Wolf18

Ok so my singled longshot is only firing about 60-70 feet but it has a spring that is about as strong as a [k26] plus the longshot spring (first gen).
I've checked all the seals and they are air tight I can plug the barrel and it will hold air for a while so that's not the problem.
I tested many barrel lenghts the one that seems to work the best is 10in. that one hits 75ft. some times 80ft. but I want at least a 100ft. I can't figure out why it is not hitting 100ft. anyone have this problem? I need to fix it for SPANO that is in about a week. I am using best materials black foam. also the barrel is not glued in yet but when i shoot it i hold the breech closed. It has a 12in barrel in the pics what is the best size?
it is shooting through a nerf box. here is a vid of pics
The handle is filled with foam and hot glue