Odd severe bullet drop yesterday

Gonna toss this one out as I am trying to figure out what the deal is. Shot a local iron sight service rifle match yesterday and could not hit anything past 400 yards. not near misses, but with three spotters watching my lane, determined my bullets were impacting several feet low.

Here are the details. Rifle is a Bushmaster CMP/DCM rifle done by Compass Lake, 1/4 MOA elevation, 1/2 MOA windage, 20 inch heavy barrel, shooting 69g SMK match loads at 2775 fps. Shooting position is prone, front bag only. 5-10 mph headwind.

100-400 at 50 yard intervals, come-ups were dead on, 100% first round hits dead center on 3/4 sized silhouettes. Every shot past 400 hit feet below the targets, windage was dead on.

Range topagraphy has trees for the first 400 yards that block the wind, more open 400-600 where wind has more affect. Elevation slightly dips out to 400 then rises the rest of the disatance.

Shot the same rifle, ammo lot, shooting position last month and had no troubles connecting at 400-600 yard plates. Only cleaning I did last month was a field strip, wipe down excess oil, re-assemble. Same as I do between each shoot for the season.

I know the head wind affected hits, but to forecully drop the bullets like that? At one point I maxed my elevation and was still hitting feet below the 500 where last month I was tagging the 600 with my come-ups with no problem.

If I can get away, going to the 200 yard line and confirm zero.

What are your thoughts on this condundrum?

ya, they are crony'd avg 2775 with spread of 20 fps for this batch. All ammo was out of the same lot and kept in the same can so pretty darn certain nothing changed in the ammo. Temp was a bit cooler by about 20 degrees. Can't think it would put it off that far. Range today only went to 100 but 10 rounds in 3/4 MOA in the center ring like always. Next week is vintage sniper so will have to wait till next month to see what happens.