I would like to know How big a box, You know 12x30 or so, I would need to be able to shoot threw. Say A 300 Win going 3030fps with 180 bullet. And me standing back 50 yds from the box

It will depend on the bullet you use. Some do better than others. Do you want the bullet to exit or do you want to stop and recover it?

IIRC in gunsmithing school they said 6' of cotton waste would stop them . A heavy slower bullet like a 458 Win is more difficult to stop than a high velocity lighter bullet such as the 300 Win.

Dave blew through 6' of wet newprint and through the backboard...
a 300 win is NOT a bigbore

I didn't know that a 300 win was a big bore...

Six feet of wet newspring? 6' (that is feet)

How about 6 inches. I don't think any bullet will penetrate 6 ft. of wet newsprint, however I have not shot a 50 BMG into wet paper.

Wet news paper is a piss poor bullet stopper.

Can't remember the last time I was atttacked by a bunch of angry phonebooks in a rain storm.