Short version: Explain the first round issue given that the barrel doesn't heat up much at all, even after five shots.
I've started to work up a 147gr load for the Kimber 6.5 PRC 20" simply to see what the rifle/suppressor are capable of. Since Hornady refers to the bullet as "match" I'm assuming that it should fly a little better than the 143gr ELD-X and Nosler 142gr ABLR. After shooting a few groups the results are promising but a little bit odd. Two days ago I shot five rounds of the Hornady factory 147gr load and got a typical group for that load of 1.087", an ES of 49 fps and an average velocity of 2,715 fps with the third and fourth shots being the fastest. Basically I'm getting around 2,720 fps for the 147gr factory load that shot at around 2,856 fps from the 24" barrel which is a huge loss of 136 fps. I don't plan on shooting factory loads so it's not a deal breaker but it is a bit disappointing.
So now on to the weird stuff. Yesterday I put together 5 rounds using once fired Hornady brass, 58.0gr of H1000, GM210M primers and a COAL of 3.000" (.020" off the lands). It was cold yesterday (20F) and so the rifle and suppressor were cold at the start. The barrel hadn't been cleaned from the previous day shooting Hornady factory ammo so when the first shot impacted 1" high on the target compared to the rest of the shots I figured it was due to the change in powder, a somewhat well-established phenomenon. Even when I saw that the velocity was higher compared to the other four shots I put it down to not cleaning the barrel for the new powder. It was a bit windy yesterday so the shot to the right could have been shooter error, wind, mirage etc., but the elevation on the target is the same as the three shots to the left so I need to shoot that load again to see if it's a good one.
2,887 fps
2,842 fps
2,855 fps
2,860 fps
2,862 fps
Discounting the first shot the ES is 20 fps, not great but not terrible given the limited prep, and still less than half of the factory stuff.
147gr ELD-Match, once fired Hornady brass, 58.0gr H1000, GM210M, COAL of 3.000",
So this morning I put together five rounds using more of the once fired Hornady brass, 58.3gr of H1000, GM210M primers and a COAL of 3.000" (.020" off the lands). It was 28F today, the barrel wasn't cleaned after yesterday's group and once again the first shot landed in virtually the same spot as the target above, followed by four shots in a small group in a similar location to the three shots in the center of the target above. All five shots felt very good, I was holding 1/4 moa or better, and had the first shot gone where I was aiming that would have been a 0.5 moa group. Looking at the LabRadar data, the first shot was significantly faster than the last four (I'll update numbers tonight). My working theory at the moment is that the SilencerCo Harvester suppressor is boosting the velocity of the first round due to more oxygen inside the suppressor which is somehow allowing a bit more powder to burn. According to QuickLoad the 20" barrel is burning about 98% of the powder. I can't explain why I don't see this phenomena with the factory 147gr ammunition which is much slower. So what are your thoughts? Am I way out in left field? I am pleased though with the 58.0gr and 58.3gr loads. Velocity is about what I was getting with factory 147gr loads from the 24" barrel and if I can resolve the first round issue accuracy/precision is excellent.
Added in Edit: Here are the velocities for the five shots shown below. ES discounting the first shot is 16 fps.
2,897 fps
2,875 fps
2,859 fps
2,868 fps
2,866 fps
147gr ELD-Match, once fired Hornady brass, 58.3gr H1000, GM210M, COAL of 3.000"