Powder Ignition Failure

[Parashooter]

. . . My theory is that the weak neck tension and probable worn throat is causing a condition akin to a low density load in that the initial pressure pulse is blowing out the bullet into the bore and increasing the size of the apparent chamber. The rapid increase in volume is dropping the pressure to the point where, for some reason, the propellant is not igniting! I just think this is odd as the pressure in the chamber should not be affecting the primer performance - at least I have never seen this reported in the literature...!

The theory sounds good if we add the relatively large volume of the Lebel case and consequent low load density. With light loads of relatively slow powder, pressure and temperature simply never rise high enough to light up the charge and the powder flame extinguishes prematurely.

Experience with .323" bullets and IMR4895 in my 1886/93 rifle (modified for Balle N} demonstrated to me that the few published loads for the 8x50R are much too conservative for consistent ignition and burning. Significantly increasing charges eliminated these problems completely.

With N140 and the .323" 200-grain Sierra, QuickLOAD estimates it would take 47 grains to reach normal working pressure in this cartridge -

Cartridge : 8 mm Lebel M/93 (8x50R)
Bullet : .323, 200, Sierra HPBT MK 2415(2430)
Cartridge O.A.L. L6: 2.849 inch or 72.37 mm
Barrel Length : 30.0 inch or 762.0 mm
Powder : Vihtavuori N140

Predicted data by increasing and decreasing the given charge,
incremented in steps of 1.064% of nominal charge.
CAUTION: Figures exceed maximum and minimum recommended loads !

Step Fill Chg. Vel. Energy Pmax Pmuz Burnt B_Time
..%...%..Grain fps. ft.lbs psi. psi. % .... ms

-10.6 82 42.00 2382 2519 36390 4814 100.0 1.709
-09.6 83 42.50 2404 2567 37504 4845 100.0 1.690
-08.5 84 43.00 2426 2615 38650 4876 100.0 1.667
-07.4 85 43.50 2449 2663 39828 4906 100.0 1.645
-06.4 86 44.00 2471 2711 41040 4936 100.0 1.624
-05.3 87 44.50 2492 2759 42287 4966 100.0 1.603
-04.3 88 45.00 2514 2807 43569 4995 100.0 1.582 ! Near Maximum !
-03.2 89 45.50 2536 2855 44887 5025 100.0 1.562 ! Near Maximum !
-02.1 90 46.00 2557 2904 46244 5054 100.0 1.543 ! Near Maximum !
-01.1 91 46.50 2578 2952 47639 5082 100.0 1.523 ! Near Maximum !
+00.0 92 47.00 2600 3001 49074 5111 100.0 1.504 ! Near Maximum !