If its in good working order there is a ratchet on the opening (you should hear it) and on fully open there is a an anti bear trap device so even if you touched the trigger it can not snap shut, if that is not operative then its dangerous to use any way and needs repair.
Its simple to check, pull the lever back, you should hear clicks, and if you stop at any point it should stay put, and not be able to be returned, only when it is fully open, that is the only point when you can now shut it. To check, with it open, hold the lever, (Just in case the anti bear trap is inoperative!) and pull the trigger, nothing should happen. If that is the case then your 300 is Ok and safe, any thing else, i.e. it tries to fire, then take it to a competent repairer (Wonky Donkey on here).

Many thousands of shooters have safely used the 300 for 50 years, and back in the day there was little else that was competitive, and I've been to Internationals back in the 70's when they were near enough the only rifles there, and I've never seen one snap close, as long as its not been in the hands of a "garden shed expert" it should be OK.

Have Fun
Robin