I've been working on various 5.56 loads and ran into some strange observations.

I developed loads this summer with a 20" Ballistic Advantage 1:7 5.56 barrel.  Most shooting was moderate weather in the 70's or so.  

Confirmed the data in a Ballistic Advantage fluted 20" 223 Wylde 1:8.  The velocities were somewhat lower in that barrel and appeared to be somewhat lower pressures.

I figured it made sense that the Wylde chamber was designed, in fact, to improve the cartridge and the lower pressures were a benefit.  Made sense to me and I was happy with the performance from both barrels.  Similar accuracy.

Now, it's February and I'm testing in 20 to 25 degree weather.  Much colder that the last work and added a 20" Wilson Combat heavy 223 Wylde 1:8.  Figured it would give me a little more accuracy and less sensitive to the barrel warming up to give me a better idea of group sizes.

So, same loads.  Almost everything that seemed fine in the other two barrels have significant over pressure signs in this Wilson Combat barrel.  These loads are within the powder manufacturer's suggestion - but near the upper end.  Even when I'm dropping, say, 5. or so - it helps but still show marginal signs of excess pressure.  Once I get down several tenths it settles back down.  And - the velocities from the Wilson Combat at the reduce loads are the same as the heavier loads in the other barrels.

I noted this with IMR 8208 XBR and Vit N540.  I've used several other powders but have not yet done this comparison with them.  In particular, the projectile was 77 SMK bthp.  

I did not compare these loads to the BA barrels - but was way over pressure with maximum loads in 55 VArmageddon with A2200 and .5 under max with CFE223.

I'm not complaining about the Wilson Combat barrel - just trying to understand.  It is possibly an anomaly in this individual barrel or is it more likely a good barrel to keep loads conservative and could recur in other 223 Wylde chambers?

You didn't list your powder charges, only the powders you used.

So many things contribute to velocity and pressure (they are related). Changing primers, brass, powder lots and a dozen other inputs can change things dramatically.

22.5 to 23.2 grains of IMR-8208-XBR is a working range with 75/77 grain bullets seated to magazine length in my experience.

I used 23.5 grains of VihtaVuori N540 under a JLK 75 grain VLD seated around 2.400" for the 600 yard line for one year in competition. Velocity was just under 2800 fps from a 26" barrel. That's fast. I think the only reason I was able to get that speed without pressure signs is because of the increased case capacity from loading long and the 26" barrel which already adds 100 to 125 fps over a 20" for free.

Reference VihtaVuori's online data and compare it to your current load.

Winchester standard small rifle primers have given people fits, piercing primers when other brands did not, using identical powder charges.

Your 20" velocities should be similar, probably not identical, but similar. Have you chronographed your current load?