You can grab test development builds here -
http://megamek.org:9080/BUT they will break Campaign files, will crash, do unexpected things so make sure of the the following.
1) Always install the daily build to a separate folder than your normal MegaMek install.
2) Only
copy your customs and save to the Development build folder. Once those files are copied
don't ever move them back to a regular build.
3) If you do find bugs or weirdness you should be part of the Slack channel to ask if it really a bug or not (
http://megamek.org:3000/)
4) If you need to report a bug please follow this -
http://megamek.org:9080/ AND make sure you put the day of the build you used.
5) Lastly DON"T EVER CROSS the streams ;)
Other than that have fun...