Well nothing EVER beats range time, however…
Realize that every time you load a round into a pistol and it strikes the feed ramp it will almost certainly set the boolit back in the case a bit.
For magazine function to be even close to comparable the slide must be cycled at a high velocity. The problem with that is our grubby fingers like to grab onto things when we are doing things quickly and sometimes that can be a trigger.
You didn’t mention what type of firearm you have but here are a few options
a: If you have room behind the trigger, put a bolt and a nut behind the trigger, tape it in place
b: If you can remove the firing pin, do so.
c: If you cannot physically disable your firearm, wrap electrical tape from the grip over the trigger around the trigger guard until there is no way you can get to the trigger.
Pull however many rounds you need to fill a magazine.
Measure each one to ensure that they are just about the same length. Write that measurement down.
Now take the full magazine of your FMJ and cycle them manually through the new magazine as fast and violent as you can.
Measure each round to ensure it has not shrunk more than 1/16th of an inch (a$$uMEing 9mm)
Continue until the boolit set back is 1/16" or less, put those rounds into the range ammo box.
Repeat as needed to make you confident in the magazine function.
Remove all stops and such / install parts to return the firearm to working order.
Go to the range load 1 set back round and one JHP alternately for the duration of the mag and send them. If they gobble them up you are good. If you have FTFeed/FTFire pay attention to what round is causing the issue and evaluate. Also evaluate if it is at a certain point in the mag where things happen. Test with your FMJ and check with your JHP’s. All FMJ except the last 3 . First 3 JHP and a mag full of FMJ. FMJ 3JHP and the rest FMJ. If it passes these last three tests you are golden for the cost of very few of your JHP’s.