So I bought a crossman f4 (Walmart version) and for some reason after about 50 rounds the barrel is starting to face upwards to where now you can see it in the scope. Unfortunately it almost makes it unshootable since at 15 yards it is off by a foot of not more. I took the gun apart looking for any kinds of adjustment screws or any loose screws with no avail. Any one ever heard of this or better yet have a remedy ?