Low & Inconsistent FPS with HPA (KJW)

cardkid2331 · Apr 24, 2016

I have a KJW M700 that I've been working on upgrading. First thing I did was convert it to HPA using Palmer's regulator and conversion kit for the KJW. The mag I'm using is an Action Army long mag.

Never had problems with FPS before. I never attempted to max it out, but I had seen 600-620 with .2s at chrono before.

Now, the FPS is all over the place. Using .28s, I can set it to about 485. Then 30 minutes later, I'll shoot the gun and we'll be around 420-440, with the occasional 380 mixed in. It goes even lower than that sometimes...I'm not understanding why. When I get to 480-490 (with .28s), I've found that no matter how high I turn the PSI on the regulator, it doesn't increase the FPS of the BB. But I've had instances where it was shooting 520 with .28s for 4-5 shots.

I've dunked the mag in water and there are no leaks. Is this a problem with the bolt? The gun maybe has 1500-2000 shots through it. Perhaps there is an O-ring somewhere in the bolt I need to lubricate?

Grantmac · Apr 25, 2016

Sounds to me like you are getting partial valve lock or poor magazine to bolt sealing. Did you upgrade the striker and spring?

Grant

cardkid2331 · Apr 25, 2016

Haven't upgraded these yet - did the more important stuff first. I will if I need to though.

If I have partial valve lock, would lubricating the valve aid with this, since I wasn't having the problem before?

If I had poor magazine to bolt sealing, would I have an audible hiss of air? I've tried listening and it is dead silent.

Grantmac · Apr 26, 2016

Lubrication makes no difference to valve lock. Adding more pressure will just cause it to lock harder and actually result in less FPS.
Try backing the PSI down until velocity starts decreasing again.

Grant

cardkid2331 · Apr 26, 2016

Hi Grant - I've figured it out, as I tried your suggestion and was running into the same problems.

I wasn't quite understanding how these work, but now I think I've figured out the problem - just not sure how to fix it.

I am using the Action Army 29 round magazine, and it's too loose. When I put it into the mag well, I can move it up and down and around a considerable amount. I found if I held the mag tightly and pushed it in hard, my FPS would be around 525-530 and pretty consistent. Then if I let go, I'm down to 470, 460, 430 - all over the place.

Does this sound like it explains the issue, or is it just a fluke?

If it is the problem, how can I solve something like this? My KJW has a very home-made like washer/spacer in the front where the barrel/hop up connects to the front side of the mag well (came like this from the factory). I've seen Youtube videos and their KJW didn't have this. I also have left it out before, and the magazines just fall right out of the mag well.

Is there a way to tighten up the mag well to prevent this from wobbling around so much?

cardkid2331 · Apr 27, 2016

Tinkered with it a bit more. I took out the spacer, added a cloth like tape (what goes inside bike wheels) along the rear of the mag, and it's now pretty snug...but probably a temporary fix.

You were also correct - I had the PSI a bit too high. Backed it down, and eased it back up. With .28s, I can get to about 470-475, and then I have problems. Anything higher than this, and I'll get varying FPS. I think I'll upgrade the striker spring to see if I can get that any higher, but 475 with .28s should be about 570ish with .2s....so I'm fine with that.

I know none of this really matters, but in case anyone else has a similar issue, hopefully this helps.

Low & Inconsistent FPS with HPA (KJW)

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