Weirdly enough, that G is probably the most difficult. it was for me. thing is, when you do get it the rest becomes easier. I know this isn’t the best news since you’re still at the struggling to get it stage but you have to keep working and experimenting with it.
as for some horns being tougher than others, impossible to say unless someone literally tries em all! what I can tell you is I’ve played many horns over the years and am always able to hit any note. some horns will behave slightly different but it’s a scientific thing; those altissimo notes are based on the natural harmonic series of the instrument so technically they should all work out around the same.
There is also some validity about certain reeds and mp’s making a difference. For example, a slightly stronger reed than you’re used to may help and so might a more open mp.
Spend some more time on the part where there is experimentation with your embouchure, breathing, throat and oral cavity. slight changes in one or more of these can make the difference.
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