The only military grade .303 cases still being made today are Prvi Partizan, they would be comparable to “Lake City” military .303 cases “IF” we made them here in the U.S. American SAAMI commercial cases were NEVER designed to shoot in a military Britishicon .303 Enfield.

My 30-30 Winchester has .004 head gap clearance and with an .303 Enfield at the maximum headspace of .074 you can have .016 head gap clearance with commercial cases. (More head gap clearance-headspace than these cases were designed for.

Even CIP the European counterpart to the American SAAMI states the .303 British has a problematic head space condition with commercial ammunition.

Delta L problem

The delta L problem (ΔL problem) is a condition that occurs regarding certain firearms chambers and their practical incompatibility with ammunition made for the corresponding chambering.

Conflicting industry standards

The main cause for the ΔL is that the two main civilian ammunition and firearms industry standards organizations C.I.P. and SAAMI have assigned different standards for the same cartridges. This leads to officially sanctioned conflicting differences between European and American ammunition dimensions and chamber dimensions. Since C.I.P. and SAAMI do not rule nor control civilian ammunition standards worldwide other causes for conflicting standards leading to ΔL issues are also possible.

Firearm cartridges with otherwise problematic headspace

There are also some firearm calibers with problematic headspace listed by C.I.P.
The headspace defined by:
Depth of rim recess
• .303 British
• .38 Sp AMU
• 6.35 Browning
• 7.65 Browning
• 9 mm Browning long

Delta L problem - Wikipedia, the free encyclopedia