Has anyone chronographed some standard factory loads in a 20" blr / I mean stuff like win. opower points,Remington Corelock or federal vital shock. or any 150 grn handloads.

All depends on the caliber, general rule of thumb though, the shorter the barrel the more the velocity drops!.

I agree with dd. In (general) you can figure on around 50 fps per inch loss. But it all depends on the caliber, powder used, as well as the bullet weight and makeup (lead or jackted, monolythic, etc).

48.5/748/ .308 150 Win. power points

I was recently reading an old artical that used a '06 that started at 30 inches and was cut back an inch at a time using mil match ammo of one lot .I don't recall the volumes of ammo but but it was like 100 rounds per length . The last cut was at 8" . The velocity drops were not what you would expect at all . The drops were 10-20 fps per in down to 26 inches at 24 or 25 velocity went up then down again . From 26" to 22" the drops were flat . At 18 big drops showed up at 12" the 2850 of the 30" bbl was about 1200 at 18 the rifle was still over 2550 . I think the tests were done by Akley but I can't find the artical .