In Maryland those guns require a 7 (really 8) day waiting period (here's the list
I don't think you can legally make this purchase without a waiting period and I think you should consult an attorney about it. But I'm pretty confident of 2 things. One the transaction would be illegal to do unless the FFL at the show sends it to an FFL in MD and two that you will find the dealers at the show will require proof of state residence and when you show Maryland they will not sell to you same day.
Check out Glade valley sporting goods here in Frederick. Glade Valley Sporting Goods - 301-631-9048
I bought my Saiga .308 from them and it was already converted. It's a .308 also so the ammo is very common.
If you know what gun you want, buy magazines for it in VA as you can't legally buy magazines over 20 rounds in MD but you can buy them in VA and bring them to MD.