How big an issue or important is it for a safely functioning rifle to have a matching # bolt? Seems since they have replaceable size bolt heads they anticipated there would be headspace issues and the bolt head corrects it. Would it being a rimmed cartridge be some sort of factor re head space? Could handloading for the specific rifle make a difference? In looking at Brit. websites on shooting surplus Enfields they don't seem to think headspace is that much of an issue.
My apologies for the length.
Matching bolt heads on any rifle is important, but any gunsmith with a gauge can quickly check to verify it.
It wasn't until Lee Enfields, regardless of model, were surplused out that headspacing problems started. Most shipments had their bolts removed and put in a box to be reassembled upon arrival at their new homes. This was usually done because it was unlawful to ship fully functional arms.
The civilian purchaser would take the box of bolts and begin inserting them back into the rifles. This was where the problems began. Some rifles were not correctly headspaced because they did not have their original bolts. They were subsequently sold to the public with headspace problems. There was no assembly manual or checklist followed when these rifles were being made ready for sale. Very few businesses knew enough to check for matching nos or even bothered to check using a gauge.
How many made it out the door with headspace problems? We will never know the number, but from the 1990s forward, the Internet was often the source of incorrect information. Call it a double edged sword. It brought the potential problem to more people's attention, but at the same time, the stories were exaggerated or made worse by all the so called "experts" you find on the web.
One story that still floats is that No 4 rifles had different length bolt heads to correct headspace on rifles that increased due to use. This is simply not true. The different sizes were a manufacturing step that got rifles correctly headspaced without too much fuss. Remember that the assemblers were not skilled labourers. As well, in the days before CNC machining, when parts were being pumped out to meet production demands, parts tolerances were a concern. It was easy to train unskilled workers to use gauges and fit boltheads as required.
There is additional information here. Chambers and Headspace
The second part to this story is actually about rifle chambers themselves. These days, most people use rifles chambered with rimless cartridges. With most rimless cartridges, headspace is the distance from the bolt face to the middle of the case shoulder. That's easy to understand. I suppose with years of shooting, that area could lengthen, but it is not a common event. You will shoot out the barrel first.
Rifles that shoot rimmed cartridges have a small recess where the rim sits when a cartridge is chambered. The length of this recess determines if a rifle is correctly headspaced. The 303 British is rimmed and has this area. When your gunsmith checks a rifle, the gauge he uses fits into this small space.
The area forward of the recess - the space where the body of the case sits - can be any length, but still headspace correctly. In other words, a perfectly headspaced 303 British can have an excessively long chamber, but pass a gauge check.
Excessively long chambers were usually the result of a manufacturing reamer plunging too deeply. "Too deeply" isn't very much. Just a few thou. When that extra space was created, the assemblers would gauge the rifles for headspace and the rifle passed. When the rifle fired, the case was ejected. No one looked at the case to check for stretch. No one worried about the chamber being a little long. No one picked up the fired brass and reloaded it.
Long chambers cause virtually all of the case stretching in Lee Enfield rifles. Long headspace was rarely the cause.
When I was still in uniform, I checked records, maintenance instructions and talked with other armourers about some of the things that were talked about on the Internet. I never found any issues or special inspections ordered about headspace on rifles that came out of the Long Branch factory. The problems happened after the various governments sold off their rifles to civilian businesses.