MP-661K 'Drozd'

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How Drozd works
Drozd is a CO2 air gun it mean that pellets accelerated by compressed CO2. In cylinder CO2 stores in two phases liquid and vapor. When a part of vapor phase exhausted for shot, a part of liquid phase become vapor and pressure of vapor phase become very constant, depending only of temperature. All it means, that ALL shots of CO2 gun have equal power while CO2 cylinder contains liquid CO2. Count of equal shots is something from 55 to 90 per one 12 gram CO2 cylinder, depending of fire mode and temperature.
But CO2 pneumatic isnt perfect after each shot CO2 a bit cools down when vaporizing and its pressure also become bit lower, until propellants temperature became equal to temperature of environment. It results less power of each next shot when firing rapidly. And, of course, a power of all CO2 guns heavy depended by temperature of environment at 30C vapor phase of CO2 have pressure of 75psi, at 20C 55psi, at 5C only 35

To make drozd fire bursts without heavy drop of shots power, designers install just above CO2 cylinder a big pre-vaporizing chamber, that store vapor CO2 enough to power several shots without intensive vaporizing liquid phase. With limitation of max burst length by six, it results approximately equivalent power of all six shots.

Action of gun isnt difficult. When the trigger pulled, chip-based square pulse generator becomes producing short pulses of voltage opening a power MOSFET. MOSFET connects two big capacitors charging directly from batteries to electromagnetic hammer behind magazine slot. Hammer hits valve control pin installed into magazine top producing doze of CO2. Top BB, stored in magazine, already positioned coaxial with barrel and CO2 pulls it forward. When the shot completed, next BB with power of spring comes to fire position.

Boosting the fire rate
A construction of gun enables to boosts fire rate up to 900 RPM (15 shots per second) with all drozd parts still works normal. To boost fire rate needed to do some easy modifications with electronics. If you look at bottom side of PCB, you can see a 3 flat resistors connected to pins of fire rate control switch. Using soldering iron add in parallel a 50K resistor to resistor of middle switch pin (a switch position is 450 RPM). It will result a 900 RPM fire rate, when drozd switched to 450.

More materials will be added in future

(C) ADF, 2003

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