make sure your bullets are clean and wipe them over with something (IPA, white spirit, panel wipe, whatever). also make sure they are as smooth as possible, maybe give the a very light wet/dry sanding in places. I found even the slightest mark showed through the vinyl. i would definitely recommend a heat gun for when doing the bullets too because you're going to need a lot of heat to mould the vinyl around the lettering. more heat gives more stretch in the vinyl.
remember though to use short bursts of heat so not to 'melt' the vinyl when using a gun. something blunt to push the vinyl around the lettering is also useful.