Just throwing out another option - if you want a 50m zero but only can get to a 25m range you can use this target:

 

http://www.mdtstraining.com/PFC_ZeroTarget.PDF

by setting your scope for 100 yards I assume the paralex knob?  dont forget that the paralex seting if marked for 100y is only an aproximate, adjust paralex until you do not see any reticle movement then proceed as other's have already said.

I usually laser bore sight about 2 inches low at 25 yds while at the house.  This will give me close to a 50 yd zero.  I then do a 50 yd zero at the range.

 

 

Rick you are so on the money.

 

In a pinch I've even pulled the bolt with the upper on bags and looked down the barrel at the bull then adjusted sights.

and the two inches you speak about varies depending as the height of the sights above the center line of the bore. Two is a nice number depending on

height if sights or scope.  I put the laser dot 1-1 1/2" below the sights.

Are you using the .223 mount!? It has a built in angle... of 20 moa.. so your going to have to put a a lot of adjustment into the elevation at 50 yards..

 

I mounted my Nikon using a burris PERP, at 50 yards i was on a 10x10 target first shot from factory zero (low right) adjusted the scope up and left, adjusted it every shot until i was hitting the bullseye consistently. You really shouldn't need a boresight at 50 yards, but i will absolutely agree it can save time and money on ammo..

Not that it makes much of a difference but i shoot from the bench with a bipod.

 

You can also use Nikons calculator to zero for any distance you want.. print out a target break out a ruler and make your own reference points.

 

Look for posts from "thecolter383" on AR15.com. He created a lot of different targets like this one which make 100yd zeroing rather simple to do even in an indoor 25yd range. He has others for iron sights but I'm not sure if he made one for scopes. Might not make that much of a difference if all you really want to do is get on paper then make small adjustments.

Are you using the .223 mount!? It has a built in angle... of 20 moa.. so your going to have to put a a lot of adjustment into the elevation at 50 yards..

 

I mounted my Nikon using a burris PERP, at 50 yards i was on a 10x10 target first shot from factory zero (low right) adjusted the scope up and left, adjusted it every shot until i was hitting the bullseye consistently. You really shouldn't need a boresight at 50 yards, but i will absolutely agree it can save time and money on ammo..

Not that it makes much of a difference but i shoot from the bench with a bipod.

 

You can also use Nikons calculator to zero for any distance you want.. print out a target break out a ruler and make your own reference points.

No I'm not using the 223 mount. Not sure what mount it is.But not the 223 as I know what it looks like