DWJ:
U & I have alot of the same guns. Congratulations on holding out this long!
I have a Lyman Universal case trimmer which will hold any calibre case without a shell holder & comes with all the pilots you'll need. Got it at a gunshow. The 30 cal pilot is fine for 303 Brit. IMHO, you don't need power. You will get a feel for how much brass will come off with each turn of the handle. Easy does it. I'll trim a big pile over the winter, for my 1942 Lee Enfield #4 MK1 and my #4T.
The 1 thing my "instructor" neglected to tell me about trimming should have been the first thing he told me: you CANNOT go directly to your desired length. You must "sneak up on it". That's been my experience, anyway.
1. De-cap.
2. Measure case to see how much needs to come off. ( ie: ALOT or a little bit)
3 Lock case into trimmer & turn handle 5 times, applying GENTLE right to left pressure.
4 Remove case & measure. Now you know where you're at, roughly.
5 Repeat # 3 + 4 as often as necessary until you get the length you want.
6 GENTLY chamfer casemouth inside & out.
There may well be better systems out there, but that's what I do. It moves pretty quick once you get rolling.
If your cases have only been through YOUR gun, you should neck-size only.
I don't even waste time trying to get the lock rings at the exact distance. I just feel my way into spec.
This is not the place for a .303 Brit discussion, but PM me if this is a new calibre to you. I been beatin' my head on it for a while
Regards,
Button