ok um in entity go to the model tag and click the model than go to the blue circle ithink in the upper right hand corner click it and do import as obj
heres a rather simple tutorial
1.Open your map and go to the mode tag
2.Click your model and the go to the blue circle in the right hand corner
3.Click export as obj
4.Now you saved it
For the best model of a weapon do the First person model.
5.Open the biggest numbered version