What is the scientific premise for 'lack of nickel' in the wound?

If it's anything other than the possibility of a difference of metallurgy in the bullet from the rest of the shooter's ammunition (which is not germane to whether or not the rifling marks on the bullet match the gun), I'd say a defense lawyer is attempting to blow smoke.

Assuming this is serious. Not a forensic/wound expert but spent 23 years working in a metallurgical lab. Nickel? I will assume the argument is the bullet could not have come from the Browning Nickel Steel barrel due to a lack of Nickel contamination of the wound. OMG/WTF/ROTFLMAO What method did this shylock use to determine the lack of nickel in the wound. 1. The Browning Nickel steel barrel alloy is what I would consider a low alloy steel with a Nickel content of 2.5 - 3.75%. The nickel is added as an alloy element to increase strength with the added benefit of an increase in corrosion resistance 2. All small arms barrel steels while being relatively soft to facilitate machining and rifling processes are by necessity extremely hard when compared to the lead projectile. How else can thousands of rounds of .22 caliber swaged lead bullets be fired through plain and nickel steel barrels with no appreciable wear. 99.75% lead has a Brinell Hardness Number (BHN) of 3.2 - 4.5, A nickel steel barrel will be 180 - 220 BHN.* 3. Lead is deposited in a barrel primarily from vaporization of the exposed bullet base to the hot gasses generated in the exothermic chemical reaction of the burning powder with a subsequent condensation of lead vapors onto the cooler steel. Another small amount of lead may be deposited due to frictional forces, heat and microscopic imperfections in the barrel scraping away lead. The lead projectile does not scrape away steel with each shot fired. No detectable** amounts of nickel will be transferred to the lead projectile and be evidenced in a wound. 4. ?? Can O Worms ?? Lack of nickel does not prove the Browning was not the firearm used. Not only could the bullet have come from the Browning with no detectable Nickel contamination it could have come from any .22.
* ASME Metals Handbook ** I operated spark emission spectrometers, X-Ray fluorescence spectrographs, Electron microscope with Energy Dispersive Spectrograph, and Inductively coupled plasma atomic emission spectroscope. The reliable detection limits of our equipment for a single element in a matrix was at best 0.0005%. I could find nickel in the bullet, but any nickel present could be attributed to trace elements.

Sounds like fertilizer to me.