Create a responsive grid border system - for when you want borders on all middle items, but not the last one. Assumed imports of bootstrap and the use of the common block-grid system.

grid-border.scss

@mixin grid-border($subSelector, $color, $direction: 'right'){
    @each $key, $width in $grid-breakpoints {
        @media(min-width: $width) {
            @for $i from 12 through 1 {
                &.block-grid-#{$key}-#{$i} {
                    > #{$subSelector}:not(:nth-child(#{$i}n)) {
                        border-#{$direction}: 1px solid #{$color};
                    }
                    > #{$subSelector}:nth-child(#{$i}n){
                        border-#{$direction}: 0;
                    }
                }
            }
        }
    }
}

.grid-border-right {
  @include grid-border('.block-grid-item', black, 'right');
}