I guess all these measurements are interesting if your a statistician and want to track the evolution that Parker made to their barrels over the years but how does it impact using modern 12ga ammo in these over sized bores?
Many years ago when Stan Baker was alive, he manufactured barrels that were bored out to .800 using standard 12ga chambers. If I remember correctly, pressures were only slightly reduced due to the larger bore diameter however most of the pressure is developed in the chamber or slightly forward of the chamber so pressure wasn't greatly affected. I also think velocity was slightly increased due to less barrel friction as the wad/shot moved up the barrel. Modern wads would properly seal against the barrel. Many modern guns are overbored from the factory and well known gunsmiths like Tom Wilkenson believe in improved patterns by overboring barrels but not as drastic as Baker.
To sum up what's been discussed (correct me if I'm wrong)
1. Parker made 11ga barrels with .825+ chambers and .851 bores mostly prior to 1874 in the 2000-6000 S/N range. Some of these guns may have been entered as 10ga or 12ga when they are really 11ga.
2. Many Parkers in the range of 7,000 to 70,000 have standard 12ga chambers with bores of .750 +/-. No problem at all shooting modern 12ga ammo in these guns.
3. After 70,000 bore diameters were mostly in the .730 to .734 range.
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