Some have contended that shot placement is the most important parameter in determining the lethality of a bullet. Difficulty with the 5.56×45mm NATO at long ranges has been attributed to training; Swedish ISAF units relied on .50 BMG heavy machine guns for long-range shooting due to resilience to factors such as range and wind drift. Underperformance is thus attributed to errors in range and wind estimation, target lead, firing position, and stress under fire, factors that can be resolved through training.