I do them one at a time when soldering so there is never a chance to short. As soon as one bullet is soldered, I fully heat shrink it to cover the outside. The only way left to connect is from inside the bullet. Then I do the other wire.

You can get these soft caps like used to cover nitro carbs to cap the open bullet while batt is not used. I use same size bullets and pay attention. Obviously plugging them in wrong can happen, I haven't done it. I have a healthy respect for the DC we play with. Screw it up and you're going to fry something!!!

Bullets usually have a hole drilled at 90 degrees to the bullet. I turn this hole so I can see in it when I tin the bullet and so I can see the tinned wire's solder melt when assembling. Then I know its time to remove the heat and let it all fuse together.