Shooting vs. Ballistics Cal.

[LRRPF52]

I've done very little long range shooting so when I found myself deer hunting on a stand with 400yrs of shooting I went to a ballistics calculator. My Grendel is set-up for hunting the big woods of northern WI. Due to unforseen circumstances the second half of deer hunting was in the central farmlands.

I have my Grendel zeroed at 50yrs, which gives me 1.5" high at 100yrs and 3.5" low at 200yrs. This I have shot at the range. So now my discrepancy, I wanted to find out the estimated drop at 300 & 400yds. Using Hoernady's ballistic calculator, I enter my fps and all other info and the numbers don't match up. To get the numbers to jive I have to add in an additional 272fps.

I chronographed my load at 2128fps using the MagnedoSpeed chronograph. To match my range results I need to input 2400fps. Anyone seen similar results or maybe it's the chronograph?

What are your numbers?

Barrel length
Bullet
2128fps is really slow, unless you're using a pistol/SBR, 140gr, or just a really mild load with the lighter bullets and normal barrel lengths.

What program are you using?
Does it have input data for BC, and did the decimal get entered?
Did you enter the barometric pressure with the decimal in the correct place?

I was entering BC data at the range a few months ago, and somehow the decimal kept getting skipped when I entered barometric pressure, so the numbers were way off like yours.