IBC is a physics simulator. One rule governs everything here: this supports a load work-up, it never replaces one. Start low, work up slowly, read your brass.
Every prediction is checked against published load data: 30,327 printed loads, and our velocity calls miss by about 7.5% on average. The table below shows the error rate for each powder we serve, worst rows included. Low-fill pistol and magnum loads are where we miss most. When a powder improves, the new numbers go up here with a date.
How accurate is it?
Which powders should I double-check?
What do the verdicts mean? A load is called OVER only when the predicted pressure itself reaches the cartridge's published limit. CAUTION means it is close, or its uncertainty range reaches it. UNDER means neither. None of the three is a pressure gauge: work up from published starting data and stop at the first pressure sign.
The calculator lists the powders we have measured and can stand behind; the rest are held out until they publish clean. The per-powder table on the technical appendix keeps every graded row — held powders included — because it is the evidence, not the catalogue.
Near-empty pistol cases are the hardest regime in interior ballistics. A tiny charge in a big case behaves nothing like the full-power data powders are characterized on, and error grows there. Light charges and large-bore pistol loads dominate the worst rows, and they mostly run high.
The misses run both directions, and there is no safe-side tilt to claim. Some loads read above the book — a published max charge can come back flagged MAX where the book calls it normal — while others read low, mostly against crusher-standard pressure figures. The worst rows, in both signs, are published on the technical appendix. That is what a chronograph and a careful work-up are for.
Predicted peak pressure is compared to the cartridge's SAAMI (or CIP) limit; the app reports the percentage of proof pressure. A verdict chip states what the evidence establishes: OVER only when the predicted percentage itself reaches the limit, CAUTION when the prediction is within 5% of it or its uncertainty band reaches it — the same rule on every surface. IBC is an engineering aid: chronograph your own loads, work up from published starting data, and stop at the first sticky bolt or flattened primer. No calculator replaces pressure signs on your rifle.
The per-powder table and the worst rows live on the technical appendix; the full how-we-measure story is the accuracy paper.