NO, Springfield armory saint pistol IS NOT legal in MA. To e legal, it must be on this list of Bud’s approved handguns for MA. Check here for law ( budsgunshop.com/feeds/state_reg/massachusetts_restrictions.pdf ). and here for approval list ( budsgunshop.com/search.php/exclusive/mass/page/2 ). I hope this helps.

You can go to a local ffl and see if you can find an AR pistol (an Ar15 with a barrel under 16" and a brace instead of a stock is considered a pistol)with a fixed magazine, the fixed magazine would make it technically not an "assault weapon" but you are going to be hard pressed to find one online that will meet all the requirements.
I was just watching videos on cape gunworks youtube channel he is a MA guy and does alot of videos about MA compliant firearms.
Watch this video here, https://youtu.be/FmKV7hz_zyI
he goes over what AR type guns you can legally own
They sell some online and have a brick and mortor store in Cape Gun Works
96 Airport Rd
Hayannis, MA 02601
(508) 771-3600
info@capegunworks.com
https://shop.capegunworks.com/

Christopher, unfortunately is is not. Details under Handguns here : budsgunshop.com/feeds/state_reg/massachusetts_restrictions.pdf