Ive used the gun city greek pulled projectiles (not good for hunting) and the hornady 174gr round nose ones. Both work good. The hornady ones might not follow the sight graduations that well as they aren’t exactly aerodynamic but will work well for hunting within 100m or so (realistic open sight hunting distances). The Sierra ones should work well. Ive heard non boat tail projos are more suited to the 303 for some reason.
Neck sizing can be a good idea with 303 brass to prevent overworking the brass and preventing case head separation. It can make the bolt a little hard to close sometimes as the body of the case remains fire formed. I found my No.4 shoots really well with full length sizing for some reason though. Just neck size for my SMLE (also a 1941 lithgow). Keep brass separate if you have multiple 303s.
Ive been using AR2208 powder in the SMLE around 40.5 grains and above gets speeds around 2400fps which is close to military ammo. Ive been using 41.6 grains in that one. In the No.4 I’ve been using AR2209 because 2208 is hard to buy at the moment in my area. 45 grains has been working well for 2209. Not sure of speeds but should be up around 2400. I just seat the bullet to the crimp line on the projectile because the lands seem to be miles away on .303s.
I tend to work up ladders with about 0.5 grain differences with 4 rounds per charge. Smaller increments would work too if your chasing specific speeds. Use upside down half circles as targets. A circle width of 14cm works well at 100m to match the width of the front sight. This shape works really well for consistent shooting with opens. Heres an example:
https://uploads.tapatalk-cdn.com/202...f81aabb870.jpg
As you can see most of the time 303s seem to shoot a bit high at 100 even on lowest sight setting. If you need to adjust the front sight on the SMLE you can put a bit of CRC on the sight post then take the front nose cap off. Draw a pencil line across the front of the sight blade and base to keep track of how far you’ve moved the sight. Use a brass bolt from bunnings as a drift. Use a bit of wood to support the barrel. Keep track of how many sight blade widths the shots were left or right. If the shots were left hit the blade left by that many sight widths etc.
Hopefully that helps.
Cheers