First, I find a good side shot of the weapon, a larger resolution is preferred.
I open it in photoshop. I cut the space from the left and right side of the gun.
Then I alter the dimensions, I change the length, and the height should change automatically.
Then comes the hardest part of all, I use the cut tool to cut a printable piece out of the 1:1 gun and print it, then I undo the cutting, and I cut the next part and repeat, I usually go from left to right. After that you get several pieces of the gun, then I tape the pieces together to create a 1:1 poster.