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I am curious if any of you have done any experimentation to see what the effect of altering the nose to crimp groove dimension would have on achievable velocity. Let’s take for example a Ruger Bisley 45 Colt with 7.5” barrel and standard 6 shot cylinder. Let us assume we are working with a hard cast, gas checked lead bullet of 340 grains. All bullets measure 0.860” long. The first bullet has a nose to crimp dimension of 0.400”, the second has a ntc of 0.450” and the third measures 0.500” ntc. Let us also assume that our cylinder will allow an overall cartridge length of 1.760” so all these bullets will fit in our cylinder. According to my crude measurements the volume of the 45 Colt case with the 0.400” ntc bullet seated would be approximately 0.1011 cubic inches (1.11” internal length minus 0.46” (amount of bullet intruding into case) and a diameter of 0.445”). The volume of the case with the 0.450” ntc bullet would be 0.1089 cubic inches and with the 0.500” ntc it would be 0.1166 cubic inches. So the difference from the 0.400” to the 0.450” would be about 7% and the difference between the 0.450” and the 0.500” would also be about 7%. A few years back I read of a formula that John Barsness came us with for estimating velocity from one cartridge to the next. The formula was used for bottleneck rifle cartridges so I do not know if it would be useful for the 45 Colt. Anyway according to the formula if one cartridge had 10% more case capacity than another cartridge of the same caliber then you would expect it to give 2.5% more velocity than the smaller cartridge. So in the example above with the 45 Colt you would expect only 1.75% more velocity at the same pressure from the bullet that measure 0.450” from nose to crimp groove compared to the bullet that measured 0.400” ntc. If the load with the 0.400” ntc gives 1150 fps safely then would you expect that the 0.450” bullet would only be able to be driven 20 fps faster? I suspect that it could be pushed more than 20 fps faster at the same pressure, otherwise bullet companies would not bother with this. Beartooth Bullets makes a 340 grain bullet with a 0.400” NTC and a 345 grain bullet with a 0.450” ntc and a 350 grain bullet with a 0.500” ntc. Cast Performance makes a 370 grain bullet with two crimp grooves. I am interested to know what kind of velocity difference this alteration might make. Thanks, Brian.
 

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Brian,

The velocity increase possible changes with the cartridge, at least when we are discussing straight-walled cartridges, and the return on even modest increases in case capacity yield far greater returns in % of change than when comparing bottle-necked rifle cartridges.

I have a .358"-190g WFNGC profile bullet with dual crimp grooves, (I don't catalog or offer it for sale, as it is a one-cavity custom mold made a L-O-N-G time ago!) one at .400" and the other at .450". When loaded in .357 brass with H110 I gain a full 110 fps when the bullet is seated out to the .450" NTC length, as compared to the .400" NTC length, when loaded to the same pressure levels (as determined by pressure ring measurements on the case).

When looking at the .44 Magnum, our .44-300g LMNGCDCG bullet has two crimp grooves, one set at .450" and another at .500". When loading this bullet out to the .500" NTC length in revolvers that will handle the longer length cartridge (Redhawks, Super-Redhawks, Dan Wesson & Colt Anaconda) the same type of gains are experienced, usually yielding right near 100 fps gain over the standard .450" NTC length.

In the .45 Colt similar results are to be experienced with the longer nose to crimp lengths as described above. Of course individual guns will yield somewhat varying results, but I hope this gives you some feel for what to expect from the varying NTC's.

God Bless,

Marshall
 
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