Any body have any experience? Thinking I should be able to reach 3000 F/sec with a 125 Accubond...or a 130 ttsx...
...........Guessing you have a Ruger compact.
Based on extensive side by side chrony test comparisons between my 16.5" barreled 300 WSM Ruger Frontier vs two shooting buds 24" barreled 300 WSMs using dozens of identical handloads, I ran on average between 4.3% to 4.65% behind in the velocity dept. My results were also very similar to Dick Metcalf's Shooting Times article dating back around 2005, where he chrony tested a 7/08 Ruger Frontier which ran 4.5% behind a 24" 7/08.
Imo, I cannot see why the same above percentiles could not also apply to the 300 RCM.
On the Hodgdon site, I see that max loadings of 135 gr Sierras from a 24" tubed 300 RCM go from 3224 up to 3287 fps. In this example, if 95% velocity is retained from the 16.5" tube, then the numbers
go from 3062 up to 3122 fps. You mention using the lighter 125/130 grainers.
I have no doubts that 3000 fps will be EASILY attainable using any 125 or 130 grain bullet of your choice.
Using RL17 and only RL17 max loads, I can move a 155 gr VLD at just over 3200 fps out of my Frontier (bedded and floated) with great accuracy. Assuming you are looking for some top speeds, you may want to try some RL17 and see how it does from your 300 RCM.