This is a passage for Horn in F in a moderato tempo (the key is 4 sharps):
Though it mostly moves by step, there are several leaps, one of which is by a dissonant melodic interval (diminished third). Will these intervals be awkward to play for the hornists, and what are the overall limits for leaps in the brass section?
Competent brass players can certainly play intervals of an octave or less, although these intervals get harder in extreme registers (especially with big leaps into the upper tessitura).
Octaves are often easier than they look, because the players already have the pitch in their ears.
All of this said, I would argue that the hardest part of this excerpt is the key. These sharps and double sharps are definitely doable, but they will likely be trickier for the players than the leaps.
PS: A to F-doublesharp is actually a diminished third, not an augmented second :-)
Leaps by themselves are not a problem, nor are the specific intervals — particularly for valved brass instruments.1 The limitations would just be the player's ability and range.
For example, a reasonably skilled trumpet player (or talented amateur) can make leaps of an octave, but such large leaps could quickly exceed that player's usable note range if they get too high.
For a professional trumpet player, leaps beyond an octave are possible, and if not routine, at least not all that unusual either.
Here is a downward leap — legato, no less, which is more difficult — of a diminished 12th, from written F#5 to B#3, from Hummel's trumpet concerto.
(Image source: IMSLP, Michel Rondeau edition, solo trumpet part)
And here's a section from Haydn's trumpet concerto featuring several leaps of a major 10th, and two leaps of nearly two octaves.
(Image source: IMSLP, Michel Rondeau edition, solo trumpet part)
1. The exception is the trombone, for which the practicality of leaps can be affected by slide position. But in that case, the issue would be more the speed with which the leap is to be executed.
Can competent brass players play large leaps?
Yes. Remember that brass instruments in their fundamental form can play only tones that are found in the overtone series. Large leaps are inherent in the nature of brass instruments, especially in their lower registers.
In the upper register, because the overtones are closer together, playing a note accurately is more dependent on the player's control: breath pressure, lip tension, and indeed ear. Here I answer "yes" because the question specifies "competent," but remember that it is unusual to attend a symphonic performance, even a professional one, without hearing a flubbed note from the horns.
Look at a few horn parts from the late 18th through the late 19th century. They don't just contain leaps, their characteristic style is based on leaps. This music is an important part of the horn repertoire. For a horn player, a few fourths, sixths, or octaves are like breathing.
You'll also want to look at idiomatic writing for the modern valved horn, of course, in more recent works, where you'll find leaps both in passages that evoke the classical brass idiom (Fanfare for the Common Man comes to mind) and in passages that are mostly stepwise, as with melodies written for any chromatic instrument.
In general, as an orchestrator, you will want to become more familiar with all of the instruments of the orchestra. Buy a cheap used horn, or borrow one. Learn to play it, even badly. This will give you more insight into the answer to this question than any of us can. Look at a book of orchestral excerpts that a student hornist would use. Listen to professional recordings of these pieces. Look for videos online of students playing these pieces, too: this will give you a sense of what's hard and what isn't.
While you're doing all of that, you're bound to come upon passages that are similar to the one you're writing, to serve as a model. How do other composers handle the sort of texture you want to create (for example, the part crossing in the last measure is unusual).
Finally, make friends with a horn player. Don't just ask them to look at the part but ask them to play it for you, so you can hear whether it actually sounds as you imagine it does. They'll tell you if anything doesn't make sense or if you need to be careful about anything.
Those aren't large leaps! A LARGE leap would be something like 2 octaves! Nothing here is a problem. And instrumentalists tolerate 'difficult' intervals much more than vocalists do.
Note, however, that the convention of writing horn parts in open key with accidentals is still widely followed.
No need to be scared of the double-sharps. They actually make this example EASIER to read.
When tempted to under-estimate the agility of brass instruments, Google 'Carnival of Venice'. All college-level players play it.
