i recently purchased a Bush 24in fluted flat top, mated to my Bush DCM lower with 4lb trigger, NO 3lb buttstock weight. mounted a Nikon scope.  i zeroed the scope at 25Meters. 5 shots thru the bullseye. all shots overlapping. i then placed my target at 100Meters.. 5 shots 6inches Above the bullseye.  shooting Black Hills 55gr hollowpoints.

Had to re-Zero my scope at a 100meters.

My only conclusion: 24inch barrel will Increase Velocity of bullet. Hence, higher/longer bullet arc?

i am a bit surprise, an extra 4 inches of barrel will cause that much difference of POI at 100meters.

all info appreciated..thanks in advance.

A longer barrel will increase the velocity, resulting in a flatter trajectory, not a higher one.

I assume you're comparing a 24" flattop to a 20" A2.  In that case you also have to consider the height-over-bore of your sight plane.  

Depending on optic setup, your scope is probably sitting lower than your A2 sights.  

ok, thanks for the info. i just thought zeroing a scope is teh same principal as Iron sites. 25m/100meters.

Longer barrel, increased velocity= Flatter trajectory?

i am hypothesizing... had my target been at..lets say..130M, the bullet would've landed on teh bullseye. hence, higher arc? regardless of velocity..  bullets from 16/20/24in barrel would drop at the same speed. but with higher velocity, it would drop at a farther distance.

It has nothing to do with the extra 4 inches.

Zero any AR at 25yards and you'll be 5-6" high at 100y.  It has to to with height over bore of the optic.

Lots of thing influenct your trajectory - your load (BC, mass, & MV), environmental conditions, and most importantly the relationship between the sights and bore.

This chart shows the difference that HOB of your optic will have on your external ballistics.  This info is based upon the velocity, weight and ballistic coefficient of the bullet you were using.  Notice the different trajectories at 100yds and you'll see how you got your results.  

[url]www.calvert1.com/guns/25ydzero.htm[/url]

Check your scope and measure how far over the bore your optic is mounted.