How your fired brass should look. Once fired, the brass should be opened up completely at the crimp. This will either take the form of a straight walled case for stronger walled cases, (M82 on the left) or the more "rose" like shape as seen on the weaker 8mm case on the right. Both of these examples are of well blank adapted weapons with the BFA set for the correct operating pressure.
2 rounds on left - severe under pressure
Middle 2 rounds - moderate under pressure
Case on right - correct pressure.
The round above (picture on left) has been fired with a blank adaptor set for a far higher pressure round. The pressure was released out the crimped end of the blank, but since there was no / not enough pressure to blow back the case mouth was not forced open. The case was not ejected and the weapon had to be cleared by manually pulling the charging handle. While not damaging to the weapon, it defeats the purpose of semi / full auto function as it will not cycle the action reliably. In the rare cases where the weapon will cycle and still look like this, operation in the winter will be poor at best. Moderate under pressure. As seen in the picture (above right - middle) It is possible for a weapon to function with moderate under pressure. These cases will have partially open crimps. With moderate under pressure, you are almost tuned in. Carefully reduce the orifice size in your BGA by about 1 drill bit size and re-test. (Example - most Blank adaptor orifice sizes are measured in standard index drill bit sizes - 1/8, 9/64, 5/32 etc. So for this example if you are running a 3/16 BFA you would need to drop the orifice to 11/64) You should find that the weapon ejects the cases a little farther / sounds a bit louder / runs more reliably.
Moderate under pressure is a major cause of weapon malfunction in the winter. As stated before, the cold weather will reduce the burn rate of the powder inside the blank. If you are already running the blank / weapon - under optimal pressure, odds are that it will give you problems with proper cycling in the cold winter air. Spent cases should eject 3-5 feet away in most weapons. If all the parts are in working order, weak ejection is usually a sign of under pressure. A temerpature drop of about 30 degrees will start to effect the pressure of the weapon.
Moderate under pressure as a good thing - If your weapon works fine in the warm summer with the case mouth being all the way open, but shows signs of moderate under pressure in the winter you are safe to run that weapon / blank / BFA combination all year round. If you have an adjustable BFA you can reduce it in the winter to make sure the weapon is reliable. Just be sure to change the BFA back to the larger size in the summer. For this reason, many people will try to keep one orifice size with proper pressure in the summer and moderate under pressure in the winter.
Over pressure / too small of a blank orifice.
There are several ways to tell if the pressure is too high. Many times the weapon may jam quickly, bind, blow parts off or a dozen other malfunctions.
Short of the weapon telling you that you are running the blank with too much pressure, you may also tell by the brass. Often weapons are run with moderate over pressure without weapon damage. Often you may get away with this for quite some time, but over pressure is still over pressure.
Over pressure will cause small part damage and premature wear on all operating parts reducing the life of the weapon and causing many common minor operating problems in the field.
Looking at the brass, it will be evident by the primer backing out of the primer pocket or being blown out all together. If you see the primer protrude slightly with no other signs of over pressure and see it rarely don't worry about it. If you see it often or with other signs, you risk damaging the weapon starting with the extractor along with numerous other potential problems..
Over pressure signs at the case mouth.
The picture below shows signs of over pressure at the mouth of the case. If the crimp is damaged, mangled or twisted over itself, the pressure is too high and the blank orifice should be gradually increased until the case mouth looks closer to the picture at the top of the page and the weapon functions reliably. Continued use of the weapon with pressure this high will cause excessive wear or damage to the weapon.
The below case was fired in a M-1 Garand with a .171 (11/64) BFA at 75 degrees F. Continued use at .171 or increasing the orifice to .187 (3/16) and watching for under pressure signs in the cold winter are both reasonable options.
