FPS reduction

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jonny201

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I have a king arms Thompson and the FPS seems to be much too low. It should be sitting at 320-330 FPS however my guess is 200-250 FPS at the moment as it is significantly less powerful than my new M4. I’m assuming this is an air seal problem. I took the gun apart and the only thing I can see which seems off is the air nozzle. I have attached photos of the air nozzle which seems to be chipped at the end. Could this account for the loss of FPS?

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A chipped air nozzle implies that the air exerted out of the front end will be uneven and thus would logically cause issues with fps. However it will not account for a huge discrepancy of 70 or so fps. When you put the cylinder and piston back together did you do all the usual air leak tests? Also more importantly are you sure the spring is capable of punching sub 300 fps (M95 type spec)?

 
I am only guessing the FPS so it may not be as low as I said however when fired beside an new M4 at about 328 FPS there is a noticeable difference in power. I took the gearbox apart a few months ago and it was my first time doing it and I didn’t know to check the air seal. What are the usual air leak tests? I don’t know much about spring types but the gun has not been modified so all the internal parts are standard.

 
Is it a v6 gearbox? If so look at the usual youtube videos. They will explain what to do and what checks you need to make..



 
ooh chipped nozzle will do that all right, could also be some issues with the cylinder as well but the nozzle would be my prime candidate.

it's amazing how much you can lose with even a little off the nozzle.

 
AlphaBear said:
However it will not account for a huge discrepancy of 70 or so fps
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I'd beg to differ. I had a cracked nozzle in my CYMA that dropped it from 330fps to sub 200.  A fraction of a mm of misalignment dropped my MP5K from 300 to 240.

You've either got a good air seal all the way into the hop rubber, or you don't.

 
Rogerborg said:
I'd beg to differ. I had a cracked nozzle in my CYMA that dropped it from 330fps to sub 200.  A fraction of a mm of misalignment dropped my MP5K from 300 to 240.

You've either got a good air seal all the way into the hop rubber, or you don't.
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Fair play... it's incredible how much difference a good air seal makes, but it's physics at the end of the day..... I guess an air nozzle will crack if there is a bb jam and it hits a stuck bb thereby chopping the bb and cracking itself in the process...

 
I’ve just got round to dismantling the gearbox now and it seems there is no compression at all when I cover the air nozzle and test the piston in the cylinder. Even with the air nozzle removed there is still no compression so although it may be a problem there must be something more. I have noticed a hole at the front of the cylinder which I’m pretty sure shouldn’t be there although perhaps someone could confirm that. I have attached a photo of it. Also the cylinder head isn’t a very tight fit to the cylinder and can slide quite easily. Is this a problem or is that normal?

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Try it again with the cylinder the other way around and no nozzle (just the cylinder, cylinder head and piston).

 
That hole definitely shouldn't be there. As Lozart says try turning the piston round but I would also check the inner bore of the piston, if that DIY hole is as rough on the inside as it is on the outside it'll possibly be tearing up your piston 'O' ring leading to yet another route for air loss.

Also notice in your pic that the spring looks to have been cut down but they haven't 'straightened' the end of the coil. 

Gratuitous plug here, I've used @ak2m4 several times for nozzle, spring, piston and 'o' ring parts, he's my go to for this kind of stuff.

 
Stevo4345 said:
Gratuitous plug here, I've used @ak2m4 several times for nozzle, spring, piston and 'o' ring parts, he's my go to for this kind of stuff.
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Never seen his shop before, but just checking it out now for a few minutes and I can see his prices are some of the best I've seen on certain items in the UK... 

Will add him to the retailer thread I put up a few days ago!

 
mzjango said:
Never seen his shop before, but just checking it out now for a few minutes and I can see his prices are some of the best I've seen on certain items in the UK... 

Will add him to the retailer thread I put up a few days ago!
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Not the most massive stock but good prices, even better is he's always been very helpful to me in figuring out what I need, always quick to respond to queries on his facebook page. I was recommended him by someone on here, used him ever since.

 
Stevo4345 said:
Not the most massive stock but good prices, even better is he's always been very helpful to me in figuring out what I need, always quick to respond to queries on his facebook page. I was recommended him by someone on here, used him ever since.
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Nice one, i' may put my next bits and bobs order in with him!

 
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