"Muslin" is almost a generic term. It can be very coarsely/loosely woven - or it can be very smooth and luxurious feeling.
The Legacy muslin from JoAnn feels "wonderful" - but it does shrink.
So I strongly recommend soaking it in hot water for at least a couple of hours, and then washing it gently before using it.
I also purchased some Roc-Lon muslin that said "pre-shrunk" on the label on the end of the bolt -
I did my usual soak and wash - it shrank a lot!