I've not done any serious research on the Sturmbataillone in more than twenty years. However, precis of the wartime careers of individual soldiers suggest that the Sturmbataillone did, in fact, accept volunteers from other units. In particular, if the anecdotes I remember are at all typical, former cavalrymen were well represented. Some Sturmbataillone, moreover, had originally been created at the front from men already at the front. Once they were formally established, these battalions were linked to Ersatz organizations at home. The original members of the unit, however, would probably have come from a variety of arms and regiments. The assault units that failed to become full-fledged Sturmbataillone, many of which existed for relatively short periods of time, necessarily drew their men from units already at the front. Of course, the quality of these ad hoc units must have varied greatly, but that is another story. By the way, Chis is quite right about my Storm Troop Tactics being outdated. If I were to write it today, it would be a very different book. Of course, this doesn't mean that you shouldn't go out and buy it. After all, I have two children at university! ;)