M16A1 low fps - theory?

[JinxDuh]

Hey guys, building a spare m16a1 AEG. I’ve put a Specna arms edge 2.0 gearbox and hop chamber into it which was working beautifully in the mk18 before I gutted it for the gearbox. Chronoing with the hop fully off I was getting 281 (average of 20 shots) with 0.25g bb’s, hop fully on at 118 (average of 20 shots).

The gearbox is all standard edge 2.0, nothing has been changed except the trigger to a normal style gate quantum trigger and an xt 105 spring. I’ve fitted a 590mm 6.03 Madbull inner barrel, the hop rubber and nub is standard Specna too. 
 

My main theory is the nozzle isn’t long enough for the long as heck inner barrel. I recall reading that nozzle lengths vary to the inner barrel but can’t find the source, is this true? I saw Lonex has an “M16A1” specific nozzle, which is what has given me this initial theory.

Also, I plan on fitting a maple leaf 60 and omega nub like I always do as I love the pairing (Unsure if it’ll fit the Madbull? Any help appreciated)

 

[Adolf Hamster]

hop fully on isn't a great way to test, many a pew can have low fps to the point of even being incapable of firing with a full hop applied.

the main thing is that with the hop set to an appropriate in-game level, that the fps is reasonable, fully on or fully off then things get weird.

if you want a ballpark measurement, without needing a range to set properly, then ~0.5mm of protrusion into the barrel will be at least in the same postcode.

 

[JinxDuh]

hop fully on isn't a great way to test, many a pew can have low fps to the point of even being incapable of firing with a full hop applied.

the main thing is that with the hop set to an appropriate in-game level, that the fps is reasonable, fully on or fully off then things get weird.

if you want a ballpark measurement, without needing a range to set properly, then ~0.5mm of protrusion into the barrel will be at least in the same postcode.

With the hop set to roughly the middle it’s putting out 217 average on .25g, so 0.54 joules which seems really low to me?

 

[Lozart]

Hey guys, building a spare m16a1 AEG. I’ve put a Specna arms edge 2.0 gearbox and hop chamber into it which was working beautifully in the mk18 before I gutted it for the gearbox. Chronoing with the hop fully off I was getting 281 (average of 20 shots) with 0.25g bb’s, hop fully on at 118 (average of 20 shots).

The gearbox is all standard edge 2.0, nothing has been changed except the trigger to a normal style gate quantum trigger and an xt 105 spring. I’ve fitted a 590mm 6.03 Madbull inner barrel, the hop rubber and nub is standard Specna too. 
 

My main theory is the nozzle isn’t long enough for the long as heck inner barrel. I recall reading that nozzle lengths vary to the inner barrel but can’t find the source, is this true? I saw Lonex has an “M16A1” specific nozzle, which is what has given me this initial theory.

Also, I plan on fitting a maple leaf 60 and omega nub like I always do as I love the pairing (Unsure if it’ll fit the Madbull? Any help appreciated)

What make is the M16A1 receiver? What barrel length do you now have and where is the vent on the cylinder?

 

[JinxDuh]

What make is the M16A1 receiver? What barrel length do you now have and where is the vent on the cylinder?

E&C, 590mm inner and 20" outer, no vent

 

[Hudson]

590mm and not 509?

A 590 would be longer than the standard M16 length, which tend to be 509mm.

 

[Asomodai]

Nozzle length isn't dictated by barrel length, but by the hop design. 

Sounds like you have a Hop Unit to Nozzle incompatibility which is very typical for bitza builds. Get an adjustable length nozzle and start working out what the ideal length is through trial and error. 

 

[JinxDuh]

590mm and not 509?

A 590 would be longer than the standard M16 length, which tend to be 509mm.

Just double checked, I stand corrected ? it was a long time ago that I got the barrel haha

 

[Hudson]

I'm having similar issues with my JG SG1, so i'll be keeping an eye on this thread.

I normally use 0.30s, but adding enough hop causes the power to drop precipitously.

It looks to be an issue with compatibility between the hop unit, bucking or air nozzle at a guess so I've got lots of playing about and experimenting to do!

 

[JinxDuh]

I'm having similar issues with my JG SG1, so i'll be keeping an eye on this thread.

I normally use 0.30s, but adding enough hop causes the power to drop precipitously.

It looks to be an issue with compatibility between the hop unit, bucking or air nozzle at a guess so I've got lots of playing about and experimenting to do!

Ironically, I did the exact same swap into an M16A2 build although it’s a standard 509mm inner barrel and it works flawlessly with a consistent FPS about 335 iirc. I’m going to try a different unit and bucking tomorrow and see if it fixes the issue. 

 

[gavinkempsell]

Dunno if this helps but the JGM16A1 I have shoots .84j, 297.5fps straight out of the box.

 

[JinxDuh]

Update:

The spring on the hop unit was making the hop unit sit at a slightly funny angle. Removed it and the fps is up to 271 (.25g again), which seems to be the average if Gavins JG and my other m16 is any indication.

 

[Adolf Hamster]

An option for the ar platform instead of the hop unit spring is some o-rings around the inner barrel in front of the hop, you gotta play about to get the right number.

Side benefit is it'll put some tension between the upper and lower reducing wobble.

 

[JinxDuh]

An option for the ar platform instead of the hop unit spring is some o-rings around the inner barrel in front of the hop, you gotta play about to get the right number.

Side benefit is it'll put some tension between the upper and lower reducing wobble.

Thank you, I’ll give that a try! I noticed the hop unit will occasionally turn left/right slightly too, I have to push it back straight via the mock bolt. I’m assuming it’s just the Specna hop unit not liking the E&C upper?

 

[Lozart]

An option for the ar platform instead of the hop unit spring is some o-rings around the inner barrel in front of the hop, you gotta play about to get the right number.

Side benefit is it'll put some tension between the upper and lower reducing wobble.

Might be worth looking into something like this: https://www.firesupport.uk/airsoft-pro-reinforcement-m-block-for-version-2-gearbox.html to hold the hop unit steady. Usually the wings on the side of the hop unit engage in the slots in the barrel to prevent that side to side rotation. You may be able to pack that a bit?

 

[JinxDuh]

Might be worth looking into something like this: https://www.firesupport.uk/airsoft-pro-reinforcement-m-block-for-version-2-gearbox.html to hold the hop unit steady. Usually the wings on the side of the hop unit engage in the slots in the barrel to prevent that side to side rotation. You may be able to pack that a bit?

That’ll be something to keep in mind! I’ve ordered a modify M16A1 nozzle to test, as well as a ZCI hop unit/rubber combo because it was cheap and I know they work well. Hopefully one of them should solve the issue ?

 

[JinxDuh]

Update-ish:

The nozzle was out of stock, using the standard specna hop still. I’ve put tape to remedy the hop wobble as seen in this video:

I also didn’t realise but the cylinder has a hole in it at the back, so I’m going to replace it with a brass sealed one and see if it improves the performance.



IMG_6322.mov

View attachment 134629

 

[JinxDuh]

After changing the cylinder the fps has stayed the same, so that leaves the dodgy hop unit and the nozzle. That’ll be the next changes to see if it helps. 
 

0.54j on a .25g bb isn’t ideal though, that’s for sure!

 

[Adolf Hamster]

The hole is a port, you get ones at different positions to adjust the volume of air being sent to the barrel.

Typically different port positions dont have much impact on fps (unless it's significantly under-volumed for the barrel length), counter intuitive as that may be. I suspect arcane pressure curve type shenanigans are to blame.

For a 590mm barrel, you'll be wanting a full cylinder for the volume anyway so it's not a wasted change.

 

[JinxDuh]

The hole is a port, you get ones at different positions to adjust the volume of air being sent to the barrel.

Typically different port positions dont have much impact on fps (unless it's significantly under-volumed for the barrel length), counter intuitive as that may be. I suspect arcane pressure curve type shenanigans are to blame.

For a 590mm barrel, you'll be wanting a full cylinder for the volume anyway so it's not a wasted change.

To be fair, it's a nasty old brass one so I'll be ordering a nice new shiny one along with the nozzle etc. I also realized when I was talking about nozzles needing different lengths for inner barrels I was actually thinking of the cylinder which I'd read on AK2M4's site. 

Luckily I did open it, as my piston was chewed to pieces and didn't look like it would survive much more. Lovely specna quality. Great, until it's not great. ?