A question leading on from the Hard extract L1 problem Peter, is the pressure generated by a 7.62x51mm blank (with BFA fitted) equivalent to the pressure drop, when a live round leaves the barrel and the L1 unlocks?
Printable View
A question leading on from the Hard extract L1 problem Peter, is the pressure generated by a 7.62x51mm blank (with BFA fitted) equivalent to the pressure drop, when a live round leaves the barrel and the L1 unlocks?
It is an impossible question to answer. First, as you know, I'm not an ammo person but the problem with blank is that from the moment it is fired some of the gas generated is already escaping past the BFA rod that goes into the barrel. This is a safety feature too..., more later. The gas generated by the BFA must be sufficient to impart sufficient energy to the piston and working parts sufficient for the momentum to carry the working parts through the reloading cycle. So given that gas is already leeching past the rod to atmosphere would indicate to me that it won't be anywhere near that of a live.
The rod is a loose fit in the barrel as a safety feature that ensures that if you fired a blank with the BFA fitted and the gas plug reversed to the NO-GAS position, there simply cannot ever be a massive build-up of gas in the barrel that would cause damage because the gas is always leeching past the rod. A good example is the L4 Bren where old worn out barrels had a ..25" diameter bolt vertically through the barrel. .300" - .25" = .050" difference through which the gas can vent.
Evening Peter, thanks for the comprehensive answer. It certainly gives me a good understanding of the forces involved.
It gets a bit tricky.
Blanks are blanks because there is no bullet.
This means that there is no significant resistance to the gas expanding up the barrel.
It is also the reason that blanks are loaded with VERY fast-burning powders.
To burn correctly, "proper" rifle propellant REQUIRES the mass of the bullet and the initial resistance of that bullet entering the rifling.
The BFA bore "restrictor" contains MOST of the gas within the barrel and this is directed through the gas port. Then, the regulator ( say, on an L1A1 type), VENTS excess gas and the remainder hammers the piston back a very short distance, but with sufficient force that is transferred to the breech-block carrier, and away we go.
If you are firing BRASS blanks, and the regulator is not set to correctly bleed the excess pressure, the "return wave" will often travel back to the chamber and partially crush the blank casing. Something similar happens to those horrible plastic blanks.
Pressure curves, dwell times and residual bore pressure at extraction are VERY different between ball, and that of blank fired with a bore restrictor fitted. The residual bore pressure factor, coupled with their VERY fast bolt movement, is why you need a "blank-firing shield" with the open-topped action of the M-14; to avoid a face-full of partially burnt powder residue
Thanks Bruce, I had no idea blanks used different powder, very interesting.
Basicly a pistol powder in CDN blanks...used to be ballisite in the old M1909 blanks I think but that was a bomb so they changed. I've watched what Peter said happen, an FN fired with the gas plug reversed to grenade and the rifle barely made a mouse fart...and no damage...