303 Br data is "somewhat" specific to the bullet. Sierra, Hornady and Speer make bullets for the 303 Br. They vary slightly WRT dia and shape. For the most part, data is similar.
It would be nice if you would list the bullet, ie A Sierra 180 gr Pro-Hunter #2310 for example, and you would probably get some pet loads.
Personally I prefer shooting a 150 gr bullet, and my preferred powder is Varget. It is common and readily available and usually works OK. I seldom use top loads, usually settle on a "middle" load. The 303 Br also performs well with some of the ball powders. It's hard to make any recommendations not knowing your intended use, or any firearms details.
For example Hodgdon online data shows this for 150's "Hodgdon Varget .312" 2.995" 39.0 2,458 39,600 CUP 43.0 2,656 42,700 CUP" and I started with 41 gr and fooled a bit with seating depth to get a pretty accurate load. Speer bullets worked with this combo, Hornadys, not so much. I had a nice SMLE #4 Mk 1 and a Martini. Both had nice bores, tight chambers and were without any headspace issues. Case life was good. My primer was a WLR and my OAL was 2.89". Speer bullets are 311, so obviously the Hodgdon data didn't use a Speer or Sierra.
It is also hard to ignore IMR 3031, a fairly fast burning powder that doesn't seem to get funky at the top end.
I'l quote some interweb info "During the course of World War I, as with the production of the P-14 rifle, Britain turned to the United States to help with ammunition production. This call was answered by Dupont and their IMR 16 single based nitrocellulose powder, which was officially adopted for use in 1916. IMR 16 closely duplicated the performance of the cordite loading, but had the advantage of not burning as hot. As this powder offered the same ballistic characteristics as the cordite loaded round it kept the Mark VII classification, but with the addition of a "Z" (Mk VIIZ) to denote the propellant change. The nitrocellulose powder also offered the added benefits of not having to alter existing or future rifles sights changing bullet weights and because of its cooler burning, prolonged the normal service life of the Enfield barrel. After the war, this powder was offered for public consumption as IMR 3031. This development of the nitrocellulose powder marked the last significant change to the .303 round and is still the propellant of choice for today's commercial loadings."
I've used IMR 3031, it's OK, but using it for loading the 303 Br lacks a bit of imagination, seems boring.
IF I don't have the current data, I'll contact the manufacturer, fill out the form and usually they send me the data I need. Buying the manual sometimes seems extravagant for just one caliber. I usually buy every second or third update, ie I have Lyman 46 and 50 (paper) and a pdf of 48. I have Speer 9 and 12. For the most part not much changes, but only using old data is not recommended. Components do change.
