Jason got it right in principle. The only fly in the ointment is that the Hornady case is likely to go into the AR with more like 30 grains water capacity than QuickLOAD's default 28.8, and to eject with more like 30.5 grains water capacity. I base that just on having measured cases fired through my own AR. The result I got is 3092 fps for the long barrel and 2732 fps for the short, so 360 fps difference. I also have a second, slightly faster powder file for Varget based on some other measurements I made, and it adds a couple percent to pressure and about 40 fps. Yours should wind up in that range somewhere.
How much you lose will vary a little with the powder choice. I find Varget is really a little slow burning for best work with the 55 grain bullets, though some get good accuracy from it anyway. In my guns it does best with match bullet weights from 69 grains and up. With 55 grain bullets I have an easier time getting accuracy out of something like Reloader 10X, which, at just 22-22.4 grains (less expensive to use) gives you the same pressure as my two Varget models, respectively, but adds 43 fps to the long barrel and 51 fps to the short barrel velocity. So it produces slightly less velocity difference at 352 fps.
There are other choices that nudge the velocity higher, but waste more powder. If I try to keep the charge at about 90% case fill, for loading ease, and burning at over 95% in the shorter tube, then Reloader 10X seems like the best compromise and it is a known good accuracy powder in the .223.
Your particular load may wind up needing more or less powder than predicted because of chamber tolerance differences. For example, the Lyman #49 manual has a 55 grain spire point seated all the way out at 2.26" COL (which should lower pressure as compared to 2.20"), but gives 25 grains of Varget as the maximum load using Remington cases. So, as with my two models, there seems to be a little lot-to-lot variation in Varget's burning speed or case capacity difference brand-to-brand or both working in concert.
If you decide to try RL15, Start at 20 grians and work up to an accuracy sweet spot. If you are sticking to Varget, try using a drop tube to load the cases, as that will pack the powder in better. Regardless of your powder choice, if you want a good systematic method of finding sweet spot loads, look at Dan Newberry's site
As to actual velocities, you do need a chronograph to measure them with much resolution. You can build a ballistic pendulum or two separated disks spun by a synchronous motor if you have AC power at your firing line, but it's a lot of trouble to do either and the resolution is enough lower that you have to take and average a lot of readings to get to a reasonably precise result.
I've both read of and seen enough people having error trouble with the cheap chronographs that I'm not inclined to recommend any of them. You don't need a $750 PVM-21, but I think it is worth saving up your pennies for the CED M2 ($199 last time I looked). You can get a sense of how it compares to others on this thread at the Sniper's Hide
. I also have an Oehler 35P (I set up two chronographs for velocity loss determinations to learn BC's). Like one of the posts in the SH thread, my CED and Oehler track almost perfectly.