.20 VS .25 strange hopup problem. Would like to find an explanation.
 

Here's the deal. I installed Guarder hopup sleeve on my AK couple of hours ago because TM one developed a hole in it during last game. Used KSC .2s to test fire the gun. No backspin whatsoever. BBs would have the same trajectory at any hopup setting. I tried several different things including some of left over TM .25s. For some reason .25s work perfect... Good range and normal backspin effect.
Any idea what's going on? I'm totally confused here. Used same KSC bb's with my other AEGs/Guarder hopup sleeve and everything worked well except for double feeds every now and than. I think even this same AK used to have Guarder sleeve on it back in the day.
Btw, this AEG shoots around 395-400FPS.

Any chance the .2g BBs wiped off the oil on the bucking to the point that when you switched to .25g, the hop up put enough friction on the BBs? How many .2s you shoot at first before switching?

There is still enough silicon oil around bucking/sleeve area. I fired around two locaps of .2s. Than fired one locap of .25s. At the end decided to fire another mag of .2s just in case. .2s gave me the same result as prior to firing .25s.

Ouch! Got me there.

are both .20 and .25 BBs the same brand?

That sound impossible...

.2s are KSC and .25s are TM.

chances are the TM bb's might be a wee bit larger in size and are able to make that little bit more of a contact then the KSC bb's. We're talking marginal size difference but perhaps it was enough. ( ~ .01 mm ).

What other AEG do you have the guarder hop up in? Does it have a different set up of hop up then your AK? If so that is probably why you have decent performance with that gun and not your AK with .2's.

I've used the same KSC bb's in MP5 (hopup unit is different from AK) I used to have back in the day, I also used those bb's in my second AK. All AEG's had Guarder hopup sleeve in them. Both AKs have stock TM hopup unit. I believe MP5 had stock TM hopup unit as well.
I tested my AK#2 with KSC .2s and discovered that it doesn't perform that well. First couple of shots went through fine but than, after I started firing more backspin effect decreased and eventually came up to 0, same way as AK#1.
It's probably something with hopup sleeve and bb's. I'm just curious to find out what exactly causes it. I was thinking about size of bb's as well. Maybe that what it is. I'll try to test .2s made by other manufacturers and see what's going to happen.

KSC BBs are actually larger than TM BBs.

Yeah, I was geek enough to actually measure a sample of BBs from various brands one day...

oh...well...there goes my theory out the window......

can you remove the mag and shine a light into the breech thru the magwell and see if the hop up nubbin is moving properly when you adjust the hop up?

Make SURE the breech is clear when you do this as you'd have to look down the barrel to look into the breech. Disconnect power to the motor before doing this or wear safety glasses.