54R Bren Issues

[Bruce_in_Oz]

My story regarding steel-cases ammo involves severe damage to a SiG AMT.

The steel-cased Bulgarian? and Chinese 7.62 NATO ammo does NOT "grip" the chamber the same way, NOR does it spring back the same way.

If you ever examine brass cases tossed out of a SiG or a H&K 91, for example, you will notice a series of dark "stains" along the front two thirds of the case. (16 for a SiG and twelve for the H&K).

The brass under these stains is RAISED above the normal fired case surface.

What happens is that the brass starts to "flow" into the flutes in the chamber at peak pressure, momentarily "gripping" the chamber a bit harder, (remember we are talking about "semi-locked mechanisms here).

Then, as the bullet moves up the barrel, the pressure drops enough for the case to "relax" slightly, contract a tiny bit and allow the gas, still at relatively high pressure, to bleed between the case and the flutes, thus starting to "float" the case.

The cumulative rearward forces exert pressure on the front of the bolt assembly. This in turn acts on the "wedge" that holds the rollers in place, and under extreme mechanical disadvantage, forces the wedge, and thus the carrier to move rearwards.

This system was first used on the old German StG 45, VERY late in WW2. It was designed around STEEL-cases 7.92 x 33 ammo.

HOWEVER, unlike the StG 45, the mechanism of the SiG was specifically designed to operate with BRASS cased ammo, first in an experimental job that used a special short cartridge, and then in the Stg57.

The AMT, or at least its "military variant" is just a "NATO-ised" 57, for export.

Firing steel cased ammo in one of these is a recipe for disaster. The steel does NOT "flow" fast or far enough to provide the essential "grip" on the chamber wall, and thus, starts to open too early. The result is that the working parts move rearwards too soon and too FAST, thus damaging the rifle internally. There is also NO guarantee that said ammo is anywhere near the pressure curve and PEAK pressure as specified for "real" NATO fodder. Some will have noticed that our Chinese cousins even made steel-cased "7.62 x 51 NATO" ammo, complete with the NATO "Circle X" mark; not sure who they thought they were fooling.

It is POSSIBLE that a change of "wedge" angle and / or return spring may solve that issue, but who has a couple of "spare" rifles for the experiments?

Remember also that, post WW2, the Stg 45, and designer "retired" to Spain for a little Rest and Re-Development. The result was the CETME series that started out with its own "special" cartridge and moved up to a "mildly" loaded 7.62 NATO case and ultimately went back to Germany to become the G3, firing "full-strength" NATO ammo.

Whilst both the SiG and the H&K rifles use mechanisms derived from the same grand-parent weapon, they are quite different to fire. The recoil of the SiG has a smooth, "rolling" push, whereas the H&K has a distinct "jab".

Back when Oz still had a sense of humour, I had both types and loved them dearly; The SiG for its pleasant and precise shooting and the H&K for its robustness and ability to handle dust and dirt, (and the fact that I had the proper scope for it).

If you own a SiG AMT, (lucky you), "cheap" ammo may ultimately be VERY expensive to use.