Rimfire Ammo
Lot Testing

I’ve been involved in competitive rimfire shooting for over 30 years, and I’ve watched talented shooters plateau without any obvious explanation. Sometimes the issue was technique. But more often, the problem was the ammunition, more specifically, the “lot” they were shooting.

Every case of rimfire ammunition carries a lot number printed on the packaging. It’s a long string of digits that most recreational shooters never pay any attention to, but for competitive shooters it carries real meaning.

That number identifies a specific production run at the manufacturing facility. The brand is the same, the product name is the same, the bullet weight are the same. What changes, subtly, from one run to the next are the variables that the factory can’t hold perfectly constant: powder charges, priming compound consistency, brass dimensions, and even environmental conditions in the plant on a given day.

Those shifts are small in absolute terms. In rimfire competition, however, small is everything. At 50 meters on a match-grade target, a variation that would go unnoticed in casual shooting can mean the difference between a shot that stays in the 10 ring and one that drops out.

Centerfire shooters dealing with accuracy issues have an option that rimfire shooters simply do not have: reloading. If the rifle is not performing the way it should, the reloader can adjust the powder charge, change the seating depth, try a different primer, and work backward from the target to find a load the barrel prefers. The ammunition can be tuned to the rifle.

That is not possible with rimfire cartridges. The priming compound is sealed into the case during manufacturing and the cartridge arrives complete from the factory. There is no component to adjust after the fact. A competitive rimfire shooter is working entirely with what the manufacturer produced, which means the only way to optimize is to find the factory lot that already agrees with that particular barrel.

Lot testing is the rimfire equivalent of load development
Since the ammunition cannot be tuned to the rifle, the shooter finds the lot the rifle already likes. The principle is the same even if the method is completely different.

Every barrel has its own character. The precise dimensions of the chamber, the throat geometry, the rifling, and the natural harmonic behavior of the barrel under fire all affect how a given round performs in that particular rifle. Even two rifles produced on the same day from the same production run will respond differently to the same lot of ammunition.

The person who runs the Lapua Rimfire Performance Center in Mesa, Arizona has tested hundreds of rifles and says that the more he tests, the less the results follow any predictable logic. He has seen lots with poor velocity standard deviations produce tight groups, and lots with very consistent velocities that opened up badly on target. The numbers are useful context, but they do not reliably predict what will actually happen downrange.

There is a meaningful difference between testing lots at home on an outdoor range and what happens at a professional test facility. Both approaches can give you useful information, and both have a legitimate place in a competitive shooter’s toolkit. They are not, however, the same thing.

The operator at the Mesa facility has noted that after testing, the results are usually not ambiguous. The top one to three lots generally stand out clearly from the rest, and even when two lots are close, the simultaneous 50-meter and 100-meter data often separates them at the longer distance.

A lot of guides about rimfire ammo testing spend all their time on the testing process itself and almost none on what comes next, which is where things get practical in a hurry.

Once you know which lot your rifle prefers, you need to purchase enough of it to last a reasonable competitive season. Lots do not remain available indefinitely at the supply level, and once a lot sells through at the distributor, it is gone. The standard recommendation for serious competitors is to purchase a full case of 5,000 rounds from the matched lot while it is still available.