AJ,
All I can think of if you have taken accurate measurements around the chamber, and case wall thickness is the amount of jump to the rifling is reducing pressure some 5 Kpsi, which is normal.
For peace of mind I'd try a box or two of factory ammo to see where it registers peak psi, do what you have to in order to prep it to fire in your chamber, little more work but I think I'd go there if put in this position myself.
I have MV and pressure data with Remington factory loaded 180gr Partitions and 180 Sciroccos in the 26" factory barrel of mine to compare if you do indeed get to that point.
3100 fps is reasonable to think it about maximum load for the 200gr Accubond, and adding .003 to the case diameter should have just reduced the indicated pressure.
Tell me how you determine the case's precise location under the gage exactly, in order to get the actual point to measure the case diameter at?
With an empty case in the chamber and bolt closed I insert a cleaning rod down the bore and twist it a bit to make sure it works its in as far as it will go centered up in the dish shape of the flash hole, hold it there and place a piece of tape on the rod "perfectly flush" with the muzzle.
Now I remove the case and insert it over the cleaning rod as it was in the chamber but now line the tape back up along side the bbl to determine the correct location of the gage to be glued on. The bbl and case both get marked at this point and both measurements are taken and wrote down in my log book.
One thing I do differently than most people that I know of who use these sttrain gage systems, and probably Burt or Jim too, is move the gage location forward more than it would normally be. Instead of centering the gage between the bottom of the shoulder and the base of the case, which is very close to the front edge of the receiver on the WSM and 308 length cases, I center the gage between the bottom of the shoulder and the "front of the receiver".
My reasoning for doing this is that the reinforcement that the front receiver ring gives to the chamber has a lesser impact on pressure readings for the stated barrel wall thickness. The bbl is turned down and threaded at that location, so it's smaller than the OD you enter into the program and calculate things from, then you have the fact that the receiver ring that's much larger than most barrel diameters at this location too. The lack of a tight fitting barrel to receiver thread joint would, in my mind, act the same as if the barrel diameter got smaller there, and if the thread joint were nice and tight it may act the same as if the barrel was larger in diameter, being reinforced by the larger receiver diameter... hope that makes sense.
The point I'm trying to make is that the barrel diameter at the location very "near" the receiver face is "less" of a known factor due to its questionable "effective" outside diameter. If I position the gage between the bottom of the shoulder and the receiver face I have an even "width" of "known" barrel wall thickness on "both" sides of the gage, so to me this makes way more sense.
Here you have the bottom of the shoulder and an increasingly thicker wall bbl wall thickness helping to reinforce the muzzle end of the chamber, and the receiver ring at the breech end either reinforcing it or with a sloppy barrel thread making for a "weaker" than expected pressure vessel.
The expansion real close to the receiver ring could easily be measured and compared to where I place my gages by using two gages, one at each location then switching between the two while firing enough of the same load for an average on both gages one after the other. Then one would at least have quantified the difference between them and have seen psi go up, or down near the receiver face, which would also be interesting to know.
By placing a gage on the receiver ring itself I have measured 45 Kpsi with an actual 65 Kpsi load, both being tested simultaneously with PT and the M43. So, there is quite a measured expansion on the ring itself, but it proved not linier and unpredictable to a point so it was not useful in the end.
In all reality, I think the placement of the gage foward or back 1/4" or so doesn't make a "real" big difference if the barrel wall thickness is still accurate for the actual location. If I tested it and found a 5 Kpsi difference it would shock me, really. It's probably in the order of 1-3 Kpsi at most, if that, but it would be interesting to test it though.
