(these strats apply only for XBox360 version of Limbo)

To get the Egg15 you have to switch the boxes, to get the correct box at the guns ... in these 2 videos you can see how to optimize the box switch - to get faster to the guns

Note 1: check the video descriptions for explanations about the movement and precise steps

Note 2: the gun cycle lasts ~13seconds . therefor reaching it sooner is a big time save

Note 3 : it is possible to reach an earlier gun cycle, using the "gap" strat (above) and getting carried up the slope by the box - thus not needing the second gravity switch, since you can already push the box to the gun ... but it's risky and hard to pull off

only took be 5 years :P but here's the early gun and early piston cycle I was talking about :