Benjamin Trail NP All Weather .22 Breech Seal and POI

[Hsuny12]

I am new to air rifles so learning a lot right now, not only from from forums and videos but also from experience.  I had a small problem lately where the POI seemed to be changing almost every time I went shooting.  First I thought that the scope was moving but even while keeping the scope rings tight, the problem persisted.  I opened my gun and checked the piston seal, gas ram, and piston for signs of damage and wear, everything was perfect.  I used a Chrony to measure speed and discovered that the FPS was highly variable between shots and was almost 100 fps slower than before.   I did a tissue paper test for breech seal leakage and never noticed anything, it was also very hot outside where I was shooting.  At this point I was stumped.
Last night I decided to set up a little shooting range in my hallway to test further (the loss of power was driving me nuts!)  I re did the tissue paper test and low and behold.....it blew a hole right through the paper.  The air was cooler in my house than outside so I guess the seal was smaller than it was when it was hot. 
I replaced the breech seal and put a few wraps of dental floss behind it so that it sits slightly proud of the breech.  I began shooting and everything was perfect,  FPS was back to almost 740 fps, extreme spread was less than 10 and the groupings were excellent.  Problem solved.  I hope things settle down now so that I can work on marksmanship rather than better ways to scratch my head. 

Just to check my old breech seal, I tried it in the gun with dental floss shimming and it performed the same as the new one, so I will save it as a backup in case the new one fails.  I put the new one back in anyways.

All testing was done with Crosman Premier Dome 14.3 g.