Freebore makes a difference but I’m not convinced it’s that much. Remington 700’s are notorious for a ridiculously long freebore yet they’ve been some of the fastest factory barrels I’ve ran. Tikkas on the other hand which have a longer freebore but not crazy are very slow.
The fastest barrels I’ve ever had have been 4 groove lothar walthers. I’ve had probably a dozen total and all were fast. I had a AIAT 24” 308 that got over 2950fps with 185gr Juggernauts and PP2000MR before signs of pressure. The 24” 308 on my AIAE MK1 launched 155gr Scenars nearly 3100 with 46.5gr of Varget. I’m comparison my 24” 308 Bartlein AIAX only ran 2780 with juggs where I settled the load and max was just over 2800 before seeing the first pressure signs. That same Scenar load in my 700 milspec and LTR was about 200fps slower.
I wouldn’t get too wrapped up in it though. Pick a good cut rifled blank be it Proof, Bartlein, Krieger, Rock Creek, Hawk Hill, Mullerworks or whatever and you can’t go wrong. I just stay away from Button barrels now after getting several with issues (LW’s are button but have been solid for me) and I stay away from the bargain bin shit. I’ll take a slow consistent Tikka barrel any day over a McGowen, preferred, Carbon 6 and similar crap.