Ok but let's look at the .223 round itself.
Now lets look at a Lyman Reloading book 46th edition which is what's handy for me.
Using data for 10" and 14" barrels in .223.
First thing noticed is you have a rough average drop of about 275fps when you go from a 14" to a 10" barrel.
Now for 2 educated guesses going on by that info.
1). Referencing a Accurate Arms Powder booklet I found listed a MAX load of 25.5grs of 2230 powder giving 2860 fps out of a 14"bbl. using a 55gr bullet.
Now if you subtract the velocity drop average you get 2585fps.,.
2). Referencing HODGDON reloading booklet in which is a MAX load of 27.5 grs of VARGET powder which gives 2959fps out of a listed 15"bbl again using a 55gr bullet.
Again subtracting the average you get 2684fps.,.
Now some things to consider. The later load is close to GK's claim but what is not known is the ACTUAL performance of the 2 above powders: VARGET and AA-2230 out of a 10-11" bbl.
The data used for averaging the velocity drop was with a lighter bullet using different powders than in the two above loadings. Which means they have DIFFERENT burn rates and MAY give different results in velocity, accuracy and function.
So anyone ever do any real testing using these powders in those barrel lengths? Be interesting to see what actual results are.