From my experience, these cases for bullet connectors are a pain to work with. If you spill a tiny bit of solder on the outside of the connector, it won't fit into the case anymore.
I'm using + male and - female 4 mm bullets on my batteries, but if it has to be 100% foolproof, use female ultra plugs.
Thomas, what chassis is that car in your avatar? PM
thanks
“The modern astrophysical concept that ascribes the sun’s energy to thermonuclear reactions deep in the solar interior is contradicted by nearly every observable aspect of the sun.” —Ralph E. Juergens