NP XL 725 Compression Tube

[hd833]

Is there a problem with the tube being out of round? I found a rub mark on my piston that ran the whole length. Just finished tuning it and after being disappointed by the velocity (628 fps w/27.8 grn domed Benji) I recalled the marks on the piston and will now have to tear it all down again and mike the tube and piston to see which is the culprit. If it's the tube I'll have get a new one and if it's the piston I'll have to turn it a few thousandths. I'm wandering if this has been a problem with other owners.

[Bryan Heimann]

Those numbers equate to 24 fpe, which about right for a .25 XL.  Unless you added or removed weight from the piston, or used a different, more powerful ram then I wouldn't expect more

[thekid]

Commenting on the rub on the piston.
If it is on the back of the piston, other side of cocking slot it is from cocking the gun.

[hd833]

I know the fpe of the rifle (thanks anyway) but you would think the wear marks would be the equal to the radius of the tube. In other word the whole other half of the piston, not just one little tiny strip just opposite the cocking slot. The pressure applied when cocking should be applied evenly across piston if it was machined properly. I realize there's a few thousandths difference between the outer edge of the piston seal and the shaft of the piston. Maybe I'm just being too fussy being an old machinist. One other thing is the oblong shape of the orifice of the transfer port and all the swirl marks inside of it. Either from a dull bit or drilling too fast. I might just get a .140" reamer and clean it out.

[uncle paulie]

Has the gun been  honed? As the gun is shooting exactly where it should, I would be very cautious about fiddling with the transfer port. A little too much can easily go the wrong way. JMO

pv

[mtsheron70]

My piston of my regular Trail NP has the rub too.  But nothing has affected the gun other than a nicked piston seal which can be frustrating.  But I know what you mean because the first time I broke it down I too wondered why this rub in the one spot. 

I am reminded also of, "If it ain't broke; don't fix it!"

But with that said, I love to tinker way too much so good luck with your venture.  I will suggest first and foremost a chemical effort first.  I took mine back down due to loss in fps (ie. the nicked seal which I surmized) then to make sure the cylinder and the compression tube were truly clean of grit and muck I used brake parts cleaner to saturate those parts to get it squeaky clean!

Well, when my Maccari piston seals come in then back to the shop to reassemble and hopefully not nick another seal!