I haven't dealt with the HAM'R, so it might be all that & a bag of chips, but I suspect that it is going to be a lot more limited as to what bullets you can use with it than the 300 BO or the 7.62x40. As the case gets longer there are fewer & fewer bullets you can use due to bearing surface & ogive length vis a vis mag length.
Personally, I am really liking the 7.62x40. There are plenty of useful bullets for it and I easily get 2,500fps with 125gr bullets out of an AR.
In an AR I would also consider the 7.62x39, .30 American (30/6.8 SPC), .30 Remington AR and the 30 ARX/ARk/Major (all Grendel/7.62x39 variations). None of these are handicapped by having to be useful at subsonic velocities like the .300 BO.
Lots of good info on 7.62x40 bullets here -
https://mdws.forumchitchat.com/
