I need to get the column of a dynamic grid

sp_dsBulletinGrid = new Ext.data.JsonStore({
        url: 'sp_BulletinGridProcess.jsp?Action=LoadDinamicDataGrid&airframeId='+sp_airframeId+'&auxiliaryId='+sp_auxiliaryId+'&engineId='+sp_engineId ,
        remoteSort: false,
        root: 'data',
        id: 'bulletinId',
        totalProperty: 'totalProperty',
        listeners: {
            metachange: function(store, meta) {
                var cm = Ext.getCmp('sp_GridBulletin').getColumnModel();
                Ext.getCmp('sp_GridBulletin').reconfigure(store, cm);

 var colModel = new Ext.grid.ColumnModel([]);

    sp_GridBulletin = new Ext.grid.GridPanel({
        id: 'sp_GridBulletin',
        height: 400,
        width: 820,
        title: 'Bulletins',
        store: sp_dsBulletinGrid,
        frame: true,
        resizable: true,
        stripeRows: true,
        colModel: colModel,
        loadMask: true,
        sm : sm