Does anyone make a 5 shot flush fitting magazine that fits the AR type magazine well?

Will it be ok with low rings or does it need a stock riser?

Will it shoot 55 & 60 grain bullets accurately enough with the 1-8 twist?

I don't know the answer to the first two, but the answer to the last is that an 8 twist can work with 55 and 60 grain bullets quite well. I suspect it'll be happy with anything from 50-70.

Didn't the Colt AR15 used to come from the factory with flush fitting mags, 10 shot maybe?

I have seen images of a flush 5 round, but don't know who makes it.

#1 No

#2 Scope dependent

#3 Yes, so long as it's an accurate rifle.

Mine came with a six round magazine that extends about an inch.