Because trace hit weapons are calculated using a straight line from one point to another and then see if it intersects anything along the way (namely another player or something to block the fire like a wall) This vector can't jump across the grid. When you press the fire button it marks your location, Then sees the direction you are firing and then just like on a sheet of paper, draws a straight line from your location forward. When it hits something solid it stops and then returns that location. Now that it has a start and end location it returns those values to play animations and to determine what happens at the ending location (Either to just stop, damage something, or whatever) When you fire a "Trace"hit weapon into a portal then the line again is drawn and it stops at the back wall of the portal because that is the first solid object it hits along its set straight line path. Tracehit weapons are totally done with math calculations and there is never any actual projectile. Projectiles are indendent actors that have their own code governing their physics so they are able to fly through portals on their own because they are fire and forget. Once you fire then the code for that weapon is no longer conerned with what happens to the projectile because it takes care of itself.