I went through the same thing and found the best solution was make some racks for the rifles. kind of a W formation covered in foam that goes on the front carrier.(holds two rifles) and a bungy covered in foam to hold them over rough stuff.
The problem I found with "normal" atv gun racks were they wernt wide enough for the fore ends.
At one stage the rifles were carried in pastic cases with breaks off, but this soon got tedious, for that reason and because plastic cases slide easily even tied down well.
The down side to this is that the racks dont hold sporter rifles all that well but I normally sling them over my back any way, and you have to be aware that the barrels can stick out a long way out the side.
I hope this pic fills in the blanks of my desciption, oh and the brackets are held on with 20mm U bolts.
BTW this with my Tikka Lammy T3 300WSM so it dosnt fit so well