I do not recall that they were "made" stronger - it was my impression that they were standard bolt heads, previously proofed at 303 British levels, that had been re-proofed to 7.62 NATO levels. Something like the difference between 18T and 20T - and, it really sucks not to be able to find where I got that idea!!!
Don't sweat it, I wasn't being contentious. I just find it interesting.
I've converted a few No4 rifles to 7.62 Nato with a Palma Match reamer. I even converted one to 243 Win. That was back in the day when Sterling and other mags were still available from Numrich at reasonable prices. The 243 was actually more interesting than the Palma Match 7.62.
The Palma Match rifle barrel was a NOS Parker Hale hammer forged, match or maybe L42 type/weight, 26 inches long. Came in a waxed cloth sleeve, inside a plastic sleeve. That barrel shot very well.
The 243 Win barrel was a Shilen 6 groove medium weight match barrel tapered from No4 receiver diameter to 3/4 in at the muzzle, 28 in long.
Neither of these rifles were in milspec issue furniture. Both had after market stocks that were appropriate for hunting rifles. Both had home made, side mounted scopes, fashioned from angle iron and screwed to the side of the receiver. Very sturdy set up.
The rifle with the Palma Match chamber is likely now a safe queen, sleeping patiently in the back of someone's safe. The 243 is still taking coyotes every year, north of Edmonton. It has a couple of thousand rounds through it by now and has been passed from father to son once and will likely be passed to the grandson from the sounds of it.
That's the main reason I found your comment interesting. Neither of these rifles were pristine when I got the actions. The bolts were used as is and the barrels were chambered and headspaced to suit the bolt head that was on them. The only changes made to the bolt heads was to replace the extractors with new, longer tooth models for the rimless cases.
It's also why I found the recent video, showing the No4 action that had been converted to take the 300Win Mag so interesting. That video was quite enlightening and helped alleviate some of my fears on the 243 conversion.
The owners of the 243 don't hand load. All of the ammo they use is factory loaded. Factory loads run close to 60,000psi.
Thanx for the reply. Much appreciated.