As mentioned in a couple of other posts, a hunting buddy and myself both bought suppressor ready 308s recently, mine a Howa and his a Remington SPS. I've been working up subsonic loads for both guns using Sierra 220 gr. round nose bullets and Trail Boss.
We've been twice to range to chronograph the loads in our guns. On the first outing my Howa with a 20" barrel was just over 1000 fps while his was substantially lower.
On the second outing with powder charges increased by 1/2 gr. increments, my Howa came predictably to about 1100 fps. When we put his gun on the bench though, the head scratching began. The first shot in the 800s, slower than what we had shot on the first outing. The next shot jumped some 1000 fps to just over 1800 fps. This kept happening, so our thought was something must wrong with the chronograph.
We took his rifle down, put mine up... good, predictable results were back. We also shot a 22 Hornet across the screens and got good repeatable data. Then we put his gun back on the bench and the previous results repeated themselves. So I thought there must be something about our subsonic handloads that his gun really doesn't like. To test this theory we fired two shots of PMC 150 gr. factory loads. The first shot registered 2734 fps, about right I thought, but the second shot clocked 3187 fps. Wow what a muzzle blast out of that stubby barrel!
I'm at a loss, what could be wrong here? Something causing radical pressure increases? Could it be a problem with the chamber grossly out of alignment with the bore? Any other ideas?
We've been twice to range to chronograph the loads in our guns. On the first outing my Howa with a 20" barrel was just over 1000 fps while his was substantially lower.
On the second outing with powder charges increased by 1/2 gr. increments, my Howa came predictably to about 1100 fps. When we put his gun on the bench though, the head scratching began. The first shot in the 800s, slower than what we had shot on the first outing. The next shot jumped some 1000 fps to just over 1800 fps. This kept happening, so our thought was something must wrong with the chronograph.
We took his rifle down, put mine up... good, predictable results were back. We also shot a 22 Hornet across the screens and got good repeatable data. Then we put his gun back on the bench and the previous results repeated themselves. So I thought there must be something about our subsonic handloads that his gun really doesn't like. To test this theory we fired two shots of PMC 150 gr. factory loads. The first shot registered 2734 fps, about right I thought, but the second shot clocked 3187 fps. Wow what a muzzle blast out of that stubby barrel!
I'm at a loss, what could be wrong here? Something causing radical pressure increases? Could it be a problem with the chamber grossly out of alignment with the bore? Any other ideas?