I would have the chronograph out when testing to check for this. Having one of the cartridge-shaped laser bore sighters you can pop into the chamber for a quick check is probably the most convenient thing to use at the range.ΔΆ6 grains, at just over 50% case fill is on the cusp of volume that can cause erratic pressure and velocity. But I would just keep an extra careful eye out that if anything sounds or feels off, you check that light still shows down the barrel. There certainly is no barrel/cylinder gap dropping pressure. It may be the shoulder and length of the case are enough to lower the expansion rate enough that it isn't the problem it can be in revolver cartridges. My main concern is that the case fill is below 70% with a powder that is known to have occasional squib-out issues in straight-wall cases when used at less than 88% fill. I hate coming home and having to pull/break down cartridges that did not work. I'd only load up a 2 or 3 at each charge increment the first time out just to see if your upper will cycle those charges. My point is still the same and that is the Lyman data using a 18" barrel would lead me to believe at minimum they have/had a long dwell time of at least 8" if not 11". Yet this 458 cartridge has the same exact data everywhere except in a actual manual where it's quite different. It's all likely based off of one source because none of my manuals have the same max charge and velocities as another manual for any cartridge. I see a lot of internet data that says 28gr to 32-ish grains but they all have the same max velocity in the mid 1600's fps which leads me to believe although I see that data in multiple locations. These specialty AR cartridges need to be matched with the right gas length, gas port size and burn rate to be optimized. My guess is carbine but it could be rifle length. Lyman used a 18" barrel but does not say what gas system. I'm concerned the action may not cycle at that charge. Yep that's where I'd start but based on internet data I'd be hesitant to load up a bunch that low. So at this point maybe 26 gr starting is where I need to go if that is in the Lyman 50th manual.
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