Since I broke alternative to brass savers ...

[SplatterTone]

I tried to post a link to Wikipedia for gauge sizes and apparently broke the topic. So here is my answer without the link. If the original topic gets fixed, then delete this one.

I keep my horn clean by blowing it out with a HVLP blower and spraying valve oil in all the slides and lead pipe after every practice session. Instead of a HVLP blower, you could use a shop vac on blow with a reducer.

So I don't have any actual experience with what I am about to suggest. But ... I've have often thought a shotgun cleaning rope would work well. The 12 gauge bore is .729 inch.