I recently got myself a new free float PSA upper with a 1/7 twist nitride barrel for my AR and I’m dedicating it as some sort of mid range setup (100-300ish yards) outfitted with a Burris Fullfield 2-7x35.
Right now I have it zeroed for 100 yards with 55 grain ammo (because it’s all I have) but I’m considering in some 77 grain stuff. This will be the first time I’ll be playing with different bullet weights.

Without changing the current zeroing, how much of a point of impact drop am I expecting when I shoot 77 grain at 100 yards out of a setup that’s zeroed for 55 grain at 100 yards?

At 100 yards, you shouldn't see any significant difference between the two weights.  Beyond 100 yards, as the weight causes the 77gr round to slow more rapidly than the 55gr, you'll start to see more drop.





http://gundata.org/ballistic-calculator/

** I can't vouch for the accuracy of the above calculator, but it seems to agree with 2 others I compared it to.

YMMV

Too many different factors and variables. You'll be close but it also depends on how good of a zero you consider acceptable. I've seen over a 1 min of POI at 100y with different bullets/loads out of a 556.

For example: you could take several different 77gr bullets with the same powder charge and they will all have different muzzle velocities and POI.

At 100 yards, you shouldn't see any significant difference between the two weights.  Beyond 100 yards, as the weight causes the 77gr round to slow more rapidly than the 55gr, you'll start to see more drop.

The calculator is set for a 100 yard zero for both rounds which is why it doesn't show a difference at 100 yards.  Should be like 1/2" difference.

I figured it out, set scope height to "0" and zero range to "1"

The calculator is set for a 100 yard zero for both rounds which is why it doesn't show a difference at 100 yards.  Should be like 1/2" difference.

That make sense. I was expecting a little more drop than the original graph showed. 500fps is quite significant. With that said in our Hawaii tradewinds, I'd take the 77gr even if only out to 300y. 

I figured it out, set scope height to "0" and zero range to "1"

Good.  I tried the zero setting at zero, but the numbers were way off.  Didn't think about scope height.   

Thanks for the input, I tried playing with the ballistic calculator before posting but I wanted to see input/opinions from other people besides me.
I expect to be shooting things 300 yards and in, maybe even 400+ but most likely between 100-250 is my intended purpose for this setup.

I might question the muzzle velocity of the 55gr stuff, seems a bit high, and the OP did not mention barrel length.

The popular 16" barrel does not do so well getting high velocity from what I have read.  20" maybe.

I figured it out, set scope height to "0" and zero range to "1"

I don’t think it’s programmable for barrel length variations in the first place.
I picked up two boxes of SSA 77gr custom competition for about $23 a box. It was all that was left in stock for 77 grain at my local shop.
Not sure if I’ll get around to testing the ammo this weekend but we’ll see.

I don’t think it’s programmable for barrel length variations in the first place.

Software, you mean? 

No, your own chrono measurements or what I do, read  Am. Rifleman and note the velocities for various barrel length and bullet weight.

 Never seen 3200-3300 for 55gr stuff other than longer barrels, certainly not 16".  That makes a big difference at long range, as does the BC of the round in question.