Probably Jack O'Connor.
But like most "gun writers" (including O'Connor) I eventually wound up in the job after doing many other kinds of writing. Started out publishing poetry and fiction, but then in my early 20's started selling non-fiction to various magazines, including not just the usual suspects but SPORTS ILLUSTRATED, back when they ran hunting and fishing stuff, and also some general-interest articles, including one feature for NATIONAL GEOGRAPHIC. But in those days I mostly wrote for FIELD & STREAM, GRAY'S SPORTING JOURNAL, SPORTS AFIELD and various flyfishing magazines, rather than shooting magazines.
Started selling a few gun articles in the late 1980's, and in the 1990's more and more. Eventually I was writing more gun stuff than anything else, partly because fishing articles didn't pay much, and partly because gun magazine editors started asking me to.
Most of the other gun writers I know also fell into it in some fashion after doing other stuff, but there are exceptions. Mike Venturino always wanted to be a gun writer ever since he was a teenager, and that's essentially all he's ever done, except for the usual "starter jobs" when he was really young.
