"...all I find are..." That's because most bullet makers are thinking Mk VIII ammo. don't think there are any flat based 174's.
You'd need to slug the barrel first too. Sierra and Speer use .311" diameter. Hornady makes .3105 and .312 inch bullets.
I have part of a box of Hornady, 303 174 grain, round nose, flat base.
The picture also shows flat base spitsers in 174grain.
They are marked .311.
Chinchaga bullets makes a 175gr flat base spitzer.
I take partial credit for telling him not to make the boattail he told me he planned when I asked if he made a 303 bullet.



























