A group of time-traveling soldiers ends up in 15th century Europe. They have an arsenal of either AR's or SMG's and need to use them until their resources (ammo) run out. How can they maintain the guns and make new supplies while still using them?
To clean a weapon in the field, they need gun oil, rags/patches, and a few simple tools/gadgets. A properly equipped soldier will carry those and clean the weapon after each day's hike through swamp and dust. A soldier would also carry a couple hundred rounds of ammunition per weapon -- five to ten 30-round magazines for a SMG or assault rifle, half a dozen 100 to 200 round belts for a SAW.
- If they store their weapons in a well-made box, they won't have to clean the weapons every day, so their bottle of gun oil will last longer. I would say the supplies they carry with then should be enough to clean the weapon more than a dozen times.
- Parts like springs and firing pins don't break all that often if the weapon is simply stored, and fired and then cleaned afterwards from time to time.
So cleaning supplies won't run out before the ammo does if they take the AR out of storage, fire roughly a magazine worth of ammo, and then clean the weapon.
It would be different if they somehow manage to manufacture new ammunition. This new ammo will be much more corrosive on the barrels, and there will be more of it as well.
Some hobby shooters and hunters do reload their cartridge cases. They collect the empties, replace the percussion primer, fill in powder, and put a new bullet on top. Usually this involves industrial-quality primer, propellant, and bullets ...
- The bullets would probably be unjacketed lead. Hard on the barrels and less accurate in flight.
- A home-made primer would be less reliable, and also corrosive.
- Black powder would be less powerful, further reducing accuracy, and also dirtier.
- The weapons might no longer fire on semiautomatic or automatic.
Pet Peeve: An alternate history is when a general didn't make the blunder he did in the real world, or when the heir to the throne didn't die as an infant, or when the exploration ship got blown off course and discovered a continent early. Modern-day people with modern-day gear in a historical setting are time travel, not alternate history.
Even ignoring maintenance, a modern rifle like an AK 47 can go a few years in decent conditions (open air, not in the rain or particles) before its performance becomes problematic. And even then, a quick cleanup routine would put it back in decent shape. An AK can go months in bad conditions (basically drowned) before it becomes dangerous to use. Ammo can go years before underperforming. To put it in numbers, a decade can go by without use if your characters put them away properly.
But to extend their use, mostly in terms of getting more ammo, is simply not possible using 16th century technology. Even if they could recoup and recharge the shells, medieval tech isnt up to par to get you proper gunpowder needed. A proper chemist and modern processing is needed for that.
Ammo will be the biggest issue, not cleaning supplies or replacement parts. You can use basically any oil in modern firearms, they aren't picky. Snobs will tell you you absolutely have to use XYZ premium oil specially made for firearms, but that's nonsense. Nearly any lubricant will work; vegetable oil, olive oil, even oil from rendered fat.
And modern guns don't even need to be cleaned that often. Look up some mud tests on youtube, even guns notorious for needing maintenance like the M16 are surprisingly tolerant of dirt.
As for replacement parts, once again, they aren't as immediately necessary as ammo. I would expect to get through at least 1,000 rounds before I have to replace anything at all. Probably even 10,000. And some parts should never need replaced; I'd expect an AR15 lower receiver to still be fine for my grandchildren to use for instance.