"Increasing" headspace by removing metal from the front of the bolthead can introduce another "issue": excessive striker protrusion.

If you ever get to measure a bunch of different "number" No4 bolt heads, you will find that the overall length is VERY similar across the board.

This is because striker protrusion is set by the relative lengths of the front-end of the striker (forward of the collar) and the OVERALL length of the bolthead.

Given that SMLE and No4 boltheads, (and their associated bolt bodies), have indexed threads, the mathematics and sequence of manufacture must have been quite interesting.

The "back-thrust" of the cartridge is NOT supposed to be transferred via the threads, but via the tiny flat surfaces on the back of the main body of the bolthead and on the front of the bolt body. Extreme caution is advised before fooling with either of these surfaces. The bolthead thread is "beefy" because it takes ALL of the tensile load during extraction, especially primary extraction. A delicate "UNEF" pattern would not last very long in service. (And coarse threads are harder to "cross-thread' than fine ones: i.e. more "soldier-proof").