Most missions will take you out of defensive mode, and put you at risk of player engagement. I know it's kind of beating a dead horse, but i've always been against develeoper implemented limitations like that, because it stifles player interaction. Plus, that's the way the game is meant to be played. I like the risk that missions have. Knowing that you could get shot anytime helps to keep it fresh and keeps me on my toes.
Also, level doesn't really have much to do with it. Yeah, in those first 20 or so levels you're kind of in a rut, because you don't have the experience you need to be proficient at PVP. But while carbine repeater leaves much to be desired, the cattleman is a solid option, and if you do the Land of Opportunities missions, you will have enough cash to purchase yourself a replacement long arm. I would reccomend either the lancaster or the Bolt action, as both are pretty good all rounders in PVP.