Yeah Java sucks doesn't it? It is so outdated compared to new stuff, but it will take people time to learn the new stuff so we're stuck
The good thing is that you can find cartridge diagrams from many different sources without needing Java
My Hornady manual implies the two calibers are interchangeable. It has two different listings for the cartridges, but it should be noted that the 30 Mauser listing states that Hornady produced a run of 30 Mauser ammo "at the request of Graf & Sons, and Hornady will be supporting this run with [components and data]". In other words, the current 30 Mauser ammo and data is their baby.
A few measurements:
30 Mauser / 7.62x25
.990 / .984
.393 / .392
Length to shoulder
.759 / .772
Length to neck
.831 / .828
.333 / .334
Length to shoulder will be the big one. It looks as if 7.62x25 chambers can load 30 Mauser cartridges, but .30 Mauser chambers can not load 7.62x25 cartridges. Another thought - if a military-grade chamber is used, you can be fairly certain it is "loose" to ensure functionality at the expense of sacrificing grouping/accuracy. Therefore, I would attempt to find out if they can be interchanged by hand-fitting into the barrel in question.
The 7.62x25 is a little hotter, and holds a few grains more than .30 Mauser. Thus, some lighter-weight .30 Mauser actions may be stressed by shooting 7.62x25.
Anther thing of note is that the .30 Mauser bullets are shown to be only 86 grains, and the 7.62x25 is shown to only be 90 grains. I thought there was a 115 grain as well, but I don't see it in this manual.
Food for thought.