Joel is siphoning gas out of an abandoned car. 20 year old gas would be decomposed into varnish and would quickly destroy the engine of any vehicle in which it was used.
New graves dug out of hard dirt, the size of the hardened dirt is like plus 25%. Add in the size of the body, and it's like +50% for even a fairly shallow grave. Digging into hard dirt, adding a body, and patting it down flat with no leftover? It's why even shallow graves have a body shaped rounded mound over them.
Presumably they want to directly show the audience that he is out of ammunition and his situation now hopeless. Perry's rifle audibly clicks twice from trigger pulls when out of ammunition fighting the monsters. An AR of course would not work like that. After the last shot, the bolt would lock to the rear and the trigger could not be pulled. Even if he manually released the bolt forward, which you don't see him do, the trigger could be pulled once not twice.
When Joel is using the rifle from the cul-de-sac sniper there are a few times he rapidly fires more than six shots without reloading. The rifle is a Winchester Model 70, which has a magazine capacity of either 5, 4 or 3 rounds depending on the caliber. This one appears to be a .30-06 Springfield, which has a 5+1 capacity, meaning it holds five rounds in the magazine and one in the chamber, for a total capacity of six rounds.
Joel is giving Ellie and the others covering fire on the street using the rifle he took off the sniper in a house at the end of a cul-de-sac. Joel is shown firing the rifle several times, and each time there is no visible recoil. The rifle is a Winchester Model 70, judging from the size of the cartridge cases it is chambered in .30-06 Springfield, a high-power rifle cartridge mainly used for hunting big game animals like bear, deer and elk. While it is not considered to have a high amount of recoil, it also doesn't have a light recoil either. So there would be some noticeable kickback and movement from the rifle's recoil when it's fired.