The NPAS system of nozzles for GBB rifles is an interesting design. Normally the nozzles that come stock in GBBRs are of a positive system meaning that the valve inside the nozzle is kept in an open state during the initial firing of the BB until the pressure in the barrel drops enough that the gas pressure overcomes the gas/spring pressure that is keeping it open. Once the valve moves forward and closes the system will begin to push the bolt carrier to the rear and begin the cycle. What the NPAS does is allow the distance and spring pressure that the valve has to move to be adjusted. When adjusted fully open the valve is further from the closed position and will allow more pressure to flow into the barrel and thus give a higher BB speed before the valve closes off. The reverse is true when the NPAS has been adjusted to an almost fully closed state. Less gas is directed into the barrel and before it begins the carrier cycle.

 

 

In comparison to the RA-Tech NPAS system it is easy to notice that the 5KU is not quite as smooth or refined. The molding and plastic almost appears to be cracked (as can be seen in the images) though that is simply a molding defect. The plastic itself feels a bit more brittle than the RA-Tech version as well. During the firing test the nozzle keeper cracked into two pieces due to the brittle nature of the material.

 

 

Testing was conducted at 70 degrees F. The NPAS was installed into two separate bolts. The first is a brand new AGM carrier that has less than 1,000 shots on it. The coating is still brand new in the cylinder area of the carrier. The second is an AGM carrier that has been turned .0001" over the original diameter of the cylinder then polished out with 1000 grit sandpaper and final polished to a mirror shine. This treatment generally increases consistency and overall smoothness to the firing cycle.

 

I used .25g Stealth BB's (mainly because I have a ton of them and they are super slick). Since this test was less about FPS and more about adjustability they would work fine. All FPS can be adjusted for weight by calculating the FPS using the TeamDUCK FPS calculator. Shot strings were 9 rounds with 10 seconds between shots. The magazine was reloaded and topped off with gas in between strings and allowed to return to ambient temperature for 5 minutes. Before the 1st shot was fired the bolt was fired through 2 strings of 25 shots in order to check for any inconsistencies and to make sure that the magazine was at operating temperature.

After the first 25 rounds were fired I found that the nozzle keeper had broken. The plastic cracked right in half (see picture). Luckily the stock AGM keeper worked fine as a replacement. The bolt carrier was fairly tight with the new oring on the nozzle, though that appeared to be a good thing when firing. According to the charts below, the tight bolt carrier outshot the smoother one in FPS though the smooth one was more consistent overall with less problems and tighter deviation.

 

One thing I ran into several times while firing was that the NPAS would slowly adjust itself during the shot string. This resulted in having to restart the string again to make sure it didn't move. A very small bit of blue Loc-tite would cure this problem. The only real problem that was found was that the flute valve had a tendency to get out of place during firing, resulting in a shot string to go completely awry. What appeared to be happening is that the flute was able to move into the gas opening during firing and come out of place. It was simple enough to replace it but the consistency was gone. I found that when the flute was adjusted it helped to make sure that the arms were perpendicular to the gas opening (i.e. away from the opening as much as possible) in order to keep it from slipping out the gas feed. After figuring this out and reshooting the bad strings of shots everything worked fine.

 

 

Adjustment is simple and sure. Just remove the bolt from the replica and insert the key into the nozzle opening and slowly turn it until it engages the flute valves tip. Turn the valve to clockwise to increase FPS and counterclockwise to decrease it.

 

 

In all this is a super value. Time will tell how durable it really is but for the low cost of the 5KU NPAS you can guarantee that you will be able to play at your next fields game regardless of the FPS limits.