if the horn in question is a very old one and OP is attempting to play with any modern long shanked mouthpiece (even the ubiquitous Selmer S80) these...
this Baritone was originally advertised as being playing “ in tune” NOT in the key of Eb but in E.... NOT!, now the seller has reformed his ad and is...
If you are pulling your mouthpiece too much there may be better explanation and cures than finding a mouthpiece “ which tends to play flat” (if such...