Looking for a general idea of velocity with Benchmark

[CXRs]

CXR,

Be aware, H4198 is a faster burning powder than BENCHMARK - It will give you even slower velocities within max safe limits (don't use it). Check this powder equivalent chart out from the makers of the powders you are talking about. Fastest burning at the top to slowest down the bottom;
http://www.adi-powders.com.au/powder-equivalents/

For a 100-125gn bullets out of a 20" barrel the most efficient powders in a range are 8208 to 4895. I recommend 4895, and a chronograph so you know what's going on.

Plus, access an internal ballistics program like Quickload so you can do What-if's to find the best powder for a bullet, barrel length and calibre combo without having to buy it.

I'm working on getting a chrono, right now money is a bit tight so it isnt particularly high on my list of priorities. I was thinking about the 4895, but i cant find any data on it. As for the AR-Comp, I have no idea on data since it's a new-ish powder compared to my Lee book, and it's not in the info that came with my dies, either.

You really need a chronograph for checking your velocity. I know every barrel is different, but a 200+ fps difference with the same bullet and powder with just a 2" difference in barrel length seems suspect. I understand its not a bolt gun, but I just dont see that difference being that big of a contributing factor. I could be wrong though.

I'm confused by that because its extremely low compared to the numbers I was seeing on Hodgdon's website, my Lee book, and the paperwork that came with my Hornady dies. Maybe its just the 4" missing off the tube, maybe I put the wrong numbers into the calculator, but regardless I'm getting a lot of drop at 300yds, and it's fairly sensitive to wind buffeting.