A new product on the market, and a game changer, is the Versus M1T mellophone mouthpiece.
Mellophone is, sadly, a hugely polarizing topic of the horn world, but certainly a reality that is here to stay. Among the biggest typical complaints for the horn player is the mouthpiece. Marching mellophones come with a mouthpiece that resembles very much a trumpet mouthpiece, with a trumpet rim, small inner diameter, and a shallow cup. As a result, the experience of playing an instrument with one of these is more like playing a trumpet than a middle brass instrument (such as a horn or an Eb tenor horn).
In addition, of course, marching band directors are not fond of using a horn mouthpiece with an adapter on F mellophone. Perception is that it does not sound as good, and intonation is worse.
I have long advocated for the development of something that blended a horn rim and cup into a backbore and shank that fits right into a marching mellophone receiver. These have been produced, but only on a limited basis. I am thinking specifically of the horn/mellophone hybrid mouthpiece supplied with the Jupiter Quantum mellophone (reviewed here, and also mentioned in the 3rd edition of my mellophone book) which so far as I know has never been offered for separate sale.
Finally, on the market for easy purchase is a mouthpiece of this type, the Versus M1T mellophone mouthpiece, sold by Houghton horns. I was sent this mouthpiece and the companion M1H (which is meant for Bb marching French horn, but would work fine F marching French horn or any horn really) for review, at no obligation.
I especially like the M1T on my King mellophone. Sound wise it hits a nice place, some color to the sound but with depth. The cup of this mouthpiece is shallow for a horn mouthpiece, and provides nice focus and articulation. The rim is comfortable and supportive, wide but not overly wide like a trumpet rim.
For those more interested in the nuts and bolts details, my example of the M1T mouthpiece has a #9 bore, while the companion M1H has a tighter bore at #17. Inner dimeter perceptions will vary with any rim depending on the shape, but I would put it in the 17.75-18 mm range, which should produce a big sound out on the field. The shallow cup seems to me to be a great acoustical match to the high F horn. It plays easily.
I also think this might be the first mouthpiece of this type that has been offered for separate sale.** Finally! This is such a game changer. If you are a horn player who marches mellophone, or a horn teacher with students who march, give this model a look, priced very affordably at $89. For more information on this new product, see the Houghton Horns website.
**Another mouthpiece some might think of is the Moosewood T-Rex. This was a hybrid mouthpiece intended for horn players to use on the Eb tenor horn, which is so far as I know off the market now. It is deeper yet, with a larger bore, and not very suited to the mellophone.