Around 20 years ago I purchased 2 french springer air pistols MANU-ARM they are pretty good pistols. one day my Son wanter to go to our backyard range, and fire the pistol, problem was that one of my Nephews was with him. Now I trained my son and didn't have a problem with him going alone, but I tagged along because of my Nephew, this happened a few times and I figured that they were ok by themselves at the backyard range, my son using his and my Nephew borrowing mine, all was well. then they quit going and I just attributed it to the newness wearing off.
One day I took my pistol to the range and found it shooting very low, so low in fact that I couldnt bring it up with the sights, I put a straight edge on the barrel, no problem there, I couldn't figure it out. I asked my Son if anything happened to the gun while they were shooting it, and my son said that the gasket that seals the barrel fell out and they couldn't find it in the grass, what they did was go into my shop and find an o-ring and cut it to fit, they did such a good job of it I never noticed it, had I looked at the other gun I would have seen that it was a different color. the o-ring was thicker than the original and that was the cause of it shooting low, sorry about this being long winded but I wanted to set it up right...............steg