Just a brief update on my quest for aftermarket chokes for my B525.
Briley's are available in the UK but couldn't find them in stock in the sizes and types that i was after. Decided to try Teague precision who are based in UK. Liked the look of their extended but unported type in stainless and ordered a cylinder, skeet and 3/8 (light mod)for my 18.7mm (0.736") bores. They arrived today, finished very nicely inside and out and with a good knurled extended section big enough to screw in by hand so no need for their special tapered tool.
Some measurements taken:
size/entry/exit/nom restriction/Actual restriction(bore - exit)
Cyl/0.752"/0.735"/0.000"/0.001"
Skt/0.752"/0.729"/0.005"/0.007"
3/8/0.752"/0.721"/0.015"/0.015"
So it would seem for Teague, and perhaps other makers?, the actual restriction difference between barrel bore and choke exit pretty well matches the nominal restriction for that size choke. For more open chokes anyway.
The "step" still remains, so therefore still the potential for jugging effect, so it will be interesting to see if they perform true to size or tighter. I have dropped from 1/4 and 1/2 to skt and 3/8 anticipating them to be tighter.
