To rehearse the placement of said gun and its proximity to Samuel L. Jackson’s nether regions, Pam Grier had to put her hand between his legs and, well, she accidentally felt him up.
For insurance reasons, Pam Grier couldn’t use the gun until actually filming the scene, so that necessitated her just using her hand as if she had a gun in it.
In the scene, Ordell Robbie gets the tables turned on him and Jackie Brown, as the former puts it, has “a gun pressed up against my dick.”