Bottom line:
1. A clean chamber.
2. Reduce headspace to SAAMI minimum (NOT Britishicon/Commonwealth military minimum).
3. Neck size only.
4. If possible, choose brass with thickest rims.

I haven't tried to O-ring method of fire-forming brass yet, but I will if I feel the need.

http://www.bdlltd.com/

Be advised that Brian Dickicon at BDLicon Ltd has NOS (new, old stock) unfitted #3 bolt heads for the No. 4 Lee Enfield Rifleicon. He doesn't list them on his website, but he has (or did have) them.