LAZY EYED SNIPER said:No experience with the S&J myself, but someone should be along shortly.
I use a Nordic Components +2 extension and it's been great. Just stay away from Choate, there's plenty of reading on reasons why around the forum...
LAZY EYED SNIPER said:I use the NC +2 on my 18.5" 930 and it's about 3/4" short of the barrel's end. I have the breacher barrel though, so the overall dimensions may differ from your 590a1.
My NC +2 is exactly 15cm long, so taking a measurement from the base of your tube's threads to the end of the barrel should give you an idea if it'll work for you...
By the way, welcome to the forum!