The endcap doesn't come off of the TM SD front end. The supressor itself does though, it's not threads, it's a dual lug style attachment on it that loosens after a while (plastic on plastic). On mine, if I twist it on the full 180deg, it's loose, so I only twist it about 90deg to keep it on tight.
If you need to shorten it, best case is to cut a slice out of the middle of it, then glue back together, and buy some wide black heatshrink to finish the supressor end. That'll cover up any screw ups you did and the glue seam.
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