Hydrochloric acid would be the fluid, not exactly sure what glass so let's just assume something optimal for this (I don't know my glasses very well), and the metal would be copper.
Preferably a smooth jacket around the glass spheroid.
apologies if this is already answered--
EASY. That's how much difficulty.
Process:
Modification of "silver mirror" technique, followed by electroplating.
Step 1) In a beaker, combine dextrose and distilled ammonia. In another beaker, pour some silver nitrate solution.
Step 2) Combine the beakers in a disposable bucket and stir vigorously. At first, nothing seems to happen, but then it starts to turn yellow/brown. WORK QUICKLY.
Step 3) Swish the solution over the glass spheroid for several minutes, until a suitably thick layer of metallic silver is deposited. It will be mirror shiny.
step 4) Coat the silver sphere with a copper jacket via electroplating in a copper sulfate bath. The thin coating of silver deposited previously will allow further coating via electroplating.
To fill the spherical ampoule with HCl and then seal it-- I suggest using borosilicate glass, then using a fast laser sintering technique to seal the spheroid. The above process assumes already filled and sealed glass ampoules.
However, this will not be very good for the purpose you desire. The bullets will still tend to shatter in the barrel of the gun, even with a really thick copper rind.
What you are really looking for, if you want an anti-mech round, is NOT acid. What you REALLY want is a mini-thermite charge, and those are already commercially available. High explosive incendiary rounds would also be a good choice.