Interesting that IMR4166 didn't shoot well in your .223 but every barrel (either bore or length or both) seems to create its own preference.

With my 12FV .223 with a 26-inch barrel, IMR4166 has #1 position out of 22 combinations on my Powder Bullet list with 77 TMKs (0.279 for 205 groups) and and the #5 position with 69 TMKs (0.287 for 96 groups).
It also holds the # 8 position with 69 SMKs (0.292 with 89 groups) and and #9 position with 77 grain SMKs (0.297 with 77 groups).
With those group sample sizes, these results are statistically significant.

The #2 position is with N140 and 77 TMKs (0.280 for 40 groups) and #3 position is held by N140 and 77 TMKs (0.283 for 40 groups) so the 77 grain bullet preference seems to hold for this barrel - 4 out of the top 10. But 5 out of the top 10 positions are also held by 69 grain bullets.
Position #4 is held by H4895 with Berger 70 grain VLDs (0.286 for 11 groups) - I finally figured out what jump the Bergers liked.

But IMR4166 also holds # 17 position with Nosler CC 69s (0.403) and position #20 with 60 TMKs (0.487), and position #21 with 70 Berger VLDs (0.509 - but one of the loads averaged 0.291) and Position #22 with 68 Hornady 68s (0.653).
I have never gotten the Hornady 68s to shoot well in any of my .223s.
Also I haven't shot many groups for the combinations that sort of 'bring up the rear'.
I don't like wasting my time and money on combinations that don't work well.

That is a pretty varied result history for IMR4166 with the .223 12 FV - either really good or really bad. But I think the bullets (and a sub-optimum jump in the case of one Berger 70 VLD load) were the reason for the bad performance of the 4 combinations near the bottom of the list.