Hi Norab,
I am kind of curious: What were the calibers for the guns you slugged? I would not be terribly surprised to see a .308 caliber L42 that slugged to .307 inch groove diameter, but I WOULD be surprised to find a nominal .303 caliber gun that slugged that low.
Just as a matter of note to all of us here, we should be a bit careful about terminology. The SMLE (Short, Magazine, Lee-Enfield) was the correct name for guns before the Great War. Around the Great War, the great renaming took place and the SMLE became the Rifle Number 1 Mk.III and ones that did not have the magazine cutoff became the No.1 Mk.III*. I believe the Pattern 14 became a Rifle Number 3. The Number 4 was a long ways off in the future.
Near as I can tell, the SMLE or No.1s in any of their variations never had replaceable bolt heads for head space adjustment. This is not to say that you can't find a bolt head that will adjust headspace tighter. You just can't do it predictably. Thread timing is also unpredictable. In building Frankenstein as described earlier, I had sufficient parts to find a bolt head that was about 180 degrees off time to a particular bolt body I had. I also got lucky that one nice looking bolt head timed and headspaced PERFECTLY to a new Lithgow bolt body, barrel, and receiver I was working on. It was really a matter of rolling the dice enough times by having enough parts to select from.
These guns SMLE and No.1 Mk.III and Mk.III* are also NOT terribly sturdy actions. They are NOT suitable for .308 Winchester or 7.62 NATO. The Ishapore Number 2 or 2A1 (designation depends on the rear sight installation) may have the same contour but different metallurgy and ARE sufficiently strong for 7.62 NATO. The Ishapore 7.62 NATO guns were only ever issued as reserve guns until sufficient numbers of FN FAL types were available.
Rifle No.4 Mk.I, Mk.I*, and Mk.II MAY be sufficiently strong. The British checked the actions before rebuilding to 7.62 NATO. The difference between a Mk.I and Mk.I* is that the Mk.I* has a cutout in the front of the right receiver rail instead of a spring loaded tab that blocks the opening at the REAR of the receiver rail that allows the bolt head to be rotated up to remove the bolt. The Mk.II has the trigger attached to the body (receiver) of the gun. This was to prevent dimensional changes in the wood from affecting trigger pull. The earlier guns had the trigger attached to the trigger guard which was clamped on by screw pressure.
Rifle No.4 (and presumably No.5 Jungle Carbines) had replaceable bolt heads numbered 0,1,2,3 which were progressively longer. Thus in a gun with headspace that is slightly too long, you can switch to a higher number bolt head and reduce headspace. Incidentally, it is very rare to find a military .303 British gun that headspaces correctly according to SAAMI specifications. Go to NoGo gauges only differ by about 2 thousandths of an inch. If anyone cares, I can go find my gauges to confirm my memory.
For those of you who may be checking out 7.62 NATO guns, keep in mind that 7.62 NATO ISN'T .308 Winchester. The ammunition spec is pretty close, but the chamber radial dimensions are quite different. The headspace specs also have a greater tolerance between OK and BAD. IMHO, .308 Winchester headspace gauges CAN be used to ballpark test a 7.62 NATO gun to some extent. A 7.62 gun should chamber a .308 Go gauge. A 7.62 gun MAY chamber a .308 NoGo and still be in spec. If a 7.62 gun chambers a .308 Field (Field Reject) gauge, it is equivalent (or very close) to chambering a 7.62 NoGo which means it is out of spec.
In reloading for the .303 British, I highly recommend neck sizing EVEN IF your gun has proper headspace. Because the .303 British doesn't heaspace on the case shoulder, the typical military chamber has a shoulder VERY FAR FORWARD of that of the cartridge case. Thus, if you full length size, the case shoulder gets blown forward with each firing and will separate after a few firings. If you use range pickup brass or brass from other than the gun you are reloading for, Full length size for the first reload and never after. For this reason, it is a good idea to not interchange brass between different .303s if you have more than one. My personal opinion is that the Lee Collet dies are the most cost effective neck sizing dies around. I use them in lots of different calibers.
As usual when giving this kind of advice, Your Mileage May Vary and I take NO RESPONSIBILITY if you should decide to take the advice of some unknown person on the Internet and possibly blow yourself up.
Pardon me for being so long winded.
- Ivan.