To add to your explanation.
It’s called “tara” but it’s really just ら appended to the past tense. It’s だら when the past tense ends with だ (e.g. 死んだら).
It can indeed be either “if” or “when”. English has very specific rules for these two words, Japanese has different rules and “tara” doesn’t fit into either category.
Kuno lists the following rules for it:
- The second part must always happen after the first part; it cannot be used to express something like “Please take me with you if you leave”.
- When using tara to describe past events, there can be no “self-controllable” timing. To explain this concept consider these examples:
“When I opened the door, a cat was outside.” ← no self-controllable timing
“When I opened the door, I was startled by the cat outside.” ← no self-controllable timing
“When I opened the door, I picked up the cat outside.” ← self-controllable timing.
So the third example couldn’t use “tara” in a Japanese translation. - Also with past events, tara carries a connotation that the second event is surprising or unexpected.
- As an exception to 2, past habits are considered natural courses of events.
“I always went to New York when the summer came.” ← ok with tara even though technically self-controllable timing
So considering 3 I think the best translation is something like
“We were patrolling when we suddenly heard the sound of a pistol”
In this case, reversing the “when” feels more natural in English and expresses the surprise better.