When animals are inadequately stunned before slaughter there are serious implications for animal welfare. The most common mechanism for cattle stunning is induced concussion by a captive-fired or pneumatic-fired bolt into the forehead. Careful observation of behavioral symptoms is required to determine if animals are adequately stunned. The aim of this study was to develop a standardized protocol for assessing stun quality in abattoirs. This included a list of behavioral symptoms that cattle display at stunning which classified adequate or inadequate stunning. Inadequate stun behaviors included display of corneal reflex, blinking, breathing, any combination of eyeball rotations, eyeball twitching, nostril flaring, groaning or severe righting reflex. Two-thousand-eight-hundred-and-two cattle (1250 bulls, 1330 cows, 124 steers, and 98 calves) in 5 abattoirs in Sweden were assessed. Stun quality was recorded as a binary outcome (inadequate; adequate). The effect of weapon type (captive bolt or pneumatic) and of cattle sex on stun quality in individual animals was assessed separately using z-test and Chi-square-test, without accounting for clustering by abattoir. The percentage of animals poorly stunned varied significantly between sexes (Chi2=159; 3 df; P<0.0001), with the highest percentage in bulls (13%). Percentages for cows, steers and calves were 1.4, 2.4 and 4.1%, respectively. Captive bolt gun was used in 2306 cases and pneumatic gun in 496 cases. The percentage of inadequate stun was greater with captive bolt (8.0%) than with pneumatic gun (0.81%) and the difference was statistically significant (z=5.8; P<0.0001). Reasons for poor stunning were attributed to use of worn out weapons, damp ammunition and weapons with insufficient power for stunning bulls. In one abattoir, 15% of the bulls showed poor stun quality even though they were shot accurately. The assessments provided abattoirs with valuable information on stun quality status and highlighted the need for better weapons to ensure adequate stunning in bulls.