Here’s something that surprises many shooters new to precision rimfire: two boxes of the same brand and model of .22LR can shoot very differently in the same rifle. One box prints tiny groups. The next opens up for no obvious reason.
The difference is usually the lot number. Understanding lots, and testing for them, is one of the cheapest and most effective ways to improve your rimfire accuracy.
What Is an Ammo Lot?
Ammunition is made in production runs. A lot is a batch made with the same components, on the same equipment, in the same period. Each lot gets a number, printed on the box or the case.
Within a lot, ammo is relatively consistent. Between lots, small changes in priming compound, powder, bullet weight, or case dimensions can change how the ammo performs.
Why Rimfire Is So Lot-Sensitive
Several things make .22LR especially sensitive:
- Priming. Rimfire primer compound is spun into the rim. Small variations in how much there is, and how evenly it’s spread, affect ignition and velocity.
- Velocity spread. Small velocity differences change where each bullet exits during the barrel’s vibration (see What a Rimfire Tuner Does).
- Every barrel is different. Chamber and bore dimensions vary slightly from barrel to barrel, so a lot that shoots well in one rifle may not in another.
The result: your rifle will “prefer” some lots over others. The only way to find out which is to test.
The payoff: Many shooters find that switching to a well-matched lot shrinks groups more than any accessory upgrade, and costs much less.
How to Test Ammo Lots: A Simple Process
1. Gather samples
Buy a box or two of several different lots, either of the same ammo or a few different match-grade options. Write each lot number on its box.
2. Set up for consistency
- Shoot at 50 yards from a solid front rest and rear bag.
- Pick a calm day, or shoot early.
- Foul the barrel with 10 to 20 rounds of the first lot before recording groups.
- Use a fresh aiming point for each group.
3. Shoot multiple groups per lot
One group tells you very little. Shoot at least three to five 5-shot groups, or a few 10-shot groups, per lot. Fire a few fouling shots each time you switch lots so the barrel adjusts.
4. Measure and record
For each lot, record:
- Lot number
- Date, temperature, and conditions
- Each group size, especially vertical spread
- The average group size
5. Compare averages, not best groups
The lot that shot one tiny group isn’t necessarily the winner. Look for the lot with the smallest, most consistent average. Consistency wins matches.
Found a Good Lot? Buy Deep.
When you find a lot your rifle likes, buy as much of it as you can afford: a case or more. Once it’s gone, you start testing again.
Store it properly. Keep it in a cool, dry place with stable temperatures, since heat and humidity swings can affect performance over time.
Testing Tips
- Tune and test together, carefully. If your rifle has a tuner, test lots at one fixed tuner setting first, then fine-tune the tuner for your chosen lot.
- Use a chronograph if you have one. Lower velocity spread often goes with better accuracy at distance.
- Don’t test on a windy day. Wind will hide the differences you’re looking for.
- Keep good notes. A simple spreadsheet becomes very valuable over time.
- Ask about test centers. Some ammunition makers and dealers offer lot testing services, where your rifle is shot with many lots in a controlled tunnel. This can save time if you’re serious about benchrest.
The Bottom Line
Your rifle, barrel, and tuner can only perform as well as the ammunition you feed them. Lot testing takes a range session or two, but finding the right lot is one of the biggest accuracy improvements you can make.
Every Cybex rifle is test-fired with match-grade ammunition before it ships, so you start with a proven rifle. Browse ready-to-ship rimfire rifles.
