Yes it is acrylic based.
I'm going to guess Vallejo.
The reason is I have had to wait DAYS and DAYS for their primer to properly dry.
I only use enamel based primers now in spray cans. Automotive (NON-FILLER types only! - be careful and read the can, most shops are changing over to "filler primer" as their primary use is for autobody work) grey is good.
If you are using GW Contrast Paints then use their Wraithbone (or if you are really good at fine coats you could use Krylon Fusion (Dover White is a good slightly beige colour for giving warm tones, for instance) - HOWEVER, if you are NOT confident with fine thin coats (it's paint and primer in one so it will go on heavy) just stick with GW's Wraithbone or the automotive).
Krylon Fusion is half the price of GW primers and automotive primer is half the price of Krylon Fusion; at least around here.
Oh! Vallejo has new tinted sprays that can be used for primers, now. Yes, they are enamel. There are also the Army Painter tinted primers, too (enamel).