.310. Call Ruger to verify.
Best to let a gunsmith help you.
Pull and hold the trigger, then pull the bolt back and it should release.
100-400 USD
Primarily in the magazines; Mark II has a bolt hol open also
You can request a owner's manual at the Ruger website.
100-450
$350-$500, depending on the exact version, and the condition. The rifle is a Number 1, not Mark 1. Only Ruger pistols are Marks.
Probably about $300 - $400
100-560 or so
100-650 usd
75-170 USD