This is discussed on another thread somewhere, I will try and find it. The buffer washer thing is curious, because there are other designs which don't have have and seem to be OK. I wouldn't be keen to dry-fire a BSA without one in, but then I don't dry-fire rifles anyway. The chamber has to be checked to make sure the front of the cylinder is square before removing the radius, but I am sure that this is what John Bowkett used to do on his 12 ft/lb Meteor conversions. The Bowkett Airsporter and Mercury conversions were long-stroked and were, according to the stories, almost recoilless and had very short springs. I will do some searching and see if I can find some more info.
There is also the PTFE piston head sold by T.R.Robb, which is supposed to give good results, but which can deform if the rifle is not used for long periods. John Budd got much better consistency with one in his rifle.