pojim:
You are absolutely right, of course, that one ought to practice the vital first shot, which will be DA.
At the same time, since the subsequent shots will be SA, one really ought to practice the SA as well, IMHO.
What is more, if one is really serious, one ought to practice the transition (first shot DA, second shot SA).
Either way, as you point out, SIG's do not require partial retraction of the slide like Glocks to dry-fire.
Skorzeny
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For to win one hundred victories in one hundred battles is not the acme of skill. To subdue the enemy without fighting is the supreme excellence. Sun Tzu