My "too short" was in reference to my desire to use that configuration. When I went to a shorter barrel (actually 8.5", not the 7.5" that I previously stated), length of pull went from 10.75" to 12.75".
I originally built this gun as a legal "braced pistol" consistent with my understanding of ATF rules and practices at the time. With either "new" configuration the brace location against my arm "works" and the gun is now "more appropriate" for one handed use. As I read the "new rules" this will allow me to keep the brace on the pistol. I have no interest in dealing with the extra rules that would come with making it an NFA SBR.
The 300 BO has more than recoil than a 223/5.56. I am shooting 165 gr cast bullets with a case full of WC 680 powder. I get close to 1740 fps in a 10.5" and 1660 fps in a 8.5". The brace / gun combination has let me know that a sloppy hold can easily result in a sore shoulder even with my configuration before the new rules were proposed.
With the 10.5" barrel, if I try to go for a two handed hold and no contact with my shoulder, sooner or later the brace will be too close and will impact my shoulder when fired (ouch). With the brace in contact with my shoulder, the 10.75" length of pull puts the charging handle right at my cheekbone. Again, sooner or later it will hurt.
With the 8.5" barrel (photo below), the extra 2" for length of pull makes a big difference for how the gun feels to me.
I do not know if I will keep it this way, but I know that I am not going with the brace adjusted for the new rule and a 10.5" barrel.
And from what I can tell the "extended" does not apply to guns with the brace secured in place with a setscrew.