Factual error: Walt's medical records contain an error - on the left side of the folder, his address is listed as "Detroit, CA" and it's correctly listed as "Detroit, MI" on the right side, although as pointed out in another entry, the house number is completely wrong in the medical records.
Other mistake: When Walt and Thao go into the barbershop, the barber gets up and puts on his barber jacket and zips it up. As the camera returns to the barber each time, the jacket zipper is at a different location.
Character mistake: The information on Walter's "patient information sheet" lists his address as 5962 Delco St but numerous shots indicate his house number is 238. (01:03:25)
Continuity mistake: When Tau is working under the rain, Walt is staring at him over his house. It's pouring heavily, but on the reflection of the window just behind him you can see the bluest sky.
Continuity mistake: Eastwood is chatting with the priest at the bar and holds his hands together on the table. When the angle changes they're spread apart.
Continuity mistake: While Eastwood talks with the priest at the bar he grabs a glass, raises his right arm and lowers his head towards the drink. From the angle behind his arm is down and the head is up, as if he wasn't holding anything.
Character mistake: When Walt is fixing the washing machine in the basement, he adjusts the wrong corner of it to stop it from wobbling.
Continuity mistake: Near the end of the scene where Walt scares the Asian gangsters away from his lawn, when they are running away to their car, there's a shot behind Walt where he's seen putting down his rifle. However, immediately after that, Walt is shown from the front, still holding up his rifle and pointing at the Asians.
Continuity mistake: When the neighbours leave presents by Eastwood's door, a pink parcel appears tilted at first but straight a second later.
Continuity mistake: Near the end of the movie, Tau comes over to Walt 's house at 4pm. After locking Tau up in the room and leaving it is broad daylight. However, by the time Walt gets over to the gang banger's apartment it is pitch dark. Couldn't have taken more than 60 minutes to get there even with stopping to call Tau's sister. Detroit is in Eastern time so it doesn't get pitch dark until at least 7pm, later in daylight savings time, which is probably what time of year it was since it was obviously warm weather.
Answer: Well, a lot of veterans actually buy their weapons once they have retired, since they have the constitutional right to keep and bear those weapons. However, I don't think it is said in the movie that those arms were exactly the ones Kowalsky used during the war.
Actually, yes he did say that it was the one he carried. When Tao asked what it was like to kill a man, Walt says I shot the kid in the face with that rifle you were holding earlier.