I dont really know if the sandbags would be all that effective against bullets, against shrapnel sure. The jack plates are a 1/4 inch steel plate they would be very effective.
We were told that there is no uparmor kits for the M931, either get the welder to weld scrap metal re-enforcement or improvise for safety, we chose improvise. This was in March of 03 at the beginning of the war so some things I am sure changed.
Sandbags are VERY effective, somewhere on the internet are guidelines, if I recall 12 inches is supposed to stop anything below .50cal