Now, the Big Guns: First Bank System Inc. and Wells Fargo & Co. have stopped sniping at each other in their takeover fight for First Interstate Bancorp. Now they’re firing cannons. First Interstate two weeks ago chose Minneapolis-based First Bank as its “white knight” merger partner to fend off a hostile advance by San Francisco-based Wells Fargo. As the rivals jockey to line up shareholder support for their bids, the level of invective is sure to rise. This week, Wells Fargo is expected to start buying lots of First Interstate stock, having won permission from federal regulators to acquire up to 4.9% of the Los Angeles-based bank. Both Wells and First Bank will be lobbying investors in ways public and private. Last Friday, for example, First Bank took out full-page ads in California’s biggest newspapers to attack Wells Fargo. With a shareholder vote unlikely before late winter, the war has barely begun.