Most likely, yes. Start with the 5.3gr load, then proceed with the 5.9gr load assuming no issues with the first batch. Watch for pressure signs & if ANYTHING seems out of place, STOP AND INVESTIGATE before continuing!
Although it sounds like this advice is a little too late, I would highly recommend working with much smaller batches to start with, *especially* when testing out new recipes for the first time. 5-10 rounds at each charge weight you want to test should give you a useful qty to function test with, without having to worry about tearing apart the rest of your 50 or 100-round batch if they
don't function properly. You'll really grow to appreciate that the first time you experience having to pull down the
other 49 (or 98) rounds that you just loaded when you go to fire them & find that they don't perform as expected...
That said, the magnum primers tend to increase pressures (at least somewhat) beyond that of a standard primer, so your 'hotter' primer combined with the slightly lower-than-recommended-starting charge should counterbalance one another to some degree.
It is a standard safety practice to begin working loads up from the starting charges again any & every time you change ANY component from the listed recipe.
With your charges below recommended starting charges you should be in safe territory as far as pressure is concerned (even with the mag primers); your potential issues will likely be:
- inefficient powder burn due to pressure too low ('dirty' burn - sooty cases & possibly unburnt powder in the barrel/action)
- stuck bullets - not so much a problem in & of itself, but potentially disastrous to you/bystanders & the firearm if followed by another round while the barrel is obstructed