Barrel Swap - drastic FPS differences

[t_jones98]

Crosman Nitro Piston Venom .177 cal

Swapped a standard N-Venom barrel for a shrouded Trail barrel the other day.  Put it on the chrony and was quite displeased. 

.177 shooting CPHPs
ave 780 fps

That can't be right.   
Tried another string and the same.  Ok, maybe it's the shroud.  I'd take a little drop in fps for a little quieter shot.

Took the shroud off, and the same, about 780 fps ave.

Hmmm, ok, back to the original barrel.  Shoot a string and 830 fps ave.  What gives?

And I know that rifle should be shooting faster than that also.  With the barrel swap, I had also replaced the piston seal.  Maybe I tore it in the re-assembly.  I'll have to take a look. 

But why the difference in speed with the swap of barrels?  All other things the same, did the breech seal test on both as well.

Thoughts?

[BenjiHunter]

Maybe the barrel is not aligned with the transfer port.

[rsterne]

or the barrel that is shooting slower is larger in diameter and the pellet is moving too easily and not building peak pressure.... Push a pellet through both barrels and compare them....

Bob

[t_jones98]

Bob,
I did the pellet test before assembly and there is a noticeable difference.  A pellet almost slides down the new barrel on it's own. 

Bad barrel?

But that still leads me to wonder about the lack of FPS with the original barrel, 830 ave?  I'm not expecting the quoted "1000 FPS with Lead" from Crosman but 900+ would be nice. 

BH, I'll check the breech seal.  That was new as well and added one shim.  I did the breech-tissue test, but that doesn't mean that there isn't a leak on either side or the bottom.

[56S]

Are these the 7.9 gr pellets?  If so, you should be getting around 940 fps with a warm Nitro Venom.  I noticed a reduced velocity and a noticable piston slam if I shoot a loose pellet.  I believe my seal needs some restriction in order to expand against the compression tube walls to get full power.  Next time I do a tear down I will test the seal by seeing if it hold air.  I block the transfer port and push on the piston with a dowel.  A good seal will trap air in the chamber.

Something else:  What lube are you using and how much?

[t_jones98]

Slight amount of moly on the back side of the piston seal.  Tube has been lightly honed.   

Yeah, they're the 7.9 CPHPs and something just doesn't seem right overall with the FPS.  But that difference in speed between the barrels doesn't make sense.

[56S]

Is the gun warm?

[t_jones98]

Is the gun warm?

Yes, shooting in the basement. 

[avator]

Hmmmm... mine shoots in the low to mid 900s as well

[Kailua]

Is the transfer port in the barrel block the same?  I would assume so being from the same production factory.  But just curious.

[56S]

I'd go in and check the seal to see if it holds pressure.  Shooting with the loose barrel may have damaged it.  I may have miss the answer to this question.  Did you check FPS prior to making the change?

[Blue]

But that still leads me to wonder about the lack of FPS with the original barrel, 830 ave?

What's your elevation?  Your signature line says you're on your way to Colorado, if you are testing significantly above sea level expect lower velocity readings from spring guns due to thinner air in the compression chamber.

Blue