Here's a quickie from Wikopedia as a reference point. Still wondering about a weaker mainspring, Tanker. See what you think. ej
Gas operated and blowback firearms
Blank Firing Adapter attached to an M16/M203
BFAs for blowback and gas-operated firearms are relatively simple. These weapons depend on high pressures in the chamber generated by the combustion of the propellant to push the breech block to the rear, allowing another round to be chambered and fired. If a blank round is used, there is no bullet to seal the barrel, and the combustion gases exit through the muzzle without building up enough pressure to rechamber the next round.
Simple BFAs for these firearms consist of a metal plug with screw threads. The adapter is attached to the muzzle of the firearm, and may attach to or replace the muzzle brake if there is one. A means is provided to allow some of the powder gases to escape. This can be adjustable, allowing it to regulate the amount of pressure used to rechamber the next round.
A drawback to the use of BFAs in gas-operated firearms is the amount of powder residue that builds up in the barrel. Since a limited amount can escape (compared to when live ammunition is in use), and the barrel can foul very quickly. Extreme care must be taken to ensure cleanliness of the barrel following BFA use to avoid damage to the weapon and injury to the operator due to fouling in the barrel.
Recoil operated firearms
Drawings from US patent 4,128,040, for a blank firing adapter for the short recoil operated M2 Browning machine gun.
Since blank cartridges generate very little recoil, far less than that produced by a live round, the recoil operation mechanism is not suitable for use with blanks. BFAs used with recoil operated firearms typically replace the locked breech of a recoil operated firearm with a simple blowback system using a restricted barrel, similar to a gas operated BFA. Short recoil operated pistols, the most common type used for self-defense and by police, are typically converted with a simple barrel replacement; the replacement barrel will lack the locking lugs to lock the slide to the frame, and will be built with an adjustable restrictor to control the chamber pressure. On designs with tilting barrels (again, the majority of modern designs are like this) there may also be a provision for tilting the barrel back, simulating the unlocking of the slide — this is visible in US Patent 5,585,589 listed below.
One notable exception to the design of BFAs for recoil operated firearms is machine guns based on short recoil designs, such as the German MG 34 and its descendants. These designs use a muzzle booster to add energy to the recoiling parts, and muzzle brakes for these designs simply replace the muzzle booster with one that provides far more boost with blank cartridges. The M2 heavy machine gun, while it does not use a muzzle booster normally, can use a similar muzzle-booster-derived BFA.