I got mine from Black holes weapons, it's a three grove polygonal barrel, my first one was very very accurate. I haven't had time to shoot my second one in yet! A 300Bo will fit in the chamber of a 6x45 and you can fire the 300BO.. you will not like what happens next!! trust me on that;. it is a very accurate round. and brass is cheap! (necked up 5.56 to .243) then annealing is next.
The 6x45 is a great easy caliber incredibly accurate and easier to reload than the 223R parent itself (more forgiving). No fireforming needed.
It just can use a bit of extra freebore with a hand throater reamer for longer bullets and more speed although it is not 100% necessary.
The 6x45 won a few world championships back int he late 60s and 70s until the PPC and BR later became king.
Another good one is the 6mm TCU. Also can use a tad extra freebore depending on what you want to do. This needs fireforming but it is easier
than other fire formed cartriges.
Ease of reloading and accuracy are my reasons for choosing the 6x45 over other "hotter" cartridges.
However, finding an affordable barrel is not so easy. I very seldom have an interest in taking a shot over 150 yards.
I prefer an 80 yard shot....for quicker travel time and less chance of target movement.