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 Post subject: Need help
PostPosted: Sat Aug 06, 2016 4:17 pm 
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Location: Central West River Nova Scotia
Ok I did the search thing, and dont want to read 200 posts to find the answer, but I know that its there somewhere. So I'm asking the members. I tried switching barrels on my SSP from a 17" long .22 cal, to an 18" long .177 cal and only gained 20 fps. Shouldn't I have gained a lot more ? Wassup ?

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 Post subject: Re: Need help
PostPosted: Sat Aug 06, 2016 6:07 pm 
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Location: San Diego, CA
Joe, you added just one inch and got an extra 20 fps. What were you expecting? What's the gun? I think you are being damn greedy.
That seems awfully good for any gun. I would have expected about 5-7 fps per inch. If you want some serious velocity put a 30" barrel on it then look for a holster.


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 Post subject: Re: Need help
PostPosted: Sat Aug 06, 2016 6:16 pm 
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I believe you missread the question. I switched from .22 cal to .177 cal, and only gained 20 fps.
I am assuming the transfer port is the culprit, but don't quite understand why. The tp is .165 dia. So already I think its too large for .22 cal, but wouldn't a larger tp be better for .177 ?

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 Post subject: Re: Need help
PostPosted: Sat Aug 06, 2016 6:48 pm 
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joe hickey wrote:
I believe you missread the question. I switched from .22 cal to .177 cal, and only gained 20 fps.
I am assuming the transfer port is the culprit, but don't quite understand why. The tp is .165 dia. So already I think its too large for .22 cal, but wouldn't a larger tp be better for .177 ?


Sorry man, my fault. I will default to Eric at stalwartarms@yahoo.com. This guy is the absolute wizard on modding airguns. He will have the answer.


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 Post subject: Re: Need help
PostPosted: Sat Aug 06, 2016 7:40 pm 
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Location: Coalmont BC
If the total volume of your transfer ports (all the porting from valve seat to base of pellet) is relatively large, then the pressure drop that creates will hurt a .177 more than a .22.... It could also be that your valve volume is great for a .22 cal but too big for .177, which would mean the gains are minimized.... Your .177 barrel is only abut 2/3rd the volume of the .177 barrel....

Bob

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 Post subject: Re: Need help
PostPosted: Sat Aug 06, 2016 8:34 pm 
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rsterne wrote:
If the total volume of your transfer ports (all the porting from valve seat to base of pellet) is relatively large, then the pressure drop that creates will hurt a .177 more than a .22.... It could also be that your valve volume is great for a .22 cal but too big for .177, which would mean the gains are minimized.... Your .177 barrel is only abut 2/3rd the volume of the .177 barrel....

Bob

Thanks Bob. Im not sure exactly what it means though. So is my tp too big for 177 ? if so, where would a good starting point be for trying different sizes ? And,, with the same pressure and volume, what fps difference should I expect when switching from .22 to .177 ?

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 Post subject: Re: Need help
PostPosted: Sat Aug 06, 2016 10:45 pm 
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Location: Eastern Townships
I'd say start at 3/32'' (0.094''), then go up in 1/64'' increments, chronying each TP size. If you have some kind of plastic rod to make the ''trial TP'', the better. Easy to work and seals rather well.JMHO.


Last edited by airmec on Sat Aug 06, 2016 11:52 pm, edited 1 time in total.

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 Post subject: Re: Need help
PostPosted: Sat Aug 06, 2016 10:53 pm 
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I would suggest a TP and Barrel port of about 0.140" (9/64") for a .177.... What I meant by extra port volume, is are the ports a lot longer than what you would find in, for example, a 2240?.... The HPA has to bring that volume up to valve pressure before it gets to the pellet, which causes a pressure drop....

Normally, the FPE would drop somewhat going down (maybe 10-20%) from .22 to .177, but the velocity should go up.... by quite a bit.... I would think you should get at least 10 FPE in .177....

Bob

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Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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 Post subject: Re: Need help
PostPosted: Sun Aug 07, 2016 6:16 am 
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Location: Central West River Nova Scotia
Ok. That all helps a lot. Firstly, my current tp is too big, at 5/32" and quite long at 1/2". So a lot of power loss or pressure loss after the valve opens. So I'll start small and measure in increments. Airmec, thanks for the suggestion of using plastic rod. I have a good selection of 2240 tp,s , but since the entire volume area of my 1/2" long tp needs to be restricted, I'll have to make new ones. One for .177 and one for .22. The 620 fps I achieved may possibly be surpassed with the right TP.

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