A chamber for the 243 LBC shoulder starts at 1.150.
Have you done a case capacity on fired brass check on yours? In mine using Lapua brass I get 35.2-36.17 depending on what batch of brass I am using.
I measured one case=36.3 (flat meniscus).
The shoulder for the 243LBC AR without the 40 deg shoulder starts at: 1.150 which is the same your 243 LBC.
Whitley's version of the 243 LBC is almost identical in chamber dimensions inmost area. That shoulder doesn't pick up much but then movement forward does. As you can see it was not dramatic but every CC counts..LOL One of the biggest things we see is lot to lot and brand to brand case capacity variations.
The best firearm for self defense? Any loaded one will do.