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Full Auto Droop

763 Views 21 Replies 4 Participants Last post by  Jimps
Current Build:
SHS 18:1 DSG (8tooth)
G&G 18:1 Bevel & Spur
SHS swiss cheese piston (teeth cut off, 3rd tooth lightly shaved down, AOE corrected)
Lonex POM Piston Head (bearings removed, spring guide has them)
Lonex Full cylinder
Lonex Cylinder Head (one with the pads at the front of the gearbox)
Retro Arms 8mm GB
Matrix Meteorite bushings
Maxx 21.25 nozzle
Tappet plate cut to 11ish mm (Guarder)
Real Deal Tappet Spring
ASG 30k (not the fancy CNC)
Perun Hybrid (in DSG mode, no AB in semi or in full auto, no precock, no ROF control)
Maxx ME Pro Hop-up
6.03 155mm brass Krytac barrel
G&G green bucking
SP150 (Guarder)

Semi is consistent at 355-360, full auto drops to ~330 and then the last shot ends around 360fps yet again. Any thoughts where i'm losing the fps in full or why the last shot returns to normal fps?
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~48rps with 11.1v It has real deal tappet springs, would I need to cut them at all? When you say place it on hop feed tube, am I supposed to turn the gun upside down and place it in the magwell practically? If so, it did not move in semi or full. Does this indicate pme? I believe it was quite consistent around 350-355 at 30rps on a 7.4v (will rechrono and post again if it wasn't consistent on 7.4)
Well it "passed" the tissue test and no air blew it off, so good seal where? Just nozzle to bucking/hop or also nozzle to cylinder head? Just trying to understand what was accomplished passing the tissue test. Where am I losing the fps then?
Rof control seemed to work great, does this mean that just some piston lightening should do the trick? Is there a good way to further swiss cheese the shs swiss cheese piston. What about expanding the holes on the lonex pom piston head? Could this shave any significant amount of weight or help the o-ring expand faster? IIRC it was about ~17g, will weigh it again tomorrow and post exact weight.

Is 350fps (.20) in correct range for a krytac brass 6.03 155mm barrel with an sp150?


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Any thoughts then? Is there a good way to test nozzle tightness, could it be too tight? Is it a possibility enough air can't refill the cylinder in time? Trying to think of everything here I can possibly test.
Piston slides freely in rails, and I wouldn't say it's tight in the cylinder. May be hitting 3rd tooth but I smacked it a bunch with heavier piston head and m150 so it should mostly be shaved down 😂. Waiting to correct everything before replacing piston body/rack.
Piston weight 18.7g

I also have already discovered the SHS tappet plate PME haha, hence the guarder I am currently using. Nozzle is lubed, I think it may be a vacuum issue as well, is there a way to know for sure if it's PME or vacuum? What did you use to enlarge the holes on the piston head? (No AB used ever, the slight overspin makes perfect precock haha)

FPS is consistent with ROF control lvl 2, practically no loss in full auto compared to semi at ~42rps. ROF control lvl 1 puts me at nearly 46 rps, but I get ~20fps loss. ROF control OFF gives me 48-49rps but fps drops all the way to ~300.

Anything look off in the gearbox? Seems like theres some wear on the rails where the piston rides, yet it slides freely without cylinder up and down in shut GB. Anyway to fix this? Sand GB? Sand piston rails on piston?

I also believe I have a slight nozzle to cylinder head leak. How should I go about fixing it? I tried to stretch the nozzle o-ring and still seems the same. Should I just switch to Maxx cylinder head? It seems to fit great with the Maxx nozzle. Just opened a friend's GB with Maxx cylinder head/nozzle combo and seemed solid.

Nozzle seems to sit slightly lower in the tappet plate, it's ~1mm "unseated," is this normal?

Also cleaned, re-shimmed and re-greased gears after pics were taken, same result. Used Molykote33 on gears, tappet plate, and piston rails. Used superlube on piston-head o-ring, and nozzle/cylinder head.


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If slowing the cycle down resolves FPS loss in FA and you're not detecting an air leak at the hop chamber, options are limited outside mild PME... Any chance the piston is a little tight in the GB rails or cylinder? Does it mesh OK with the sector gear? Could your AOE require a little more be taken off the 3rd tooth? Can't immediately think of another test or check to rule it out but I agree - PME wouldn't be my first guess with an sp150 and a ~17g piston... If you pull the piston, take a real good look at the pickup on it and hopefully it'll tell us something. What's your battery spec btw?

16:1 DSG SP140 147mm 6.03 ~45rps ~345fps on a lil 3s buffer tube stick
Battery spec 11.1v 3000mah 50C Zeee
The ASG Ultimate cylinder head is OP.

This is the ASG Cylinder Head. Sold at Brill. I also like the Tienly Cylinder Heads.
I have this ASG one
Automotive tire Automotive lighting Rim Auto part Font

Should I order the other one or is this one ok?

Not sure what piston you used.
I used the SHS swiss cheese 15T, will check for the teeth not meshing properly tomorrow, but I feel like I would have noticed that, especially considered how long i stared at the damn thing today going back and forth.

I found a clear hose that fit on the end of my nozzle and added some red water in the hose. I then would pull the piston out of the cylinder as fast as I could and measured how much the water moved. I widened the holes in my cylinder head and used a thinner o ring. I tinkered with it until the water did not react to me pulling back the piston. So no vacuum at all. I got around 6-8 FPS gain in auto. My main issue was the tappet plate PME.
I think this could potentially be adding to my problem, but do you think it'd be a big enough vacuum to lose 50fps on full? Considered with ROF control on lvl 2, at 42rps, it is lossless on full.

Anything else you can think to check for? I know damn well I'm going to be mad when I find out it's something so simple...
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Think I would still need to cut coils even if it's a real deal tappet spring. I believe it's rated at 9lbs and should be plenty strong. My nozzle still fits quite snug on my cylinder head and I don't have problems with midcap syndrome. I believe it passed the tissue test on full auto, but it's slightly hard to tell due to the vibration of the gun, it usually ends up sliding off the feed tube before it might be up to speed, but it should get up to speed quite quickly I would assume according to Chronos low bb count before top ROF
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