Malcolm in the Middle is an American television sitcom that followed the misadventures of a dysfuntional lower middle class family and their teenaged son Malcolm, a young savant with an IQ of 160
What appears to be a Colt Detective Special is seen used by One of the pair of criminals robbing Lucky Aid in "Robbery" (S2E07). His Colt is a snub nose with ivory grips.
Malcolm (Frankie Muniz) finds what appears to be a Walther TPH in an escort's purse in "Poker 2" (S3E13). A nickel plated version is first digitally imposed into the purse using special effects, but later appears in the same episode with factory standard coating and a taped grip.
Luger P08
Otto Mannkusser (Kenneth Mars) carries a Luger P08 into the desert while accompanying Hal in "Boys at Ranch" (S5E08).
Of the two shotguns used in the show, the Remington 870 appears most frequently. Shotguns are commonly used for ranch work and home defense throughout the show.
Mossberg 500
The Mossberg 500 is the second shotgun seen in Malcolm in the Middle, and is often found in tandem with the Remington 870. The props department most likely only used these two models, as they were reused many times regardless of the show's location.
Rifles
M4 Carbine
Reese(Justin Berfield) wields an M4 Carbine rifle with the MILES equipment during his training in "Reese Joins the army Part 2" (S5E22). He also uses the rifle without the training system in "Reese comes home" (S6E01).
Other
Zoo Rifle Cabinet
In "Zoo" (S4E01), numerous firearms, including a dart gun, can be seen behind Lois' ex lover.
Pawn Shop Display Cabinet
A rifle cabinet can be seen in the pawn shop in "Motivational Speaker" (S6e19).
M26C Taser
A Police officer deploys an M26C Taser on Hal in "Reese Joins the Army Part 1 (S5E21)