This might be helpful, and includes a summary of Bell's study
http://parkerguns.org/forums/showthread.php?t=34360&page=2

I would emphasize again that we have NO pressure data regarding 16g and 20g long shells in short chambers, and that, esp. 20g, run higher pressures than 12g. IMHO using longer than chamber shells in a lightweight 20g is not wise.
And BTW, in the past when lots of shells were available, it was not hard to find 16g loads that (of course after being fired) were shorter than the nominal 2 3/4"; esp. Fiocchi.

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