RE: Where to go from here?
If you are using bore butter/Wonderlube... etc... discontinue that use.
If you are using 50-grain pellets, buy some 30 grain so you can tweak powder charges at 80, 90, 110 & 120 grains. Never go beyond 120 iin pellets with Powerbelts. Generally, their skirts prefer 80-110 gr.
But it's best to use loose powder with Powerbelts because of their loose fit. You want to avoid a situation where loose fit exists with both powder & bullet. Since all ML bore diameters are different, you may now be facing a situation where both your Powerbelt and your pellets contain a bore wall gap.... creating excess blowby.
You also need to dismount every Powerbelt skirt you have from the bullet - put them back on the bullet -- twist & turn them a few times for a "loose" fit. This bullet cannot tolerate a one-thousands undersized skirt hole that slips on the bullet nub- because when that happens, the skirt rides along with the bullet too long -- creating poor shot groups and sometimes hour-glass impressions on paper targets.
As a measure of making sure those Powerbelt skirts are removing themselves properly, dab a Q-Tip in oil -- then lube around the Powerbelt bullet bottom nub before mounting the skirt. Also keep in mind that the Powerbelt was designed & developed for CVA/Winchester rifles - which use different bore measurements than T/C.
If I were you, I would stick to T/C bullets initially - until dialed-in.If the Encore has been only seeing Powerbelts si9nce new, you need to break-in that bore properly with a box or two of "full-fledged conicals like T/C Maxi-Balls, Hornady Great Plains conicalsor Buffalo Bullets.Those bullets remove tiny burrs in your bore. Then I would buy some T/C Mag-Sabot 240s and T/C Shockwave 250s to try ...... unless you hunt in Colorado where sabots are forbidden.