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Piston Advice for HI SPEED build

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Does anyone know what the SHS blue pistons are made of?


My girlfriend (yes I have a real female girlfriend) works for a plastics wholesaler and she thinks its a mixture of poly carb and some other black magic or witchcraft. Its definately thermoset but even the SHS website says plastic! What I want to know is whether a lighter 1 tooth pure polycarb piston will handle 35+rps over the traditional 15 tooth SHS blue?




I'm thinking Element M115 spring and Lonex A1 with a 13:1 Super Shooter gearset in a reinforced 9mm bearing/bushing box

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I think that the differences in density between polycarbonate, nylon, acetate, polythene, and recycled lower polymer length versions thereof, in something as small and light as a piston make very little, if any measurable, difference to the speed at which even a shitty motor can overcome their inertia. Choosing 1 steel tooth over a full rack will alter the weight far more than the composition of plastic. Personally I'm not convinced by drilled plastic either - the weight saved is, again, much less than the difference between numbers of steel teeth.


TBH, steel is so much denser than than plastic or aluminium that the number of teeth will matter more than drilled ali or solid plastic IMO. After having experimented with a few that I've acquired through people's clearouts or have been in guns, my current fav piston is the ICS MC-05 which weighs 24g:



I don't currently have a plastic one with a full rack of steel teeth, so I just weighed the steel rack from another ali one on its own = 6g, which compares to 14g for a plastic piston with 1 steel tooth. I couldn't be arsed to find a piston head to fling into the mix... especially since I favour ali ones and yours there are plastic - i'd have had to root in my 'will only see the light of day again in an emergency' bits bag.


Can't fault the high torque motor vs very high speed gears plan or large bearings (assuming you choose good bearings, but I dunno if Kanzen do 9mm?) and yeah, if you're going to run a 115 spring at 35-ish rps then a reinforced box has got to be a good idea, but... :unsure:

:wacko: ...what gun is this for?

I mean, with a decent air seal, a 115 spring will get more than 370FPS from even a medium length barrel...

Is it going to use a vented piston and short barrel... an SMG on roids?

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363mm x 6.08mm barrel should be loose enough that I can adjust fps with different weights of pellet to keep it legal on the day.


Its a spare partsspecial, I've not done a build from scratch so thought it would be an idea to do something a bit special. According to all the airsoft engineers i've spoken to, weight of the piston setup is key to prevent pre engagement over 30rps. I managed to loose just under 3 grams through swiss-cheesing the polycarb piston. The aluminium head I was considering weighed in at like 13g on its own so plastic was a no-brainer. My brother gave me a prototype polymer he acquired through work with a compressive load limit of something mad like 16,000bar, similar to ally but 1/5th the weight. So we machined an exact copy of a lightweight piston head recommended by action hobbies and there you have it.


Bearings are element but I also have terminusX bushings for when/if my seigtec gears ever arrive! Probably best to save those for something really special. I'm more worried about the tiny bearings 5hat the super shooter gears have already installed in them.


Where can I get a decent stick lipo btw? Nothing fits a standard battery stock once the mosfet is in, I am going to purchase a madbull skeleton stock for a bit of extra room but there's still only going to be 250 x 23mm space for batteries.



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Turnigy Nano-Tech LiPo from Hobby King.


You can't adjust FPS by changing BB weight site legally, mate. The word for that is 'cheating'. You're supposed to chrono with 0.2g BB's and no hop on, usually - some sites allow you to leave hop on, or with a conversion chart so they can tell if your .25's are too hot or not. Using .23's or .25's to come in under the .2 FPS limit is a serious breach of rules and may get you an instant ban.


I think that a 115 spring will be too hot with an M4 length 6.08mm barrel. If it's for a woodland site that allows 370, best go no higher than 110, or if you have your own chrono to test it bit by bit, you could start with a 115 and clip turns off... Bear in mind though that ambient air temperature and humidity alter how air behaves under compression and expansion, so if you get it set nicely at 368, you could easily find it at 373 another day, and various chrono's measure differently... a 110 is all you need in the UK.


What cylinder are you using though? Maybe if your vents are far forward a 115 will make up for low volume, and low volume increases ROF. You may be right that 3g matters to exact ROF, but in terms of skirmishing, does 34rps create less psychological impact than 35.6rps, or whatever... no - you're creating a monster, don't make it prone to breakdown every time you fire that mental full auto :lol:

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That makes sense actually, firearms are graded by muzzle energy so chrono'ing set weight bbs simplifies it for laymen. Its been about 6 years since I skirmished at a licensed site so forgive my ignorance, I'll read through the legalities section again.


It might be a case of short stroking a piston and going for a 105/110 as you say.


Sorry, think this should be in the tech section

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Yeah, tech section - doesn't get as much traffic, so unless anybody complains well, or another mod/admin thinks different, it can stay here. We are covering a whole raft of issues too, rather than sticking to one tech subject, so yeah... whatever :)


Did you do any research to determine how much plastic to drill out of that piston?


What cylinder are you using and what FPS are you aiming for?


Are you doing a build, dian11?


For the greater edification of all and lurkers...


Cylinders are typed by number or letter:


SHS-CYN-SHST-T1_1_MARK.jpg Type 1 or A has no vents SHS-CYN-SHST-T2_1_MARK.jpg Type 2 or B

SHS-CYN-SHST-T3_1_MARK.jpg Type 3 or C SHS-CYN-SHST-T4_1_MARK.jpg Type 4 or D


There are variations between manufacturer, but the basic idea is that the closer the vents are to the cylinder head and/or larger, the less air will be compressed by the forward stroke of the piston and the sooner the back stroke will no longer require air to flow through the piston head vents to prevent suction through the air seal nozzle (Edit: which thus allows the piston to move back faster, since it is doing so against less resistance).


For eg, 5KU call this: 5KU-EG-CY-03_MARK1.jpg a Type 2, but they call 'Type A' "Type 0"

And this: SYS-A12_1_mark.jpg doesn't have a Type, it's just meant for MP5's, but clearly will work with any short barrel, or medium barrel + powerful spring (as we talked about above).

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OK this is all starting to make sense now. Most of the info I have collected has come from airsoft mechanics' sites and in most countries the muzzle energies allowed are much higher than in the UK. So I need a strong (ish) spring to get the piston forward before the gear to prevent premature engagement but I also need to keep pressure down in the cylinder to keep fps low!


By gum! I think I've got it! URBAN ASSAULT limit aeg's to 345fps max whereas the other site I'm considering is woodland so limit is 370fps


But what kind of cylinder is this? Its an Element I think with one cut-outpost-7095-0-20671500-1389731046_thumb.jpg

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Looks like one of these, which is for medium length barrels.


The eternal juxtaposition: CQB vs Woodland, which FPS limit to go for?


In the absence of a split gearbox, or one which allows varying tensions on the spring, it may seem like we have to pick one or t'other, but not necessarily, because FPS isn't everything when it comes to range - my AKS-74U is set up to be CQB compliant, under 350FPS, but still good enough for woodland, better than average - ICS hop units are the way forward anyway, but i needed a new one b/c the original was only held together and in place by dirt and goodwill (but it was still firing consistently at 318FPS, as a brand new one would) and I added some gucci hop rubber (I forget now) and precision barrel.


I'm sort of distracted now so i'll come back another time and say something more useful about specific cylinders, barrel lengths, springs and FPS :)

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That's the cylinder. Surely having one cutout will imbalance the piston at high speed and cause extra wear?


Stripping the gun and splitting the gearbox takes 10minutes, something I'll be doing after each skirmish anyway to see how the internals wear. Swapping out cylinder, piston and spring is no bother between skirmishes.


What's the difference between the ICS and Madbull Ultimate hop units? I've swapped out the stock blue bucking for a softer Lonex one as the barrel wouldn't fit otherwise and I had to polish the seams away from inside the hop unit itself. Makes you wonder why Madbull are so expensive when every part has to be rethreaded or refinished and painted! Except the battle axe stock I picked up yesterday, thats well made but f*^k ugly! Will the nub make a big difference?

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Yeah, that was kind of what I was thinking, but by that stage last night my meds had severely reduced the gorms/s² I had available to devote to the task :lol:


- my AKS-74U ROF is on the low side of medium currently and i've been happy with that using mid caps (although I am in the process of upgrading her to an 11.1V LiPo for initial trigger response and naturally, unless I buy a more complex MOSFET than the simple AB I have to control it, that will result in an annoyingly higher ROF) so my choice of CQB / woodland FPS is primarily about choice of spring and barrel, but you're trying to tune your beast much more finely, so yeah, swapping out spring, piston & cylinder together to up/down from 340<=>365, if keeping the ROF so high is your main aim in either case, could be the best bet. But if you think about it, assuming you get 35rps at a consistent 365, then a simpler way to get down to 340, rather than having to start from scratch, may be to swap out the barrel...


Hang on, hang on, I know, 25FPS difference in such a short barrel...? Well, you get about 30FPS by dropping a med-long barrel from 6.08 to 6.03mm and up to another 30 by dropping down to 6.01mm diameter. So yeah, if you use a 363x6.01mm TBB for woodland and swap that for something like a 300x6.05mm for CQB, then you may well have it. That also means you can use a weaker spring, which is easier for the motor to pull, thus giving a higher ROF...




Surely having one cutout will imbalance the piston at high speed and cause extra wear?

Yeah, you'd think so wouldn't you? TBH, even though you wrote "one cut-out", I assumed you meant one on each side. I can only assume that, if they even care, Element decided that this Universal Cylinder would not be chosen by techs doing high speed box builds.


I keep meaning to build an MP5 so that I can learn the V2 GB, but I'm not a gun tech per se and M4's don't suit my body and hands, so I'm not familiar with the hop units you're on about, but yeah, by the sounds of it I'd have sent the 'kin thing back! As far as I'm aware the ICS M4 hop unit has a good rep, but I can only tell you from personal experience about their AK one in comparison to the TM / clone / compatible ones - whole different creature.


It's attached by long bolts parallel to the barrel which go through the clip that locks the barrel centralised (fitting into those side grooves on barrels) so it's much more solid than the TM which uses a poxy little 'C-clip'. It's made out of a very shiny, somewhat brittle, plastic, which is kind of surprising but it's plenty strong enough for the feed tube to resist being damaged by clumsy mag changes. I can only guess that they really wanted the surface to be resistant to wear and very smooth so it doesn't add much effect of its own when the hop rubber squeezes the BB against it.


Anyway, yeah, t'other week I strolled into a conversation, as you do, without having heard what had been said as somebody shouldered his AEG to shoot a few shots at a pole about 40mm wide at maybe 22m - I raised Phatima pretty casually, not even using the front sight pillar, just the kind of instinctive shot that you'd take at somebody who surprised you coming around a corner - ding! Hit first time. Ding, ding, ding, etc. - turns out the convo was about accuracy and how fettling guns for it isn't necessarily so easy :lol:


So that isn't me boasting, b/c I'm not a bad shot but I don't think I'm exceptional, nor do i think i'm a top drawer gun tech, but it's prelude to saying that yeah, the hop nub is crucial. The set up - standard ICS hop chamber, gucci rubber (I forget), Madbull Fish Bone Spacer (that's what they call an H-nub), Systema BS(lol) 6.05 x 225(iirc)mm (MP5 TBB = short for an AKS-74U but I got it cheap so tried it out and just haven't got around to swapping back to the one I had in her before) & Blaster 0.25g BB's. IMO it's a case of nothing in the chain is bad for accuracy, but the only thing which is good for it is the nub:


- it only just fits into the space allowed for it, so it cannot be anything but bang on central to the hop unit

and those flanges at the edges make the rubber engage with the BB equally on each side and wrapping around so it forces the BB into the centre of the hop chamber, whereas a simple cylindrical nub presses down on the centre of the BB, wherever it is in the chamber, so naturally the space needed for ease of BB feed and the tolerances that the system can be engineered to, at sensible prices, mean that the BB must be starting out on one side or the other of the chamber and as the pressure of the nub comes down central to the hop unit, there is a slight discrepancy there...


- it's difficult to explain what I mean without a diagram (I may draw one later tonight), but I hope you can imagine what I'm on about - the rubber is squashy, most nubs have some spring in them, the direction of force is off centre to a spheroid, hence it must impart more spin to the side of the BB opposite to that side of the hop chamber to which it is closest, which must be unstable since that will make the BB tend toward that side of the barrel and bounce off... from then on you'd need a supercomputer to work out what happens to the spin as the BB bounces back, forth, up, down, etc. down the barrel. The 'exit cone', iirc its name, describes the potential trajectories of a BB exiting the muzzle and the more unstable the spin, the wider it is. In an ideal world the BB would leave the hop chamber with back spin on an axis exactly perpendicular to the centre of the barrel and then pretty much only gravity and shooter movement would widen that exit cone from close to a straight line, but obviously that isn't the case... measures such as H-nubs / X-Spacers / Fish Bone Spacers, in a complex way the "biro-refill mod", and hop rubbers with a built in division such as PDI W-Hold, Falcon Dual Point, stock CYMA, all go some way towards getting closer to the ideal.


I like that stock - I mean the paint job in general is lush, but the stock makes it look like a custom weapon rather than just another (yawn) painted M4.

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I smell what you're cooking and I'll raise you a 2 peice barrel!


Just realised the bloody thing is a 2 peice so I can switch between 255mm and 363mm inners. Game changer!


I've been reading up on the Lonex A1 motor and I'm not sure I want to go for a spring under M115 so which cylinder would you suggest? Can you get a type 4 for a short barrel?


Hop unit looks identicle to your ICS but the Madbull is chromed steel with a single wheel on the back for adjustment and a cutout to attach an led on the in-feed for tracer.


Like you, I've always favoured accuracy over volume but being able to suppress and manouver in to a position where your team can flank the enemy is important, plus a hi-speed build is more challenging. But the whole point in hi-speed is accuracy through volume, hence the looser bore barrels. Imagine a rifle that can hit a 10p at 20yards repeatedly then full-auto 30+ BBs per second while you shoot-and-scoot back to cover. Thats the brief anyway.


I've encountered another problem though, I can't find an 11.1LiPo that will fit in the stock. I can fit 2 7.4v1000mAh 30c LiPos which should be fine as I'm using #14AWG wire and deans connectors but for the full hi speed effect I really want an 11.1v. Madbull do an 11.1 specifically for the stock but I can't find anywhere in the UK that sells them.




I like that stock - I mean the paint job in general is lush, but the stock makes it look like a custom weapon rather than just another (yawn) painted M4.


Believe me, the only 2 parts of this gun that fitted together first time without having to be modified were the upper and lower receivers! I had to re-thread the Madbull upper to fit the Madbull barrel nut and RIS, then I had to take it all apart again about 6 times to get the outer barrel to sit square so that the hop unit lined up with the mag. Then I had to take alot of material off the hop unit so that it sat forward far enough to line up with the mag. Then I had to machine a lot of material off the back of the lower receiver so the Madbull stock tube would fit all the way on...



...some time later...



Then I had to polish all of the internal faces of the 5KU gearbox so that the cheap paint they sprayed the thing with didn't flake constantly and clog the gears up. Then I found that the spur gear wouldn't fit because the recess in the box was the wrong diameter so I had to machine that out. Then the wires were far too big to fit the box so I had to machine out the recess for those too...


Basically, the whole gun is custom from start to finish, top to bottom. If the gearbox doesn't explode in my face and the LiPo catch fire first pull of the trigger I'll be shocked!! :D


There are so many parts available both internal and external for the M4 that it seemed like a good idea for a first build, plus it fits my tiny feminine size 8 hands pretty well and the 6point crane stock is just about long enough for my arms. If I had the choice of a VZ58 Tactical I would have gone for it but I don't know if they even make an airsoft replica.



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Almost forgot the obligatory gun porn. I'm thinking about cutting away some of the rail at the front so I can fit a flip up iron sight to the barrel to match the rear.





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mmmmmmmmm vz58 tac <3 not the same but... I mean, it sounds like you have the expertise and tools to take it to the next level.


So yeah, 2 part barrel? Never considered before but it doesn't sound good for accuracy. Dropping 38% of the length will drop some FPS, but not 25. Maybe 10... but I'm not sure, maybe not even that, do you have your own chrono? I think maybe a 2 part barrel isn't much use unless you make an option front end, like ultra short, AFV Crew stylee or helicopter deployed PMC.


Type 4 cylinders are intended for short barrels so yeah, if you go with the SHS one b/c the vents aren't that far forward, just shaped like teardrops so there's a wider bit - you can calculate the absolute air volume you're getting from it and compare that to your barrel volume. An experienced gun tech told me he always goes with 3:1 as the ratio, which sounds plausible, but then again, he also believed so much in "suck-back" that he'd convinced himself he had seen evidence of it (which is bollocks without any doubt due to the speed of the BB and position of the tappet plate). Which reminds me... what sort of sector clip do you favour? But back to cylinder vol. You could start with a Type 3, or the SHS Type 4, and mill the vents bit at a time down to reach 3:1 and see what if any effect it has on FPS - in theory it shouldn't have any, but i suspect there could be one or two FPS in it, but the less volume, the higher the ROF should be. But at least that way there'd be no chance of fucking it up...


Did you check Hobby King for a Turnigy Nano Tech LiPo? They come in plenty of shapes and sizes and so far I'm really impressed with the 1300mAh 25C constant 50C burst 11.1V one I have - it'd be in Phatima permanently but she fires full-auto on semi with it and i haven't got around to fitting the AB MOSFET yet - in fact it's hiding under a pile of crap that is building up in the "don't put this away in a box and/or cupboard because you'll need it soon" area of my flat...




Imagine a rifle that can hit a 10p at 20yards repeatedly then full-auto 30+ BBs per second while you shoot-and-scoot back to cover. Thats the brief anyway.

Sounds good. TBH I think I'd have trouble doing that 5/5 over iron sights with my SVD solidly on its bipod, but yeah, it's a goal alright!


Do you mean you have long arms? I had to put an extra hole in the adjustable stock of my G36KV to make it fit me without feeling cramped up a bit. But yeah, big hands, but only glove size 9... width! More like 10.5-11 length - has its advantages, just not when it comes to finding anything ready made to fit hands :lol:


Oops, hang on, I've got a Type 4 lurking in my bits box - i'll measure it when i finish eating my dinner.


OK so (Ø 23.87mm/2)² x Π = 447.50mm² and L from vent fwd edge to front = 46.39mm so you need to measure your cylinder head length and subtract it from Lvfe to give you an accurate volume...


- but the first one I had lying around measures 6.76mm so that's internal Length 39.63mm


Cylinder A 447.50mm² x iL 39.63mm = V 17734.43mm³


Barrel (Ø 6.08mm/2)² x Π = A 29.03mm² x L 363mm = V 10539.09mm³


Cylinder V 17734.43mm³ / Barrel V 10539.09mm³ = Ratio 1.68:1



:blink::unsure: hmmmmmmmmmmmmmm...


not 3:1 then, but let's just check something...


Type 1/A Cylinder Area 447.5mm² x (Type 1/A Length 72mm - Cylinder Head Length 6.76mm) = V 29194.9mm³


AK Barrel (Ø 6.08mm/2)² x Π = A 29.03mm² x L 455mm = V 13208.65mm³


Ratio = 2.21:1


M16 Barrel (Ø 6.08mm/2)² x Π = A 29.03mm² x L 509mm = V 14776.27mm³

Ratio = 1.98:1

AUG Barrel (Ø 6.08mm/2)² x Π = A 29.03mm² x L 550mm = V 15966.5mm³

Ratio = 1.83:1

So I think we can be sure that Mitch was completely talking out of his arse. There is also a very good possibility that I have misremembered him, but I know I haven't misremembered so badly as to think he used a whole number ratio when in fact he had used a fraction, because there was a whole debate going on and he said it a fair few times, even mentioning a quick and dirty way to proceed by using water to measure the volumes. So no, even if he said 2:1 it's bollocks, because demonstrably AUG's with stock JG barrels do work just fine, as do M16VN's and custom builds with long barrels inside suppressors...


- Lonex do an AEG barrel Ø 6.03mm x L 715mm & Volume = 20418.80mm³

Ratio = 1.43:1

So there we are... as what seems like a reasonable educated guess, I'd say that to be sure 1.5:1 is ok...


Cylinder Volume Xmm³ / Barrel Volume ((Ø 6.08mm/2)² x Π = A 29.03mm² x L 363mm)mm³ = 1.5 so:


X Volume = 15806.84mm³ / Cylinder Area 447.5mm² = 35.32mm Cylinder Internal Length



So you should be able to mill the vents 4.31mm closer to the front of the cylinder.

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mmmmmmmmm vz58 tac <3 not the same but... I mean, it sounds like you have the expertise and tools to take it to the next level.


So yeah, 2 part barrel? Never considered before but it doesn't sound good for accuracy. Dropping 38% of the length will drop some FPS, but not 25. Maybe 10... but I'm not sure, maybe not even that, do you have your own chrono? I think maybe a 2 part barrel isn't much use unless you make an option front end, like ultra short, AFV

3. 2. 1. I'm back in the room!


The VZ is sweet but at that price it would be a major investment and it sounds like they are building it from scratch so I would be concerned about reliability etc. The original version was trully aweful too which is why they've only got a few people signed up.


I mean the outer barrel is a 2 part. 14.5" or 9.75" 363mm inner or 247mm. I'm looking at a Prometheus 6.05 x 247mm inner as a starting point as I don't really fancy running round the woods atm, especially since winter might be on the way soon.


I finally received the bits I ordered from Airlab so I've installed a sorbo pad and got the AoE set and that leaves me with 2:1 volume with the 247mm barrel.


The sector gear I'm using has a separate baring on the tappet plate lug s no need for a chip.

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