SRC claims that you get 3000fps out of a 20" barrel with an 87 grain round.  I'm wondering how this is possible.The case is a 223 necked up so it either has the
same powder capacity or possibly less than a 223. . A 75 Grain 223/556(12 grains lighter) might  get you 2800fps out of a 22-24" barrel.   The only thing left is pressure, but how could you
beef up pressure to get this velocity w/o damaging the gun?.  I've looked for specs and the only spec I don't see is pressure. Does anyone have any insight
on this?

Thanks

The .257 barrel has a more efficient burn. The larger the diameter gives more volume for combustion. The .223 really was designed around a long barrel and requires a 24" barrel to get the same burn profile as a 20" 25-45 sharps.
Powders play a role somewhat but in the end this equation is constant.

We saw 2885fps (average over 10

It will be interesting to see your chrono results.  I have always wondered why Eugene Stoner did not select some variant to the 24 cal bullet for the AR-15, as I'm sure others have.  Then you remember the Army's spec for the new ammo
that it had to pierce both sides of  a metal helmet at 100 yards.  Maybe an 70-80 grain .24 cal would have too much cross section and not enough velocity  to do this, who knows.  But one thing I'm sure many have thought about is how well an .24-.25 caliber
bullet would solve some of the inadequacies of the 5.56/.223 round.

It was a continuation of the scamp and other programs that led to the .224 diameter. Mainly lofty weight requiremnts and burst dispersion goals required minimal overall weight/recoil. I think the 6mm or higher is better as well.