restrain vs shackle

restrain is not a noun while shackle is a noun.

Word	Noun	Adjective	Verb	Adverb
restrain	No	No	Yes	No
shackle	Yes	No	Yes	No

As verbs, shackle is a hyponym of restrain; that is, shackle is a word with a more specific, narrower meaning than restrain:

restrain: to close within bounds, or otherwise limit or deprive of free movement
shackle: restrain with fetters

Other hyponyms of restrain include bind, tie down, tie up, truss, fetter, enchain, pinion, impound, pound, pound, pound up, fold, pen up, ground.

restrain (noun)	shackle (noun)
	a restraint that confines or restricts freedom (especially something used to tie down or restrain a prisoner)
	a U-shaped bar; the open end can be passed through chain links and closed with a bar

restrain (verb)	shackle (verb)
prevent the action or expression of	restrain with fetters
to close within bounds, or otherwise limit or deprive of free movement	bind the arms of
restrict (someone or something) so as to make free movement difficult
prevent (someone or something) from doing something