How many FPS lost from short stroking?

[marcusr]

Hi all,

I've recently made some significant improvements to the air seal in my G&G CM16 SRL and the FPS has jumped from 314+/-7 up to 332+/-2 with 0.25s. I would like this to be down at 305-310 for my local field limits. I am about to put in some 13:1 gears as well and intended to shave teeth anyway to avoid pre-engagement, so it fits well with my goal to reduce power!

My main question is how many teeth should I remove (I was thinking 1 or 2?) to get the power I want, however there is a slight complication. Part of the air seal upgrades was adding an R-hop, and in doing so I have obviously installed it with zero hop applied and tested like this. I don't have anywhere to shoot it until I go to a game so I can't go and test properly in the garden to dial-in the hop adjustment.

So with that in mind, the slightly more complex question is "how many teeth should I remove given that I currently have no hop applied at 332 but will at the field when I want it to be 305-310?"

Other relevant factors:

• M110 spring
  
• 380mm barrel
  
• 4/5 cylinder
  

Thanks in advance!

 

[Pseudotectonic]

332 FPS of a 0.25 BB is 1.28 joules, or same energy as a 371 FPS .2 BB. To get to 310 FPS (1.12 joules) you need to decrease 12.5% of power.

Assuming your sector gear has 14 teeth, 2 teeth out of 14 is ~14.3%. 1 tooth is ~7.1%. So I would say 2 teeth is probably better than 1 because you get extra headroom for PME. And better trigger response.

Not sure if your hop makes a huge difference so maybe try to gradually apply hop and chrono it to see if it drops quick or slow?

 

[marcusr]

Ah that's good to hear. I'll go for 2 teeth off and report back here to report the results for others to know for the future.

 

[marcusr]

So for anyone else asking a similar question in the future, I've got the empirical data. I shot 20 times before and after shaving off 2 teeth, with the spring/barrel/cylinder combo from my post above & both with no hop applied. I've got pretty excellent air-seal/consistency and based on the spread and standard deviation of the shots this wasn't negatively impacted during the upgrade (not that it should be by gears, but there's always the possibility of not putting it together quite as well as I did last time).

Before the average was 332.7 fps and after is 302.2 fps.

Little more than I'd like off the power, but I'm getting no PME on full auto and a super-slo-mo video shows it's close enough to the point where I'm better off safe than sorry! Once I've got the hop adjusted properly and measured the power again, I might get an M120 spring to make up the difference.

 

[Rogerborg]

Thanks, it's good to get the numbers.  380mm is quite a long barrel, so I'm not surprised you're seeing a significant drop by reducing the volume.

Have you checked how far back the piston is going now?  I await corrections with an airsoft physics explanation, but can't see any point to having the cylinder cut out further back than the piston rings(s) just before it releases.

 

[Pseudotectonic]

super-slo-mo video

Inside the assembled gearbox? How do you do that?

 

[marcusr]

Just by looking at the position of the piston through the cylinder port while firing in full auto. I'm looking for the piston to be temporarily stationary after slamming forwards. I assume with PME the piston is caught by the sector gear while still moving forward and therefore never stops moving.

I don't have the video from yesterday or access to my kit right now, but here are some I took a few months ago before all these upgrades. The second one is in full auto: https://imgur.io/a/cNyhP4o

 

[Pseudotectonic]

That is clever, very clear too so no more guessing, and I agree it looks like it is just on the edge of PME!