I thought I might share this experience to save others some time, and avoid potential injuries.
I am working on a .223 load which I can use out to 600yds.
I have two bullets I am testing.
69grn SMK and 68grn Hornady 2278.
I opted for N140 as I use it in the .308 with 168grn AMax to very good effect.
Four days ago I shot a 0.289 moa three shot group at 100yds using the 2278.
As this looked like it was working I ran another test this morning with a five shot group, and only managed to hit (if I can call it that) the paper target three times.
The only difference was the temperature was 22 Celsius lower.
I ran the figures through the various programs I have and it appears the drop is around 50fps, and as this is a long bullet with a 1.9 twist, it is on the edge of stability.
I guess I should have remembered the effect of temperature on N140 as this was the very reason I used Varget last time I worked on this load (about four years ago).
Luckily this is not a hot load, and I was working in the right direction.
Previously I had a load for a 60Grn V Max which I worked up in winter, come summer it started flattening primers.
All part of the learning curve I guess.