Yes, I'm 100% sure. A lot of dealers aren't experts on the law, so they error on the side of caution. Trust me, the Maryland State Police and the Maryland Legislature are very clear on this. As long as the AR15 has a factory installed heavy barrel (HBAR), it's non-regulated.
There are at least two dealers I know of are up on the law enough to sell you what you want. One is Shooter's Discount @ (410)247-6757 and the other is Valley Gun @ (410)668-2171.
If all else fails in your search, I have a Colt 6600DH and a Colt 6721 I'd be willing to part with for the right price. Both have bayo lugs and flash hiders, but only the 6721 has a 16" barrel and collapseable stock.
The next gunshow is at the Pikeville Armory on Nov. 6&7, you'd be smart to look there as well.