GRT and QuickLOAD are models, not real life. Any modest inaccuracy in the input results in incorrect results on the basis of GIGO.
The three most common errors are:
1) using the standard default case water capacity value rather than measuring it with an unsized fireformed case from your own chamber and brass. (But not here apparently.)
2) COALs. If you were loading to have the bullet touch, or nearly touch, the lands, is that a longer COAL than you were loading to? In many cases, factory rifle chambers have such long throats that the bullet is outside of the case-mouth and the actual combustion chamber volume of when the bullet reaches that point is what contributes to pressure, NOT what the handloader seats the bullet at to, for example fit the rifle magazine.
3) Default values supplied by the powder company to the program compiler (then often modified subsequently by the compiler in the light of reported returns). I can't comment on GRT never having used it, but QuickLOAD is WAY 'out' on N135 IME. (But actually in the other direction here as QL severely under-reports pressures and hence apparently 'allows' higher charges than it should.)
Fired, unsized Lapua 308 Win cases, the make which Viht uses in its tables, hold c. 0.4gn more water than Federal on the basis of your measurements, so will increase pressures slightly. (Based on QL Modelling and past capacity measurements, I reduce 'book maximum' charges for the commonly used R-P and Winchester by 0.6gn when using Lapua or RWS in the 308, but this capacity discrepancy is much greater than your 0.4gn. So, powder / bullet company maxima need finessing down marginally, but only by a couple or three-tenths grains, and we're really splitting hairs here. Other factors such as chamber dimensions and barrel bore/groove dimensions in your Ruger rifle will almost certainly have a much greater effect on pressure.
The final back-check which you don't mention is actual MV. If the MV is much different from predictions, whether in the loading manual or in an internal ballistics modelling program, then pressures are also much different from predictions from whatever source. Otherwise, it's down to looking at / for pressure signs. As you're apparently OK here, then almost certainly so are your loads. Remember, bullet/ powder / ammunition companies use high-quality barrels exactly as per SAAMI / CIP barrel and chamber specs in their pressure testing. Very few (if any) factory rifles are produced with these dimensions, and in a litigious age, they are far more likely to tend towards larger pressure-reducing dimensions than the other way.
