Hi notsure,first glance it does look like the heel of a trainer,but using my magnifier it resembles a small jug if you're looking head-on you can make out the pouring end----also it has a sheen.
If it is an ornamental jug it could have been on the window sill,then swiped off while putting the rifle there.There seemed to have been all kinds of jumble in that room. Disorganised like the minds of the women. Kitchen/pantry was the same.