All as previously detailed here but again this will probably be ignored or just plain not believed.
In the early to mid 1990's Bowkett was working on was was to become the first BSA pcp, the SuperTen. This came about as a result of his work with El Gamo on a pcp suitable for the UK market as CO2 was subject to FAC in the UK killing sales.
JB worked closely with the rifling man who was also factory floor manager and had been responsible for the production of barrels since the year dot when he emerged from the Military Police. He ruled with a rod of iron and most of the rifling procedures were only known to a few. It was regarded as a black art. Brian was the daddy behind all of the pre-Gamo buyout (1986) rifling machines which were of the cut rifling type cutting one groove at a time. Imagine that with the sheer volumes of Meteors that went out of the gates. One of the hammer forged rifling machines that were just starting to be used on some centrefire rifles pre-Gamo was included in the buyout and then used for spring air rifles.
With the hammer forged set up using mandrels these had to be changed from time to time due to breakages and wear. JB wanted a different type of rifling for pcps to that used on the springers and worked closely with Mr Barrel Maker to develop it. He had large mandrels made to give a loose bore and once the blank emerged had a small amount trimmed from the end which had a choke formed by the rifling process and the breech end cut to suit the overall length required.
This was a different direction from previous and initially against Mr Barrel Makers first thoughts. The rifling for the spring air rifle production carried out by the hammer forging method had been changed to use a tight bore, easy by making a small mandrel, chop off the choke and pick the end that looked the most concentric as the breech.
The reason for the change to a smaller bore on the springers was nothing to do with going metric but simply a way of consistently controlling the power output of their products. Many alternatives had been tried but most were either uneconomical or not suitable for production. Situations occurred when the gods of muzzle energy were shining on the springers and BSA would get a batch that went over 12 ft/lbs. For production changing springs was not on as was altering piston weight even drilling tiny holes through the piston seals was tried. By reducing bore size the "base" power output could be consistently lowered and if the gods shone again there was more of a safety margin.

All this and more info on the Facebook John Bowkett Fanpage.