When we think of a masculine trait the first one most will think of is Strong. Our men must be strong. There are many meanings behind this one word. They need to be mentally strong, as in showing no weakness emotionally and physically. Crying is a huge no no and shouldn't even be considered. This is an emotional weakness that shouldn't be shown and therefore by not crying they are seen as strong.
Now there is also the physical and literal meaning of strong as in how much can you lift. This translates into gym work, muscles, and violent tendencies. These violent tendencies are cultivating through their media consumtion.
http://www.huffingtonpost.com/emma-gray/bushmaster-rifle-ad-masculinity-gun-violence-newtown-adam-lanza_b_2317924.html
This ad specifically targets men. This is obvious through the usage of the 'man card' comment. Now, what does this teach young boys? This teaches them that firearms are manly. What are firearms primary purpose? Specifically, what are assault weapons primary purpose? Its violence incarnate. We are teaching young boys to pursue violent hobbies in order to be a man. That one must love guns and own them in order to be a man and to have a 'man card'.
There are many other facets of this problem. The genderization of movie genres, the popularization of war games/first person shooters, the expectation of men to be strong and fight other men to be seen as manly are all other things that turn young boys into violent men. But this is a start to get your feet wet after all.
