I've been reloading 300 BO for a few years now. My rifle is a pistol-gas 8.5" SBR AR. The buffer is H2, but other than that nothing special has been installed to make it better function at subsonic speeds.
I have been working on loads that get unsupressed subsonic loads to fully function the action (cycle the action AND hold the bolt open after the last round). I figure when I eventually install a suppressor the loads with STILL be subsonic and the added back pressure will not cause a function issue (slightly over gassed is better than undergassed). If you ONLY expect to use your gun with a suppressor installed, it may open up additional powder options as these can leverage on the additional back pressure the suppressor provides to better cycle the action. But if you then need to use the loads w/o the suppressor you may run into an under-gassed situation. For me, I am looking for loads that will work with or without the suppressor so I have only worked up loads w/o a suppressor.
Also realize that barrel length makes a difference too. A subsonic round that has enough pressure to make it out of an 8.5" barrel at 1050fps should have a higher MV out of a longer barrel. This means a load created to be subsonic for a short barrel may no longer be subsonic out of a longer one.
When taking suggestions for 300 BO load data, make sure it is clear if they were tested on an AR and whether they produce enough gas to function the action fully without a suppressor. There are MANY powders that can be used from bolt action rifles that will reliably send the bullet down range at 1050fps. But MOST of these will NOT cycle an AR even with a suppressor.
A1680 and
CFE-BLK are the two powders that I have found reliably work with subsonic heavy bullets and AR pistol-gas actions. If I had to pick one... I'd go with A1680. A1680 used be hard to find, but now it is relatively easy. CFE-BLK is VERY similar but I've found A1680 more consistently works as you get down to subsonic speeds. The kicker for me is the last round bolt hold open. A1680 always seems to fully work the action. I have been surprised a few times when A1680 cycled the action and locked the bolt open with MVs BELOW 1000fps!
A1680 has load data down to 125gr bullets and CFE-BLK has 110gr bullet load data, but most are compressed loads. I try to stay way from compressed loads just to leave a margin of safety.
For lighter bullets (below 168gr) at supersonic speeds I switch to H110. In the past IMR4227 has also worked very well for me. My favorite light bullet load is 17.7gr of IMR4227 below a Barnes 110gr TAC-TX bullet. My rifle REALLY likes this load and this bullet. For plinking, 150gr FMJ Hornady bullets are the cheapest I can find but I can't get them down to subsonic speeds (not sure I even tried?)
As far as cases, I initially converted FC cases as I had hundreds of once-fired cases stashed. That seemed the most economical way at the time as no one was yet selling 300 BO cases yet and no one was selling converted cases. The process is a bit time consuming and you will need extra tools to cut and trim the cases to length, but for me it let me be involved in one more step in the reloading process. There is a
list of cases that have the correct case thickness so you don't need to trim the outside of the neck. Stick with these and you should be fine. I supposed I lucked out because my Lee FL sizing die has no problem reforming the cases. I may rotate the case 90deg and run the case thru the sizing die twice, but I don't need to remove the decapping pin. Just one extra stroke on the press the first time it is formed.
When Carolina Brass started selling converted LC cases I bought a batch to save some time. They work well but most were cut to a length a tad shorter than min trim length. Their current ad says the cases may be as short as 1.350". Maybe they have improved their QA but some of my cases were definitely shorter than that. So far this has not caused any problems. If you want to skip the time and effort involved with converting cases, I can recommend the Carolina Brass option.
As far as needing to trim 300 BO cases after firing... because the cases are almost straight walled (very small shoulder), I have not yet had to re-trim any cases because they have exceeded the max case length of 1.368". This is a bonus. All my other rifle cases eventually stretch and need to be trimmed.
If you are seriously considering reloading 300 BO, you should go
here. Lots of good info. Have fun!