X-barrel and lead build up

Hi, just registered to this forum, but have been reading it allot as none registered. Allot of good information here:) Hope my first post is not to long.
I have a strange issue with my fx crown in .22 which I bought last year:
When I bought it new it was a sub 10 joule version, wich I later installed a high power kit. I mainly used jsb 18 grains which worked great for several thins of pellets, after the full power upgrade. I then decided to try the regular 16 grains JSB`s since I mostly shoot below 30 meters outside my house. 
At first they worked great, but after maybe a after 100-300 shots I got random POI changes at 20m. 2-4 shots would go almost hole on hole, and then the next 2-4 would change poi maybe a cm to another location. Went back to 18 grains, but the problem persisted. After trying several things first I then found out the x-liner had lead buildup the last couples of centimeters at the end. Cleaned the liner, and everything was fine again with the 18 grain jsbs. Suspected it might was the 16 grain pellets it did not like, since I had shoot several thousand 18 grains with no issue before that. But after a couple of boxes I once tried the 16 grains. And it did not took more than 50-100 shots before random POI changes appeared. Since it seems to occur at the end of barrel, I can actually see it with naked eye when I look down the barrel using a flashlight. Took the liner out and cleaned it once again, and the everything has been fine so far. But will not use any of the 16 grain JSBs I have, since I am almost sure they are the reason the barrel get lead buildup. 
It probably sound strange, but does anyone of you have similiar experiance? 
 
interesting ....seems like the 16grs would be smaller causing the opposite effect? Maybe they are not as clean from factory? I wash all my ammo now but you know how that is I cant really tell you for sure it matters.Ive had good groups washed and lubed and good groups straight from the tin.I figure it cant hurt to wash and lube them.I use dish soap and power pellet lube (napier)
 
I did notice while inspecting both the 18, and 16 grains, was that the 16 grains did seems to have some small lead particles in the hollow back end on the tail. Took at cotton brush, and twisted it at the back of the pellets, and some of 16 grains did leave small lead particles on the cotton. Maybe they just are dirtier as you suggest? The 18 grains seems to be cleaner. If I try the same couple of tins I have left of the 16 grains again, I will follow your advise and wash them first. But before I do that I will get myself a proper cleaning kit, first, just in case:)
 
I will add this, I to have a .22cal that seems to prefer the 18gr JSB’s, have shot both 16 & 18gr jsb pellets and this barrel leads up amazingly fast I can’t keep it clean? I know what u guys are thinking ( cleaning to much- shooting to fast etc.) but have tried everything and only way so far to keep it shooting like I feel it should ( maybe to much pre- expectation or HYPE! ) is to keep barrel very clean, this gun is accurate but it’s alot of work to keep it that way... have never liked washing pellets but might give it a go and see what happens?