Battery capacity discharge test
Its no good trying to test the capability or current state of your battery with a volt meters.. Volts are only a measure of potential difference and not power.
I would suggest in the first instance you take your battery to a recognised battery supplier and ask him to test it properly under load,,,
You must ensure your battery is fully charged before he can test it....
You might also try swapping the battery out on your Shogun as you know that works.. But be careful because if it is a newer model vehicle your might lose both radio codes and alarm setting by removing the battery.
Finally any battery that is say over 55 amp / hour should be enough to turn your engine over......
Can you turn the motor by hand ?????