How close is the artillery to the logistics and resupply chain anyway? Considering its preferred placement in the field, far enough behind the front line to not be threatened by direct attack but close enough that its (formidable) gun ranges can put shells on the enemy, I'd think you'd be a lot closer to the supply side of things and a lot less concerned about what's available at hand than, for example, said tanks or infantry. Granted, there's counterbattery fire, and EVERYONE has to worry about air attack, but I'd expect a battery to have its own resupply sites nearby and a close connection to the mainline.
Again, perfect world, at least.
Now that is an astute question as a lot of it gets into, organic capability combat trains, field trains, logistics units, distribution nodes, etc. Years ago on these forums, I posted a picture of my battery getting resupplied at Nasiriyah in '03. If I recall, you could see two guns (M198s) with prime movers (7-tons), their ammo trucks (also 7-tons but trailers), and the battalion combat train (of which an LVS with Mk14(?) trailer was featured). Basically, my battery carried a basic load of ammunition (800 to 900 rounds), and so did the battalion which worked out to a three battery battalion have four batteries worth of ammunition with the last being in its combat train. The regimental field train would likely have a similar ratio and that would increase going further back the logistics train. Factored into that is the rate of consumption which is an average over time by activity.
If you are wondering, "aren't they guessing at this point," well honestly I've thought the same but if there is enough depth further up the log chain you end up gaining time to make up the difference. Case in point, "we" guessed wrong at Nasiriyah and ended up loaded for a medium to long range anti-armor fight but ended up in a short ranged anti-personnel fight. Welp, in that instance you end up shooting Rocket Assisted Projectiles with the rocket
OFF with your shortest ranged propellants because shooting all of the anti-armor ammunition would be less effective.
Anyway, fun times.
Please note, I haven't even touched on your (paraphrasing), "how do I protect it question," because making it happen is the first objective. It is at this point, I wonder if I've glazed some eyes and/or not been helpful.