Well I always thought the general rule was 25 fps per inch, usually up to 26" in most guns. I've never done a chamber casting although I do have some cerrosafe lying around. I could slug the bores too just to see. I thought it was odd that my white oak rifle(18" spr)was as fast as most 20s considering the deeper throat they cut.
When my 20" Howa didn't gain much over the 18" AR I got to thinking maybe it's the nature of a stainless barrel. I'm not trying to answer the question of which is more accurate,etc. Just curious, simply a coincidence or just the way stainless shoots. I've read tons of info but no where out there is this addressed. Maybe cause it's not a real thing other than an anomaly with a couple of my guns? IDK for sure.