Got myself a new Makita 2630 SDS+ Hammer drill. (First experience of sds) When I wish to use the quick change drill attachment for normal hss drilling I push the attachment into the sds chuck. It positively engages and locks but can move along the longitudinal axis by 0.250″. Is the movement normal before I start using it and have to send it back? If so what is the purpose of this float?

Normal for an sds

If so what is the purpose of this float?

It doest have a purpose its just a byproduct of what the SDS system is – holding a drill bit loosely so that it can be rattled about. The float is part of how the whole bit/chuck system operates. Sticking an additional chuck in there means that chuck itself is held the same way and SDS bit would be – with freedom to move back and forth. Its of no consequence as for non-hammer drilling (which is the only drilling you’ll be doing with the chuck installed) you’ll always have weight behind the drill so the chuck won’t actually move about in use exempt when you are backing out of the hole.

Surely you’ve turn the switch to just drill. To the left in the pic.

Slop is fine if it was friction tight you would never get the drill bit out after using the hammer option.

Surely you’ve turn the switch to just drill. To the left in the pic.

Indeed but not all drill bits are SDS compatible so the attachment the OP is (I’m presuming) referring a standard chuck on an SDS adaptor like so

When I wish to use the quick change drill attachment

it has the same sliding keyway which is what allows the SDS drill bit to move back and forth. Even when set to non-hammer the bits (and any adaptor) have that freedom to move.