Very unlikely a load that shoots 1moa at 100 is going to shoot sub 3/4moa at 300 . My experience shows the opposite. The further i shoot the larger the groups get but if i have a load that shoots sub 3/4 moa or better at 100 . I usually can keep them sub moa out to 300 but anything further and groups start opening up .
When you start shooting further distances , it’s really important you lock in as the shooter . Anything past 200 and even your heart beat starts to show up in the reticle so breath , trigger squeeze, cheek weld , well everything starts to matter more then it did at 100 yards . Lets not forget about the wind . Even a 3 to 5mph wind gust can blow up your sub moa group at 300+ yards . My recommendation is work up a load to sub moa at 100yds and that load should do well for you at any distance if you do your part .
Example , here is a 10 shot .85moa group I shot with my 20" NM AR at 100yds with 8 of those shots in a 1/2 moa cluster . I'm tickled pink if I can shoot a 5 shot moa group at 300 with that same load .
I don't have the article on hand but keep in mind weighing the cases will do nothing for your load development . internal case volume is what matters and that is measured by water . Unless your case weight differences are 8+ grains you really won't know if the internal volume is different from case to case .
Here it is , scroll about 1/4 to 1/3 down the page to see the 223 case volume to case weight test .
http://www.6mmbr.com/223Rem.html