Попадание было в сектор 25.

Here попадание means a hit, a strike, or an impact—the event of something successfully hitting a target or area.

It comes from the verb попасть / попадать. In other contexts those verbs can also mean to get into, to end up in, or to arrive somewhere by chance, but in this sentence the meaning is clearly to hit.

Было is the past-tense form of быть (to be).

Russian past tense agrees with the subject in gender and number:

• он был
• она была
• оно было
• они были

Since попадание is a neuter singular noun, the sentence uses было.

Сектор is in the accusative singular.

That is because попадание commonly takes the pattern в + accusative to show what was hit or where the hit landed:

• попадание в цель = a hit on the target
• попадание в дом = a hit on/into the house
• попадание в сектор 25 = a hit in/into sector 25

Also, сектор is masculine and inanimate, so its accusative form looks the same as its nominative form: сектор.

Because this is not just a normal location phrase after to be. The noun попадание itself naturally goes with в + accusative.

So попадание в сектор 25 means a hit into/in sector 25 as a single idea.

If you said в секторе 25, that would be the prepositional case and would focus more on simple location: in sector 25. That is possible in some contexts, but with попадание the pattern в сектор 25 is more idiomatic when talking about where the hit landed.

Here 25 works like a label or identifier: sector 25.

In technical, military, map, game, or report-style Russian, the noun often changes for case, but the numeral used as a label stays as a numeral:

• в сектор 25
• из сектора 25
• по сектору 25

You could also express it as an ordinal:

• в 25-й сектор
• в двадцать пятый сектор

That version sounds more like the 25th sector, while сектор 25 sounds more like a named zone or marked sector on a map.

Yes, there can be a small nuance.

• сектор 25 = sector 25 as a label, code, or marked area
• 25-й сектор = the 25th sector, using an ordinal number

In many situations they may refer to the same place, but сектор 25 sounds more like map/report terminology, while 25-й сектор sounds more like ordinary counting or ordering.

Yes. Попадание было в сектор 25 sounds concise and report-like, as if from:

• military communication
• a targeting report
• a game interface or battle log
• a range or simulation description

In more everyday speech, Russian might use a fuller verb:

• Снаряд попал в сектор 25.
• Попали в сектор 25.

Those sound more natural in conversation.

Yes. Russian word order is flexible.

Попадание было в сектор 25 is a neutral, report-style order.

Other orders are possible for emphasis:

• В сектор 25 было попадание.
  This puts more focus on the location.
• Было попадание в сектор 25.
  This sounds a bit like There was a hit in sector 25.

The basic meaning stays similar, but the focus changes.

Sometimes, yes, especially in headlines, notes, logs, or very compressed reporting style:

• Попадание в сектор 25.

That would sound like a short record or caption: Hit in sector 25.

But in a full sentence, было makes it complete and clearly past-tense.