Real Carcano bullet diameters
I have been pondering on how to load "Roma", my "bella figura" Fucile 91/42.
The best results in my limited testing to date were with 31 gns of RL 15 behind the Hornady bullets marked on the box as 0.268", but which are not 0.268". The results with Norma ammo were dire.
Having read a lot about the Carcano bore question, I measured the bullet diameter on some samples of original ammo, Norma ammo, and Hornady bullets:
Original: (2 clips)
0.2662
0.2664
0.2665
0.2664
0.2665
0.2666
0.2667
0.2665
0.2662
0.2663
0.2664
0.2669 (yes, I checked that one again!)
Norma:
0.2640
0.2639
0.2641
0.2642
0.2641
0.2640
0.2642
0.2642
0.2642
0.2643
Hornady
0.2672
0.2671
0.2671
0.2671
0.2671
0.2672
0.2671
0.2672
0.2672
0.2672
If I have done the maths correctly, then:
Average for Norma: 0.26412
Average for original: 0.26647
Average for Hornady: 0.26715
These measurements were NOT made with a DIY-market micrometer, but with a Moore & Wright bench micrometer with a fiducial indicator that really does have 1/10000" scale marks.
So what do I conclude from the above? - That, at least for my box, the Hornady bullet has an extremely narrow variation in a diameter that is actually a gnat's whisker over 0.267", regardless of what is printed on the box. (Thought: I wonder if Hornady reduced the diameter from 0.268" without telling us?).
Furthermore, this diameter is only 0.0005" above the original Carcano samples. In fact, it would seem to be only half a gnat's whisker larger than the upper range of the Carcano bullets. In many cases, it is probably closer to the original bullet diameter than a worn bore is to the original bore diameter.
So I reckon that the Hornady bullet is OK for real-life milsurps that are a bit worn. But I would not wish to push my luck, and so, as always, I seat my bullets so that they are clear of the lands (about 0.2 mm = 0.008") to avoid any extra pressure from bullets being pressed into the throat when chambered. In the past, I have read some fairly implausible stuff from people who claim to seat their bullets as close as 0.003" to the lands. Well, they are presumably unaware of the simple fact that brass has a much higher coefficient of expansion than steel. And I presume that they have bullets and reloading setups with tighter tolerances than I can measure with a bench micrometer. So real-life seating tolerances plus warm ammo taken out of your pocket and chambered in a cold rifle could mean that the 0.003" has disappeared or even become negative. Or maybe they keep both rifle and ammo at a constant temperature...or heat up the rifle before loading the first shot on a cold day?
So clean the barrel properly, removing any copper or lead fouling, keep the Hornadys clear of the lands, follow the table presented above, and it all should work. It does for me.
:wave:
Patrick
Measured to a tenth of a thou - plus/minus an inch or two?
Quote:
Originally Posted by
jmoore
that there's enough temperature induced change to account for the range in sizes?
That is certainly not the explanation in this case. All components and the bench micrometer were, of course, at the same temperature of around 20 degrees C, having been in the same room (in fact on the same bench) for months. But you are right to point it out. I also have a Mitutoyo measuring head that reads to 1 micron. that is, in inch terms, truly 5/8 of F.A. To use this sensibly, you need to have all surfaces clean and dry, and must avoid touching the items being measured as far as possible, And have all components at a constant known temperature. And avoid draughts etc. etc. And wear clean gloves, otherwise your have a device that is in effect measuring temperature changes and surface dirt.
The point I wanted to make (among others) is that if you get into serious measurement, you realize that the "I set my bullets a couple of thou off the lands" stuff is an empty claim unless the claimants are using much more sophisticated measuring equipment than that used by the typical reloader AND keep and use cartridges and gun at constant temperatures. An aim that is invalidated as soon as you fire the first shot! You personally will be aware that just because one can read an instrument to a certain level of resolution, that does not mean that the instrument is accurate to the same level. But many people are not aware of this.
Time to stop banging on about this bee in my bonnet - I have a lot of hungry muzzles to feed!
:wave:
Patrick
Milli-gnat's whisker measurement
Quote:
Originally Posted by
jmoore
We used to have a 400 lb micrometer in the metrology lab that had an optical viewer which would show a test piece grow in real time as you placed a finger on it for a few seconds.
Must have been down at the micro-inch level. Definitely the sort of thing that can detect draughts!
:wave:
Patrick
