rcv'd my D10 today. totally different in the "feel" of the operation of this light.
lubed up the threads on the head & the o-ring.
w/o removing any o-rings, or lubing up the piston, this light is so very smooth and MUCH, MUCH easier to operate cp. to my EX10 as regards the amt. of force req'd to actuate the piston. plus, there is NO BINDING of the piston protruding through the body of the light.
the reduction in force req'd to operate it is largely (or entirely???) due to the lack of binding/sticking when trying to get the piston to begin moving fr/its resting position. i do NOT believe that it is an issue of o-ring diameter and surface area related to that diameter.
the head is also easier to turn than my EX10 which was a "bear" also (a two-hand job as cp. to this D10 which is oh, so easy with just thumb and forefinger; but NOT too easy so as to accidentally turn ON) which might be o-ring related. my thoughts are more toward o-ring & head I.D. (inner diameter) tolerances in my particular light than o-ring diameter cp. to the larger diameter EX10 o-ring since torque differences based upon the head diameter makes this seem less likely to my way of thinking - which could be wrong; wouldn't be the first time!.
the differences b/t my EX10 and D10 are so great in both head twisting force and the force req'd to initiate the piston travel, that there is definitely something amiss w/my EX10. as stated earlier, it is binding (and i forgot to mention "clicking" when it first starts to move, as well as when it returns to its resting position) where the piston top protrudes through the light's body. FWIW, my NE head is also very, very tight and even after lubing, if the o-ring is left in place, is a two-hand twist operation.
just thought i would update my experiences w/the latest NiteCore offerings.