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Try sewing a large button onto the shoulder of your coat, or top of daypack strap to prevent sling from sliding off your shoulder.
I just front carry mine like a 2pt AR tactical sling. I zip the sling up tight to my chest, the gun comes up very quickly, and I use the sling to stabilize. Its like a v-tac with a stretchy part so you can punch out. Only draw back is you can punch a longer barreled gun into the mud, so wise to cap it with electrical tape. Pretty much never sling it over one shoulder.
When you want hands free simply run your pack sternum strap under your rifle sling.
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