As many threads on here have already stated, physical barrel profile has little to do with the “HBAR” status, effectively anyway. If the manufacturers market the rifle, or actually physically stamp the barrel “HBAR”, then it can be legally sold in MD. Ask your LGS or FFL, if they sell it to him as an HBAR, then he can sell it to you as an HBAR as long as it’s not specifically banned in the text of the law.