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For example, why the correct spelling is トンネル rather than トッネル ?

Is it an arbitrary/historical choice, or is there a rule that explains it?
Would the pronunciation differ?

[This wiki page](https://en.wikipedia.org/wiki/Sokuon#Use_in_Japanese) says
that ッ " _rarely_ appears before a syllable that begins with the consonants n
(...)" Why it doesn't in this case?
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question
katakana loanwords spelling
When to use ンネ vs ッネ?
The character っ (the small つ) represents a blocking of the airflow that lasts
for more or less the duration of one mora (or one _beat_ ) before a plosive or
an affricate, or a prolonged hissing in a voiceless sibilant (or very rarely a
voiceless fricative that is not a sibilant, such as /h/ and /ç/).

See [this](https://japanese.stackexchange.com/a/86530/43676) for the
consonants that meet these conditions.

Neither phenomenon happens before a nasal such as /n/. The ン in トンネル is
produced by passing the air through the nose. (If not トンネル, _tunnel_ would be
transcribed as トネル, not トッネル.)
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answer
I have no actual sources to back this up with (if anyone knows of any, please
feel free to adduce), but it appears to me to be simply a matter of convention
based in having to make a choice between two options.

In Japanese, there is a complete merger of all non-nasal consonantal
morphophonemes in position before another consonant: they all assimilate
completely to the following consonant. Some transcribe the result of this
merger phonemically as a generic ‘moraic obstruent’ phoneme written /Q/, while
others transcribe it according to the following consonant, but the outcome is
the same.

That is, for any sequence of morphophonemes |TC| (where T is any consonant
that is not nasal and C is any consonant at all), the phonetic output is
always [Cː], a geminate of the second consonant. In phonemic writing, some
will write this /QC/, others as /CC/.

The ‘moraic obstruent’ approach is practical because it fits with Japanese
writing, which is fully moraic: the phoneme /Q/ corresponds precisely to the
_sokuon_ っ.

If the first consonant is a nasal consonant, however, there is only partial
assimilation: the nasal assimilates to the following consonant’s place of
articulation, but remains nasal and voiced. That is, for any sequence of |NC|
(where N is any nasal consonant and C is any consonant at all), the output
form is [NC] as well.

To summarise:

Morphophonemes | Phonemes | Output | Kana  
---|---|---|---  
|tp| | /Qp/ | [pː] | っぱ  
|rs| | /Qs/ | [sː] | っさ  
|pb| | /Qb/ | [bː]¹ | っば  
|nk| | /Nk/ | [ŋk] | んか  
|md| | /Nd/ | [nd] | んだ  
  
Notice how all the cases where the first consonant is non-nasal have っ, while
the ones where the first consonant is nasal have ん. Or, if we go by the
phonetic output form, how all the cases where the output is a single, geminate
sound have っ, while the ones beginning with a nasal sound have ん.

In principle, these developments hold true for all values of C, whether nasal
or not, so the phonetic outcome of |pn| → /Qn/ is [nː], matching |pb| → /Qb/ →
[bː] perfectly. The trouble is that the outcome of |mn| → /Nn/ is _also_ [nː],
which is completely identical. In other words, before nasals, the merger of
the first morphophoneme in the sequence is actually complete: **all**
consonants assimilate completely to the following nasal.

So how do you represent that in writing?

In theory, you could be totally stringent and write っな when the first
consonant is underlyingly a non-nasal morphophoneme and んな when it is nasal.
But that’s easier said than done, because it’s not always clear what the
underlying morphophoneme is even to an etymologist, much less to the average
person who’s just writing a word. So it makes more sense to just use one or
the other consistently.

There’s no single way of choosing that inherently makes more sense than the
other here. You could argue that, since the output is always a single,
geminate sound, っ should be used, just as it is for the other geminates; or
that, since the output begins with a nasal sound, ん should be used. When
Japanese spelling became fixed, the latter argument apparently weighed heavier
than the former.

Another factor may be the restriction on voiced geminates in Japanese (see
note below): settling on んな rather than っな happened quite a while ago, before
the influx of (primarily English) loan words containing voiced geminates, so
at the time, sequences like っば っが っだ were virtually nonexistent. っ itself was
then more obviously associated with _voiceless_ geminates specifically, and it
may have felt odd at the time to use it for a fully voiced, resonant geminate
like [nː].

* * *

#### Notes

1. In native Japanese words, voiced geminates do not occur, and the outcome would possibly be /Nb/ → [mb] instead. In loan words, though, voiced geminates are perfectly allowable. A bit more detail is available in [the Wikipedia article on Japanese phonology](https://en.wikipedia.org/wiki/Japanese_phonology#Gemination).
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