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Old December 7, 2009, 01:40 PM   #12
Unclenick
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Join Date: March 4, 2005
Location: Ohio
Posts: 17,925
The Lee rifle FCD's impart a deeper and firmer crimp than standard die crimps. Its consistency is also immune to case length variance. The deep crimp can usefully increase start pressure, especially with slower powders. The idea was inspired partly by observations made at the National Matches that military ammo was often more accurate than its commercial counterparts and than ammunition hand loaded using the same components. This was traced to the pitch seal used by the military when seating their bullets. It glues the bullet in enough to increase start pressure, and that improves combustion uniformity. This is also why some loaders observe better accuracy using the rifle FCD.

I have not tested this personally, but having looked at a lot of shadow graphs of shockwaves forming on bullet surfaces, and knowing that energy has to go into their creation, I would expect the ballistic coefficients of such indented bullets to be lowered over that of their smooth counterparts, and that their long range accuracy would therefore be adversely impacted, even if they did better at shorter ranges. There is also the issue of how symmetrically the crimp can be formed, and how much helical motion any asymmetry adds to the bullet's flight path, which likewise affects effective ballistic coefficient, as well as long range POI scatter.

So the deal with the collet-type rifle FCD is that it is another tool in the kit that you can try when developing loads to see what it does or does not do for you at the ranges you intend the ammo for? It will improve some ammunition and do nothing or worsen others. You just have to experiment with your bullets in your gun.

Military ball ammo, M2 for example, often has such a crimp in addition to the pitch seal. On the M2 bullets I've pulled it is not very uniform, though I have not seen the equipment that forms it, so I don't know why? M2 is not remarkably accurate. The crimp is apparently considered useful in making the ammunition more rugged, particularly in machine gun feeding. It may be worth noting that it is not used on military match ammunition.
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