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Weird screech on full auto

409 Views 16 Replies 3 Participants Last post by  Leo Greer
Current Build:
SHS 18:1 DSG (8tooth)
G&G 18:1 Bevel & Spur
SHS 14T (8 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 12ish 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)
SP150 (Guarder)
2 Barrel setups:
6.03 155mm brass Krytac barrel & purple prommy w/ maxx hop
6.03 275mm lambda barrel &G&G green bucking w/ maxx hop
11.1v 3000mah 50c Zeee
11.1v 3000mah 15c Titan
7.4v 2200mah 120c Zeee
7.4v 2200mah 30c some RC car brand

Gun works fine on full auto on big boy 11.1v, put a mag in and it screeches like hell once it's up to speed after a few bbs. Doesn't make the noise on super short bursts, also won't make the noise with ROF control to ~35rps. It was shooting ~48rps before whatever I changed made it happened, but would lose ~40-50fps on full so I was trying to figure that out and ended up with a non feeding gun unless ROF control is on.

I went to a tappet plate ~.5mm longer than the one that dropped on full thinking it might be a weird feeding issue and nothing changed.

I read that someone had their sector gear shimmed too high and was rubbing on the tappet plate. I doubt this is my case as there is only a .2mm shim on the bottom of the sector gear. (Tried 1-4mm below the sector and sounds the same)
My bushing under the sector also happened to start spinning (was pressed in with a C-clamp originally w/o glue) around when I believe this problem started occurring, but I haven't checked under it in quite a while. It is now glued in place and doesn't move, sector gear shimming was rechecked and it stayed the same.

Any ideas as to what could've gone crazy?
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Easy. Your BBs are jamming either partially or fully and it’s causing a complete or partial compression jam, which is giving you PME at higher RPS.

When you lose FPS on full it means you’re having mild PME. When it screeches because you put a mag in, it means the compression resistance is pushing you over the edge.

More knowledgeable techs, am I crazy?
A compression block will create PME under those circumstances, depending on how bad it is. I suggest inspecting your hop rubber carefully, and trying to push BBs through the lips while the barrel is installed into the hop unit.

Check and make sure your spring isn’t binding.

Make sure that the Perun is putting your gears at the right spot.

Check for midcap syndrome—it occurs when the nozzle is pushed upwards by the mag spring and this can potentially cause blocks as well.

Make sure your cylinder doesn’t have hardly any friction with the piston head—I had an issue where my gun was hitting PME even when it shouldn’t have, because the piston was off spec and was slowing the return down.

Make sure your gearbox is aligned perfectly with your hop unit/upper receiver. I notice you have the RA shell—since it‘s an aftermarket shell, there’s a chance it’s causing issues. I have a continuing build that uses an E&C shell in a Classic Army body, and though it fed perfectly as an SSG, as a DSG it became apparent that the shell wasn’t perfectly aligned with the HU, causing nozzle stick and issues.

Airsoft is weird man—things have funky issues. Try testing everything you can possibly think of: look at and cycle the gearbox with just the motor grip attached. If you have zero issue, it means the barrel group and feeding assembly may be at fault. Simulate a mag being inserted by pushing around on the hop unit and gearbox with your fingers. If anything starts screaming, it’ll tell you stuff.
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Spring binding is when either the spring sticks to the spring guide for some weird reason, or your spring is too long for the gearbox. This was the most random of my suggestions, though—I generally don’t expect this issue.

Midcap syndrome can be checked for by: using a hicap, trying different mags, and just trying to shoot the last few round of a mag.

I have to say, I think it almost has to be BBs causing a compression block. It’s possible someone smarter than myself will swoop in with the answer, but I don’t see why you’d only hear the sound when actually firing unless it was related to jamming. Both of your barrels are normal—I trust the Lambda to be the right inner bore, and the Krytac is actually a 6.05mm, so that leaves your hop unit, hop rubber, and potentially your tensioner and nozzle. The best way to try and diagnose a jam ime is to just get inside the thing and take a look, searching for wear, damage, and other indicators.

One note is that your Maxx nozzle is actually shorter than standard M4, which could possibly be related. Obviously every gun is different, but stock M4 is 21.4mm, and I tend to find works well.

What model of gun are you using?
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That would make sense—good deduction.
Feeding is annoying man—keep on fiddling! The Maxx adjustable nozzle may be of some use to you here. When guns aren’t feeding right, I always pop one in to find the perfect nozz length, and then either use the right nozzle, or shave a plastic one to fit.
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