Range Report Published velocities and barrel length

[hink001]

Hi, I've been lurking for a while. Learned a lot, I'm self taught, naturally a decent shot but I've never had access to a range longer than 100 yards. A couple weeks ago I visited a 300 yard range and it was boringly easy. I'm not sure of the size of the dongs, 18" or so and I was quickly able to dial in and make repeated hits. It wasn't exactly set up as a precision range. Had dongs at 225, 245,265, and 300. 556 and x51 w/ light wind was right on dope wise.

I'm going to a 500 yard range tomorrow and from what I've read that is where things start to get a little more tricky. I don't have access to a chronograph and my question is how do you adjust for published velocities? and for different barrel lengths? I'll be shooting 3 calibers. 556 w/ an 18" barrel, 308 w/ 20" barrel and a 6.5 Grendel w/ a 20" barrel. All med to upper quality di guns. 6.5g upper is new to me and I'll to sight it in first but the 556 and 308 were pretty much on the money @ 200-300 yards. I'm wondering if there is a formula for barrel length to adjust published velocities. Here is an example; last night I went online and the Hornady 6.5 Grendel ELD had a published velocity of 2580 fps. My upper is 20", what should I put down as a velocity to get me started? thanks and any other advice is appreciated.

 

[Steel head]

15-25 FPS per inch usually works.