Does rim thickness sorting actually improve accuracy?
Yes, measurably, but modestly and only in rifles capable of showing it. The effect appears mainly as reduced vertical dispersion, because rim thickness affects both ignition consistency and how far the bullet sits from the rifling. In a sporter shooting bulk ammunition, the rifle and the ammunition contribute far more error than the rims do, so the improvement disappears into the noise.
Why does rim thickness affect ignition?
Because in a rimfire cartridge the rim is the primer — the priming compound is spun into the hollow rim during manufacture. The firing pin crushes that rim against the chamber edge to ignite it, so a thicker rim means more material to crush and a different ignition impulse. Variation in ignition becomes variation in muzzle velocity, which shows up as vertical spread on target.
Should I sort match ammunition or only bulk?
Bulk gives the largest relative improvement because it starts with the widest spread, but it still finishes well short of unsorted match ammunition. Match ammunition is manufactured to tighter tolerances — that consistency is much of what you are paying for — so sorting it yields less, but the shooters who benefit most from small gains are the ones already using it. Sort match ammunition if you shoot benchrest; sort bulk mainly as an experiment.
Can I use calipers to sort rims?
Poorly. The difficulty is not resolution, it is indexing — getting the caliper jaws onto exactly the same part of a small rim every single time by hand is close to impossible, and inconsistent indexing produces readings that reflect your technique rather than the ammunition. A dedicated rim gauge or a dial indicator in a proper fixture solves that. Test your own repeatability on one round before trusting any data.
Is it safe to handle live rounds this much?
It requires care specifically because the priming compound is in the rim, which is the part you are handling. Press rather than strike, never drop rounds or let them tumble loose in a container, work over a soft surface with a small number of rounds at a time, and wear eye protection. Any tool that could impact the rim rather than gently contact it is the wrong tool.
How many bins should I sort into?
Three to five is plenty. More bins means smaller batches and more handling for diminishing returns, and beyond a certain point you are sorting within your own measurement error rather than within the ammunition. The goal is to remove the outliers and shoot from a consistent middle, not to rank every round individually.
What should I do before I bother sorting?
Find out which ammunition your rifle actually prefers, from a proper bench rest with a rear bag. The difference between a load a barrel likes and one it dislikes dwarfs anything sorting will recover. After that, address the trigger, the bedding and the optic mount. Sorting is the last few percent, and chasing it before the larger variables are settled is effort spent in the wrong place.