Even though the .303 british cartridge is listed at 2.22 inches in length, most if not all military rifles in this caliber are chambered for longer cases. Triming your brass to 2.20 inches is "correct" but my rifles shoot better with cases trimmed to individual chamber length. I have two No3Mk1* rifles with chambers of 2.35". I make cases from fireformed .30-40 brass then trim cases to chamber length -.002". With fireformed cases and using the Lee collet die, I never have to trim again. Accuracy improves, and case stretching is non-existant. I segragate my cases and designate for each rifle. I shoot mostly cast bullets and keep pressures at or below 40,000psi. A lot of case "stretch" comes form pulling expander plugs through case necks after sizing. Expecting cases to headspace on the case rim is the other source of stretching. Headspace on the case shoulder and start with new cases not once fired (one-size-fits-all) factory ammo and cases will last and not stretch all that much if at all.