Hi,
Personally, regardless of similar case capacity, I would never use .308 data in a .303. Coupla reasons:
Like w/ running "hot" .45 Colt ammo thru guns we "think" or "assume" will take it, the gun mfrs have never said it's ok. It's quite possible someone, like Ruger, does build in a big enough safety margin to get away w/ it just fine, as w/ older model Blackhawk family revolvers. However, they're perfectly within their rights not to, as we see w/ the New Vaquero. Who's to say they might not save a buck along the way and downgrade the safety margin in a No. 1 in .303 British as well? We don't know.
Then, if one IS gonna load "hotter" ammo for a given caliber, it's MANDATORY they never, ever, let that ammo get out where it's possibly gonna be used in a gun not designed for it! For example, what if you've been shooting ".308 grade" ammo in your No. 1, then "get the bug" to acquire an old Enfield, and "accidentally" grab the wrong ammo? That ol' Enfield's gonna be far less happy w/ it than the No. 1.
Now, perhaps those concerns are a bit excessive, but I've learned that to err on the side of caution is far less dangerous in the reloading game than walking on the edge. Perhaps that's just me...
As such, I'm gonna join those who say "read the book(s)." There are plenty of recipes for the .303 out there. Several of the IMR powders have produced good ammo for me in the past w/ the bullet weights suggested. (See Hodgdon's site for current recommendations.) H4831 and Reloder 15 have also worked well (see Hodgdon's again, and Alliant.)
There's really no need to improvise w/ this caliber...
Rick C
