Oh Husky, you silly, silly kid you.
Hey, so here's a tip when using SAI! If you wanna get straight lines for things like weapons, there's a vector tool you can use. If you look at the layers tab, there should be a new layer option that has a pen tool over it - that's called a vector layer. It will allow you to use the vector tool, which will let you do straight lines, curved lines, or just normally drawn lines that the program will smooth out for you. It's pretty useful!
But if you can't get that to match up with your normal drawing, there's also the option of turning the picture. I don't know about you, but I have an easier time drawing a straight line when it's vertical. So, if you look at the window that shows the overview of your picture, there should be a line that says "Angle" above it. You can use this to adjust the angle so you can draw comfortably, and then simply hit the square button to put it back at its original angle again.
But that's just what I do
Keep playing around with it! It looks like you're learning fast~