barrrel llength bullet drop

[TACTICAL]

So I set up my sps tactical .308 with a zeiss rapid z 600 reticle. I plugged my ammunition in their on-line calculater to get the proper yardage for the reticle.
Their yardages are based on a 24" barrel where mine has a 20" barrel. Will the calculations remain the same or close to, or am I going to have to go another route to obtain the correct yardage drops.

thanks ahead of time for your responses.

[CoyoteHunter555]

It will be close. Your barrel is shorter, so the pressure from the gases spend less time pushing the bullet down the barrel to further develop velocity, thus leaving the muzzle going slower. You will be hitting slightly lower. You won't be able to tell/human error wouldn't fix the difference until out to mid-long range shots +. I wouldn't read into it too much. How far are you shooting? 400-1000 yards would be where you will notice, depending on how accurate you plan to be

EDIT* Chronograph at a few different distances, record all of your conditions, etc at the place the shots were taken, and get your own BC for your gun, load, avg. conditions, etc. There's a lot that goes into it of course but just start here

http://www.jbmballistics.com/ballistics/calculators/calculators.shtml

[TACTICAL]

The scope reticle has marks for shooting out to 600 yards.
I'm probably not going to be shooting past 450 yards. This gun will be used for deer hunting for the time being with most of the shots being in the 100-300 yard range. There is one area that I hunt where I've ranged deer around 420 yards without a way of getting any closer due to the openess of the terrain. This is where I would like the gun/scope combo to come through. I really don't have a range which goes past 200 yards in the area. So the 400+ yard shot would only be out in the field and only if I'm sure of the capabilities of the equipment and finger behind the trigger.
I've yet to get into the reloading scene and only shoot factory loads for now, which wil be Federal premium with 150 grain barnes tsx (no choice since I live in the so called condoer range)