I have a .303 smle that blows powder back in my face unless I use heavy bullets with hot loads. I have been trying to shoot cast bullets with 2400 powder but get sprayed with powder. The case mouth is .339 when sized and .344 when shot with hot loads. could that be the problem? By the way Clymer reamers are.341.

Roundbutt--

The pressure is too low to seal the chamber with the case. You'll need to either fireform the cases with full loads then try the squibs using the least amount of resizing possible or raise pressures high enough to obtruate the case.

Thanks for the help This rifle must have a verry lose chamber.I will neck size and use a heavy crimp and see what happens. I sent E R Shaw an email asking him on a price of rebarreling his reply was he dose not work on SMLE,s. I may look for a better SMLE rifle.
Steve

Most wartime SMLE models have loose chambers to work in trench warfare conditions.
As it headspaces on the rim, you may get some benefit from using a larger bolt head. That is how headspace is adjusted on these rifles, easy, and cheap, just find the right thickness.

Canuck is point on - read the number on the bolt head and move up one number if you don't have a "Go - No Go" guage to test headspace with. The headspace is changed by unscrewing one and screwing in the other.
Regards,