KSC/KWA USP .45 Blowback Repair Tutorial

[(un)cooldude]

Greetings all,

 

This is sort of an 'informal' attempt to explain to some fellow USP owners why they can sometimes run into the double-feed, or no BB shooting out issues, when running these guns. I've run into this myself a few times, and after much poking, prodding, and experimentation, I believe I have a possible solution to one of the possible problems.

 

In the past, most, including myself, have merely suggested 'lubing the bejebus' out of the blowback mechanism, as this seems to alleviate the problem...albeit temporarily. After looking over how the system works, I found that the valve system in the blowback mechanism seems to be the culprit. Mainly; too much lube is causing the inner tube, which directs gas to propel the BB out the barel, to stick slightly to the outer tube, which prevents the gas from going down the inner tube, and only cycling the slide, but not actually shooting the BB.

 

This problem also manifests itself at times as someone shooting the USP for 5-6 shots, only to see the BBs roll out the barrel of the gun. Even though the slide cycles.

 

I decided to tear my blowback mech completely apart, and give it a thorough cleaning of all excess lube, as well as possibly polish out any minor burrs between the two tubes that could cause the inner tube to hang up. Below is a photo of the completely torn down blowback assembly, and I've circled the areas of the inner tube, and outer gas tube, where the binding is occuring:

 

 

In the second photo below, I'm showing the partially re-assembled blowback unit, with the inner and outer tubes circled where they slide into one another, as well as the roll pin that secures the rear aluminum retaining nub that goes behind the inner gas tube shown in the photo above. At this point, if you press on the end of the inner gas tube, sticking out of the outer gas tube, circled in red below, it should POP in and out with NO resistance...other than the resistance of the spring behind it. If you feel any drag, this will cause problems.

 

 

The next photo shows the gas assembly bolted back onto the blowback chassis. I've circled the o-ring that the outer plastic sleeve to the right slides on. I believe ONLY this o-ring, and the inside of the outer plastic sleeve, including the front end of it, should be VERY lightly lubed. Don't go crazy here, or overlube it! You'll end up having to clean it all out again later if you do. I seriously just sprayed a MINUTE amount of lube inside the plastic sleeve, and then lubed the o-ring with a q-tip soaked in silicone oil. That's it.

 

 

This last photo shows the completely re-assembled blowback mechanism, with the lubed plastic sleeve assemby re-attached. Note the springs you need to remember to resecure for the sleeve, which I've circled in red. I've also again circled the inner gas tube, where it comes out from the outer gas tube. Again, make sure this has a nice popping type action when pushed in with your finger. DO NOT LUBE THIS! I lightly polished the outside of the inner gas tube, and the inside of the outer gas tube on mine with croakus cloth, to ensure a nice, smooth action here. They're lightly chromed, so honestly, this area should stay dry. This is where the gas comes out to propel your BB out the barrel folks.

 

 

At this point, I re-installed the blowback mechanism into the slide, which is a snap, but pay attention to the rear fake firing pin install. I neglected to take photos of this, so just pay attention to the way it comes out when you pop the blowback mech out of the slide. Most of it only goes in one way, and the main screw to secure the blowback mech to the slide goes through it, so if it won't go back together right, you know you've got parts mis-aligned. I'll also note that the main screw that secures the blowback mechanism to the slide, screws directly into the bottom of the rear sight, so my recommendation to people is NOT to ever try to move the rear sight...it could cause problems with your blowback assembly!

 

I also want to just take a quick moment to mention that in order to get the blowback assembly OUT of the slide, if you take it apart, all you need to do is remove the one screw you can see at the bottom, with the slide turned upside down, and then gently spread the slide apart with your fingers to pop the mechanism out. When it goes back in, you'll hear it snap back into place in the slide.

 

I hope this will help some USP owners to alleviate the minor, in my eyes, problem with the BB feed issues. Don't just lube the ###### out of it, keep it dry. Don't automatically assume that the hop-up is buggered either. Chances are good that if this mechanism is all goey with lube, it's the source of your problems. I'll also apologise for the somewhat crappy photos, as I took this at work on my bench, where the ambient lighting sucks, and my flash just floods the photos with too much light.

 

Please feel to ask me any questions you may have regarding this, and I'll do my best to answer them. Comments, and critiques are welcomed!

 

 

Cheers!

[Jimmypie]

bump, sorry about this but had probs linking it to someone, this made it easy to find for them.

[SigPro552]

How do you take out that pin you have circled in the second picture? I am hoping to revive my Compact using your method, but I cannot get that pin out.

[Sgt_raptor]

Greetings all,

 

This is sort of an 'informal' attempt to explain to some fellow USP owners why they can sometimes run into the double-feed, or no BB shooting out issues, when running these guns. I've run into this myself a few times, and after much poking, prodding, and experimentation, I believe I have a possible solution to one of the possible problems.

 

In the past, most, including myself, have merely suggested 'lubing the bejebus' out of the blowback mechanism, as this seems to alleviate the problem...albeit temporarily. After looking over how the system works, I found that the valve system in the blowback mechanism seems to be the culprit. Mainly; too much lube is causing the inner tube, which directs gas to propel the BB out the barel, to stick slightly to the outer tube, which prevents the gas from going down the inner tube, and only cycling the slide, but not actually shooting the BB.

 

This problem also manifests itself at times as someone shooting the USP for 5-6 shots, only to see the BBs roll out the barrel of the gun. Even though the slide cycles.

 

I decided to tear my blowback mech completely apart, and give it a thorough cleaning of all excess lube, as well as possibly polish out any minor burrs between the two tubes that could cause the inner tube to hang up. Below is a photo of the completely torn down blowback assembly, and I've circled the areas of the inner tube, and outer gas tube, where the binding is occuring:

 

 

In the second photo below, I'm showing the partially re-assembled blowback unit, with the inner and outer tubes circled where they slide into one another, as well as the roll pin that secures the rear aluminum retaining nub that goes behind the inner gas tube shown in the photo above. At this point, if you press on the end of the inner gas tube, sticking out of the outer gas tube, circled in red below, it should POP in and out with NO resistance...other than the resistance of the spring behind it. If you feel any drag, this will cause problems.

 

 

The next photo shows the gas assembly bolted back onto the blowback chassis. I've circled the o-ring that the outer plastic sleeve to the right slides on. I believe ONLY this o-ring, and the inside of the outer plastic sleeve, including the front end of it, should be VERY lightly lubed. Don't go crazy here, or overlube it! You'll end up having to clean it all out again later if you do. I seriously just sprayed a MINUTE amount of lube inside the plastic sleeve, and then lubed the o-ring with a q-tip soaked in silicone oil. That's it.

 

 

This last photo shows the completely re-assembled blowback mechanism, with the lubed plastic sleeve assemby re-attached. Note the springs you need to remember to resecure for the sleeve, which I've circled in red. I've also again circled the inner gas tube, where it comes out from the outer gas tube. Again, make sure this has a nice popping type action when pushed in with your finger. DO NOT LUBE THIS! I lightly polished the outside of the inner gas tube, and the inside of the outer gas tube on mine with croakus cloth, to ensure a nice, smooth action here. They're lightly chromed, so honestly, this area should stay dry. This is where the gas comes out to propel your BB out the barrel folks.

 

 

At this point, I re-installed the blowback mechanism into the slide, which is a snap, but pay attention to the rear fake firing pin install. I neglected to take photos of this, so just pay attention to the way it comes out when you pop the blowback mech out of the slide. Most of it only goes in one way, and the main screw to secure the blowback mech to the slide goes through it, so if it won't go back together right, you know you've got parts mis-aligned. I'll also note that the main screw that secures the blowback mechanism to the slide, screws directly into the bottom of the rear sight, so my recommendation to people is NOT to ever try to move the rear sight...it could cause problems with your blowback assembly!

 

I also want to just take a quick moment to mention that in order to get the blowback assembly OUT of the slide, if you take it apart, all you need to do is remove the one screw you can see at the bottom, with the slide turned upside down, and then gently spread the slide apart with your fingers to pop the mechanism out. When it goes back in, you'll hear it snap back into place in the slide.

 

I hope this will help some USP owners to alleviate the minor, in my eyes, problem with the BB feed issues. Don't just lube the ###### out of it, keep it dry. Don't automatically assume that the hop-up is buggered either. Chances are good that if this mechanism is all goey with lube, it's the source of your problems. I'll also apologise for the somewhat crappy photos, as I took this at work on my bench, where the ambient lighting sucks, and my flash just floods the photos with too much light.

 

Please feel to ask me any questions you may have regarding this, and I'll do my best to answer them. Comments, and critiques are welcomed!

 

 

Cheers!

Hi This is a great bit of info, thank you very much