Typical original (top) and current stock (bottom) bullet connectors. The original have rubber sleeves slightly smaller than the metal part so is a tight fit. Modern stuff uses plastic sleeves of these same size so the metal part can easily slide out of the ensulator and short out on surrounding components.

Original connectors have a pronounced dimple that the head of the bullet must be pushed past to snap into position, which also retains it very firmly. Modern connectors have a much less pronounced projection so the bullet can be pulled out more easily.

Original connectors have another projection in the middle of the metal part (arrowed) which helps prevent one bullet being pushed too far in, so preventing its opposite number being pushed in far enough. Modern connectors don't have this.

For easy of assembling cut a notch in the handles of a pair of pliers just large enough to slip over the wire but press on the back of the bullet.

Make sure the closed distance between the handles is less than the length of the metal part of the connector or the bullets won't be pushed in far enough.

Preparing to push the bullets in ...

... squeeze until the outer covering is compressed ...

... and the back of the bullet is flush with the end of the metal part.

'Odd' bullets i.e. only 3 wires are dealt with in the same way. Finally make sure the metal part is centralised in the outer covering

Various after-market crimp bullets (which don't fit the original bullet connectors) and spades, and an after-market solder bullet which does fit the original connectors.