New mods with chrono results!

[cpu77]

Well I added my flat top valve and piston with a extended bolt...    It shoots allot harder...   No where near advertised though:)  Wheres my 600fps on 10 pumps!

The only pallets I have are Gamo Rocket .177 Cal 9.6 Grains.  Not sure what else I could do to get s few more FPS.  I know a longer barrel I am to add a 14".   Maybe do the Port tube mod.  What kind of FPS can I expect? Results are a average of 5 shots. Green Chrono with LED Kit.

Current mods
Stock 10" Barrel
Crosman Steel Breech
Charlies Flat Top Valve
Charlies Adjustable Delrin Piston
Charlies Extended bolt

Stock with Steel Breech
5 Pumps -392
10 Pumps-491
15 Pumps -557

After Mods
5 Pumps -436
10 Pumps-566
15 Pumps - 637

[BerkshireHunter]

That's a nice increase. I think you might be around 600 @ 10 pumps with a 7 gr pellet, those rockets are heavy.
On the plus side, that's 6.8 fpe.

[cpu77]

I was hoping to see around 110fps gain!  My mods would suggest that?   Maybe I am leaking air?  Not dumping all my air?

[breakfastchef]

I was hoping to see around 110fps gain!  My mods would suggest that?   Maybe I am leaking air?  Not dumping all my air?

Strip the pistol again, clean, re-lube and test. I got 100 fps+ on two 1377's with a flat top setup. Sometimes a simple strip down and rebuild ferrets out small problems.

Also, check the o-rings on the valve and piton as well as the bolt for damage or deformation.

[NorthStaR]

If Melons FT valve is adjustable then that may also help by tweaking it to the sweet spot (if possible).

The longer barrel WILL add more power....more depends on how long you decide to go. I see 10", I went to 18".

I get 11.5FPE on 10 pumps with .22 14.3GRN DIABOLO pellets with an FT and an 18" barrel.

So in theory (apart from .22 being more efficient with gas) it is really back to your barrel length assuming as others
suggest that there's no damage/leaks to the rest of the setup.

If it isn't dumping all the gas per shot, a small preload on the hammer spring may help that.

Also the weight of the pellet will factor some of the numbers, how fast you pump and the air temp all play factors.

IMHO! 

[cpu77]

I adjusted the piston.  Not much to mention there.  Helped a bunch on my 760 though.   Ok so I always thought longer barrel more FPS.  So a 24" is the way to go.  How ever I hear people saying they think they can get more power with less than 24"  say 20"  Please explain:)

[MartinDWhite]

The 20 vs 24 argument is that the volume of air in the valve is not enough to fill such a long barrel and the pellet starts to drag in the last four inches. It really takes experimentation to know for sure the optimal barrel length with what other changes you have made.

[breakfastchef]

The 20 vs 24 argument is that the volume of air in the valve is not enough to fill such a long barrel and the pellet starts to drag in the last four inches. It really takes experimentation to know for sure the optimal barrel length with what other changes you have made.

A longer barrel yields increased fps. In my personal experience, each additional one inch of barrel length (over stock barrel) increased pellet velocity in .177caliber @6-8 fps/inch.

[cpu77]

MartinDwhite

I could see how that could be true.  Hmm....  I will do some tuning resealing ect.  Then test again.  The real bummer was my buddies Daisy power line with the same pumps right from walmart beat me with a 18" barrel by almost 50fps!   I have FT Piston and and FT valve.  I got 565 after I adjusted the piston.   I am new to air gunning.   I have learned alot.  Before cutting a barrel I paid a bunch to have custom made I think I will try the following.

Disassemble the hole gun clean and debur.
Then polish the pumper tub to a mirror finish
Then Mod my exhaust port on the Valve to 45 degrees
Maybe change the inlet spring to a lighter one if I can find one.
Put new o-rings in everything
Line up and if possible open up the transfer port
Play around with a slightly heavier hammer spring

Two things to try
Customize the bolt with a single o-ring for better sealing.  They have no o-ring
Machine the plastic bolt back to add a piece of delrin to act as a extended probe?

A huge issue with the gun is once the triggers pull the bolt bounces back.  If I hold it shut I pick up almost 30fps! 


If all this does not get me over 700fps cut the barrel a 1 inch at a time:)   Then crown it:)

[breakfastchef]

Customize the bolt with a single o-ring for better sealing.

No o-ring on your bolt? You lose a lot of air because the barrel is not sealed! Before you do anything, get an o-ring on the bolt.

[NorthStaR]

If all this does not get me over 700fps cut the barrel a 1 inch at a time:)   Then crown it:)

Remember; once cut, well........ it's cut! 

It may be cheaper and quicker to either use the 'search' function or start a thread on 'optimum barrel length for a 13xx'.

There will be a whole bunch of members which will have already been down that road.

With that knowledge the 'measure twice,cut once' will work. 

[pirouge]

the cheapest way to increase fps is to get a longer barrel, you need 300fps to kill a rabbit at 6" or 60' humanely, for that you need a springer or pcp for distance.

[cpu77]

I just read a study showing that anything over 19" on a 2240 actually lost FPS.  Then there was only 18fps more from 16" to 19".

Article is located here....
http://www.pyramydair.com/blog/2007/07/how-barrel-length-affects-velocity-in.html

I thought especially with CO2 this would not be the case.   As the gas expands on a larger barrel.   So after reading this I am thinking 14-16".   I have a 24" on my 760.  I may try the stock 18" barrel  and see what if it improves my fps on .177 pellets.

I get confused by all these things.  I think we come up with longer is better but maybe not always?

[NorthStaR]

There's only a certain amount of gas released (per shot) into a certain amount of space for it to expand, eg; the barrel.

Once it's expanded - it's job done. The rest of the barrel will be likely to cause drag/friction and slow the pellet back down due to the pressure drop.

[arkmaker †]

I thought especially with CO2 this would not be the case.   As the gas expands on a larger barrel.   So after reading this I am thinking 14-16".   I have a 24" on my 760.  I may try the stock 18" barrel  and see what if it improves my fps on .177 pellets.

I get confused by all these things.  I think we come up with longer is better but maybe not always?

   Ok, you are comparing apples to oranges........2240 is a co2 gun and the 760 is a basically HPA gun. With a co2 gun, about 18" has proven optimal for a .22 cal, but changes when going to .177 where you can use the whole 24" At least that has been my luck with experimenting.
   Now the 760 uses compressed air, which is a different animal all together than co2. I am thinking longer is better with higher pressure, but someone else really needs to jump in here so I don't mess you up to bad..............BUT, hold off on cutting that barrel down for a bit!

Rich