Hello, I just built my first AR-15 and I had been trying to evaluate some ballistics for a 55gr FMJ 5.56 load, based on a 16" barrel 1:8 twist.
I have created a pdf from my spreadsheet here:
PDF of Spreadsheet
My questions are:

Is 2940 fps a reasonable assumption for a true muzzle velocity?
Is the .255 G1 BC typical for FMJ rounds?
What range do people typically use for their Zero?

I used this site to generate the ballistic table
http://www.shooterscalculator.com/ballistic-trajectory-chart.php

I evaluated 50, 75, & 100 yard zero points.
I always assumed 100 yards for zero'ing a rifle just becuase that was how long the ranges I have been to typically are.

however 50 yards seemed interesting as it had a second zero around 160-165 yards on the back side of the arc, with only a .85 rise between 50 and 160ish.
that seemed pretty practical to me since I dont see me using this firearm much past 300-350 yards.
any input is appreciated and please let me know if this would be better posted somewhere else.
thank you and have a good day.

http://www.ar15.com/forums/t_3_118/698502_Zeroing.html

thank you for the quick response. I find your link interesting as that data does not correlate to what I had found.
the 3100 fps used in that link with the target silhouette does not seem to align with what I have read for a 16" barrel.
also down that post the computer simulation there used a BC of .243

as I read that more it seems several different solutions are found depending on the user posting.
is there a 55 FMJ flat back and a 55 FMJ boat tail that would lead to different BC numbers in the market?

The gist of that post seems to prefer the 50 yard zero, if I am reading it correctly.
However, the post seems to find that the back side of the arc is much farther out than I am seeing. Some of that could be the starting muzzle velocity.

Thanks again for the link.

most people use a 50 yd zero

Yes, there are several bullet designs that all can weigh 55 grains. They can have enough difference in their BC to be significant.

There are a few sources for BC data, they are easy to find. Some are more accurate then others, but until your skill levels get high enough to go out past 300 yards, those little inaccuracies are not important.

Muzzle velocity for barrels from 16" to 20" isn't hard to come by either. For now you can assume anything from roughly 2950 to 3200 and be close enough for work out to 300 yards.

Here is a web site that lists BC data and also has a ballistic calculator that is known to be reliable. You can play with this for free.

Applied Ballistics

As for zeroing, the type of sights you select affect the way you use those choices. If your sightline is based on typical sight heights of 1.365 to 1.465 for iron sights, you will use clicks to adjust from 100 to 600 yards.

Depending on your click value, you may use 0.25 MOA per click or more. So, you have to know your sights.

Telescopic optical sights are still dependent on the height above the bore, and some use hold-over reticles while others are for clicks. Those systems have to be selected first, then you can decide how to zero.

Start with 100 for a general purpose, then you can study while you work up.

thank you all. Its funny I should have just waited for your response. Before bed last night I discovered that some of the differences in my numbers vs the first link posted were coming from the hight of the scope above bore centerline.
Thank you for the link to the BC coefficients. Its such a common round that I easily found several differnt BC numbers.
I saw one as low as 1.51 and as high as 2.7.
I used this for the source of my FPS
http://rifleshooter.com/2014/04/223-remington5-56-nato-velocity-versus-barrel-length-a-man-his-chop-box-and-his-friends-rifle/
and I took off a little becuase it listed 16.5 not 16" for the UMC stuff.

I am going to use 50 yards for now, and thank you all for your help and information.
have a good day

I think most use the 50 yd zero with irons on carbines (Colonel Santoz modified battlesight zero) or red-dot sights (or both in a co-witness set-up). I use the 100 yd zero with a magnified scope (4X ACOG). Different trajectories for different reasons. Your muzzle velocity is very low on that 55 gr as already stated by others, which as you surmised is the reason your far zero was predicted closer in using that figure in the calculation. I don't think there is any 55 gr sold that is as slow as that.
What I do on the dot sight is zero at 50. Then I go to the 100 yd range and see where I'm hitting. Ideally it would be on-center and 1.4" high (69 gr). Naturally it really will be to one side or the other so I'll make a windage adjustment which will fine-tune it. However, you do not make an elevation adjustment unless it is clearly too low or high by at least an inch. Let's say I find it to be high and I bring it down to just about the 1.4" line. At this point you can expect to put that dot on a target and hit it out to around 225 or so, just what you need for close-quarter out to intermediate range shooting with no adjustments or hold-overs. Paint it and shoot.
On the scope, you put the reticle on anything out to 150 and hold over with the appropriate range line beyond that. Now, I'm using 77 gr in that one and I haven't seen a chart on that so what I would have to do is zero the 200 yd range line at 200 yards and adjust as needed. This will make a small difference at 100 which probably will be of no concern. Since I don't have access to a 200 yd range, I'll have to be content with my 1.5" at 100 and cross my fingers at 200. :)

The furthest I can get to be able to check targets is 65 yards. I run about .5" low at 65yd and that seems to mesh with a 100yd zero for my 2.75" (approx) height over bore from a lower 1/3 co-witness optic. I can shoot out to about 350 by laser range finder and hit the 12x12" steel.

I usually zero irons at 50 for a 50/200 with irons as the elevation drum doesn't work for a 100. My optic has a reticle calibrated for a 100yd zero, hence the difference.

Both sighting systems allow me to hit what I aim at, within reason.

For the record, I'm not a precision shooter. Just a guy that calls it good enough to hit a man size target at distance.