Why is does my French Berthier 8mm carbine have flyers always to the right

[Patrick Chadwick]

Convention wisdom would suggest that for a carbine one should use a powder somewhat faster than for the corresponding rifle.
However, if you study the construction of the Berthier bolt you will see that it is at bottom still the same as for the Lebel, which was itself a modification of the Gras bolt. In other words, basically a blackpowder bolt with a small-bore head attached. And this head is neither integral (as on Mausers) nor screwed on (as on Enfields) but simply turned into a slot. In other words, it is woefully sloppy compared with later bolt systems designed from the outset for smokeless powders. It was an anachronism, even for its day.

As a result, a powder like Alliant 2400 is far too vivacious for this action. Although it may be plausible for firing cast or copper-plated lead bullets in a modern carbine, the pressure onset is too harsh for the old design. In non-technical terms, it is generating too much "whack" too early. The system is being subjected to a shock wave that rattles up and down the barrel several times before the bullet exits the muzzle. If the bullet exits the muzzle at the same instant as the shock wave hits it, the grouping will be poor and extremely sensitive to very small variations in the load, seating depth, neck tension etc etc which can produce flyers.

If this is indeed the major cause of trouble, then you should be able to improve matters by making a series of targets with loads varying over a wide range, right through from a minimum up to the maximum. Do not worry about position on the target, just measure the group sizes. As one can always have a flyer caused by personal error, make a graph with 2 curves.
1. A graph showing the group size against powder charge.
2. Then strike out the worst shot from each group, and then draw the graph again.

If you have done this very carefully - you really need to shoot all targets in identical conditions, which means ideally on the same day, in one long session - you will have two curves that do not cross and show at least two humps of bad grouping, with a valley in between.

The "peaks" are easier to see than the "troughs". As the peaks represent the worst loads, simply take a load that is midway between two peaks, and you will have a load that insensitive to small variations and is probably the best you can do with that powder. If you are still not satisfied, try again with a slightly slower powder*.

Patrick


* You are at present using a very fast powder with a very heavy bullet!