constrain vs shackle

constrain is not a noun while shackle is a noun.

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

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

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

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

constrain (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

constrain (verb)	shackle (verb)
severely restrict in scope or extent	restrain with fetters
to close within bounds, or otherwise limit or deprive of free movement	bind the arms of
compel to behave in a certain way