Quick.
As you might be aware of, pressure readings are obtained from the very best of pressure barrels with the most consistent dimensions for the chamber throat and bore consistency with SAAMI standards.
They are state of the art and very expensive only the very best barrel makers are engaged to produce them. Do suppose they compare with a production barrel such as the Handi and or with a whole lot of others? It is virtual impossible to get the same readings as a laboritory pressure barrel.
Some barrels come very close if they are made to SAAMI specs.
You are also wrong by saying that only my 25-06 had and over size barrel.
The 223 I converted also had an oversized barrel not as much but more than standard .224. The other 223 barrel I have has not been measured but I will, since Mitchell told me it did not shoot too well. I know of two others.
When you start using a chronograph you will quickly become aware of that. Another thing you will see the fluctuations of velocities even in the same loads. (velocity= pressure) 4kpsi is quite normal, 10% difference is what you can find in a high tensity cartridges. Then comes the difference in powder lots that can change things 5% pluse or minus mostly plus.
Ok lets do a little math 51.5 K( my parameter) mean pressure plus 10%= 56650psi you can add another 5% 2575 for powder variation and what do you get 59225 psi. That is not a remote senario. You know the saying start low and work up this is not a joke.
What do you think happens when a load you make that supped to do 3000ft ft makes 3150 or 2850. Do just wonder and carry on? I don't I want to know why?
There are loads and powder that have lower variations and these come mostly with a sightly copmpressed powder charge in a range of perhaps 104% loading density. But this can only be established with a chronograph and each rifle will be different.
The ball of wax is, loading data can not be taken as gospel and nor can pressure readings. I never use them only to compare.
I am very happy to read when you have Handi rifles that shoot well and meet you expectation. So far I had to work at it, even the 257 Roberts toke very careful load development, and too I had sticky cases and I full size only with a fitted die.
Now why would a case stick in a finly polished chamber and a full sized case? You don't need a crystal ball for that one.
:-D