Brand new Cyma Mp5k running over power

[Mattcrx]

So… moving the hop made it fire a hell of a lot faster. 
 

So here is the results from my Acetech tracer BT. 
 

I have tried leaving the spring engaged  for a while and put a fair few mags through the gun and it’s still firing high. 
 

As you can see from the results the 352 FPS is with no hop at all as soon as I start moving the hop slide in towards the magazine it fire a hell of a lot faster.
 

So what options do I have now? Use 0.25g BBs?? 
 

Thanks again guys! 
 

View attachment 104255

 

[Rogerborg]

Of course with it being airsoft you'll get the occasional exception where a gun wants a little bit of pressure in order to get a perfect airseal

Well, am I glad I added that caveat.  Phew, that's a spicy little boi and no mistake.  I honestly wouldn't have expected that from such a short barrel, although CYMAs M4 and AKs do run hotter than UK limits by design, and need down-sprung for the UK market.

So what I'd assume here is that you've got one with something like an M120 spring in it that should have been down-sprung by the importer or retailer. No harm in asking them if they want to sort it as a warranty issue - it isn't fit for the purpose sold.

If they laugh it off, it does seem like you'll need to replace or cut the spring, and disappointingly it doesn't have a quick-change spring, so it'll be a gearbox out and open job.

It can be quite intimidating the first time you do this, but it's not really that complicated. Just take it slowly and keep all the parts safe.

Disassembly to the gearbox:

 

[Mattcrx]

That’s a brilliant guide… not sure what spring I’d need to replace with… 

would simply using 0.25bb not bring it down under the 350?

 

[Cannonfodder]

would simply using 0.25bb not bring it down under the 350?

No. The muzzle energy would be the same as increasing the weight means your max fps is lower. If a site is allowing players to use guns firing at 350 fps on heavier bbs then it needs to be avoided as the people running it obviously don't know what they're doing

 

[Nick G]

I was just about to answer that , beaten to it ! ?

 

[Mattcrx]

Yeah so if I ran 0.25g BBs my muzzle energy would need to be even lower to equal the max 350 of the site. I get it. 
 

so looks like iv got to change the spring! How annoying ?

 

[Nick G]

350 fps on .20's is 1.132 j    on .25's youd need to be at about 315 fps or lower

 

[Cannonfodder]

Yeah so if I ran 0.25g BBs my muzzle energy would need to be even lower to equal the max 350 of the site. I get it. 
 

so looks like iv got to change the spring! How annoying ?

That's correct. Here's a handy chart to show what the muzzle energy is for different weight bbs at different fps limitsView attachment 104281

 

[Rogerborg]

No

Maybe. Short barrelled guns can experience what I'm just about to coin as "Joule leak".  I get sharply dropping energy in my MP5K as I move up to heavier BBs.  It's worth a try, although I still reckon it'll want the spring cut or swapped.

Since we don't know what's in it, I can't recommend a replacement. I'd guess that it's got an M120, so anything in the M100-M110 range (maybe up to M115) might do it.

Always remembering that there are no standards in airsoft, so one brand/production run of "M100" could be radically different from another.

 

[Cannonfodder]

While it might happen I wouldn't want to rely on it to get the gun site legal

 

[Rogerborg]

While it might happen I wouldn't want to rely on it to get the gun site legal

Oh, I totally agree. It's shooting so hot that I'd want to get it down substantially with a spring change or cut, particularly if it's going to be used in CQB.

 

[Mattcrx]

It’s a Cyma CM.041PDW blue edition. It’s actually crazy that the power it is running is with the hop OFF! If I put the hop fully on it’s hitting 372s… ? 

I shot 1001 rounds last night and it did drop to 348s (hop off) and I thought brilliant it just needed bedding in… however this morning it’s straight back to 352s. Maybe the drop in power was battery going flat or everything starting to get warm etc… I’m not sure. 
 

Thanks again for all the advice guys! Looks like I’ll be stripping it myself or sending it to davescustomairsoft… 

Here some readings when I shot the 1001 BBs last night 

you can see where it hits 360s 370s as I’m putting the hop on. 
 

The last image is when it settled quite nicely but that’s after 1001 BBs

View attachment 104293

View attachment 104294

View attachment 104295

 

[Rogerborg]

It’s actually crazy that the power it is running is with the hop OFF! If I put the hop fully on it’s hitting 372s

Just to be absolutely sure, which way are you sliding to add hop?

 

[Colin Allen]

Yeah so if I ran 0.25g BBs my muzzle energy would need to be even lower to equal the max 350 of the site. I get it. 
 

so looks like iv got to change the spring! How annoying ?

No,  the muzzle energy, measured in Joules, could be the same; the velocity would have to be below 313 FPS when using a 0.25g BB.

313 FPS on 0.25g = 1.13J
350 FPS on 0.20g = 1.13J

Sites should really measure in Joules rather than quoting a velocity based on a 0.20g BB, which hardly anyone uses.

 

[Rogerborg]

Sites should really measure in Joules rather than quoting a velocity based on a 0.20g BB, which hardly anyone uses.

Insert Stock Rant #3 about "lol m8 evry1 noews wut 350 meens m8".

 

[Cannonfodder]

lol m8 evry1 noews wut 350 meens m8".

You'd be surprised 

 

[Rogerborg]

You'd be surprised 

I wouldn't be at all surprised, I've seen plenty of folk on Facebook ask "wut bb wait 2 get undar 350?" entirely sincerely.

 

[Colin Allen]

I wouldn't be at all surprised, I've seen plenty of folk on Facebook ask "wut bb wait 2 get undar 350?" entirely sincerely.

That would include the chap that I met at a site the other week; I tried to explain to him that using 0.28g BBs at 340fps was not acceptable but he was not having it.  Watching him try to argue it with the chrono marshal and then the site owner was amusing.

 

[Mattcrx]

Just to be absolutely sure, which way are you sliding to add hop?

I’m sliding the hop towards the magazine as the slide says HOP-> which I assume put hop on.. pullin out towards the muzzle is hop off

 

[Rogerborg]

I’m sliding the hop towards the magazine as the slide says HOP-> which I assume put hop on.. pullin out towards the muzzle is hop off

Yup, that's right. I didn't want to prompt the answer.