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Englander,
If you've any cases to spare, I would set one up to take this measurement for this bullet and any others you may wish to use. I simply take a case that is fired in my chamber and neck size it and cut a slot in the neck/shoulder with a abrasive cutting wheel in a Dremel tool. You can partially seat a bullet, a bit longer than you anticipate the OAL to be, and chamber it to determine where it contacts the rifling. I typically do this 5 times and take the average although it seldom varies more than a thousandth or two. This method is very accurate. I would seat the bullet .020-.025 back from the measurements you take here unless using a Barnes X-Bullet, in which case you should follow the reccomendations of Barnes. This technique will eliminate several unknown factors in order for you to come up with the measurement that you are looking for.

I'm assuming, always dangerous, that the flat point has a nose taper that is not THAT much less than a spitzer type bullet. The shorter overall length, compared to a pointed bulllet of the same weight, likely acounts for the fact that you can seat it out further. The good rule of thumb, as stated by BCStocker, would be to use a caliber length in the neck and to let that determine your OAL. If magazine length prohibits this, load it out to the max length that will reliably function through your magazine. If the cartridge seems long but functions perfectly there is no big deal, just some increased powder capacity. You can easily deal with this by comparing your velocity to loading manuals to be on the safe side. You could also do as I would and just load up until pressure signs are apparent and back off a grain. Unless anticipating shots at long range, which I doubt since you're using a FN, I would load to safe pressures and optimum accuracy.
 

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MikeG,
it's not the width of the thumb, it's the diameter that the "rule of thumb" is based on. That said, I'll advocate your notion of experimenting with what works. I have also had experience in single shot pistols in which I had to seat the bullet far less than a caliber and not even come close to the rifling. I simply used the additional cartridge capacity to increase velocity as I was using the rounds for hunting. One thing to be careful of using this technique is to load several rounds, handle them semi-roughly as might be expected in a reasonable hunting situation, and compare the bullet runout as compared to the runout when the rounds where reloaded. This will also give you a, field based, idea of what will be acceptable for your weapon/cartridge combination.

Englander,
your idea of the Lee crimp die may be a good one, but I'd base my results on the resulting bullet runout and actual accuracy of the weapon. I guess it all depends on how much you WANT that bullet to work. If you don't already have a RCBS Case-Master, or similar tool, I recommend it in order to measure bullet runout in a effective and inexpensive way. Excessive bullet runout is a sure fire way to make a very accurate rifle look mediocre.

Good luck with your project.
 
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