Welcome aboard!
You are right that a better fit between the bore and BB helps accuracy and MV. A couple ideas to improve the fit have been mentioned. There was plating the bore with nickle which no one here has reported doing, and choking the barrel which is found in some production airguns as well as having been done elsewhere by hobbyists to improve their unchoked barrels.
Then there's the option of replacing the barrel with one having a tighter bore. That's been done here and was spearheaded by a member, hinz57, using a Daisy model 499B shot tube. Hinz also went a step further and bought a Lothar Walther smoothbore barrel that is choked. Using that, along with various other custom parts allowed his gun to reach the highest MV seen so far. The project was interrupted by his area being flooded, so no formal accuracy testing was done with that gun but hopefully he will be able to get back to it soon. We all miss him being here!
But there's an easier way than modifying or replacing barrels and that is to use larger diameter BBs! One type that has proven to be a better fit is the Daisy Match Grade Avanti Precision Ground Shot. The other is Marksman Premium Grade Steel BBs but they're quite a bit larger than ordinary BBs so the fit has to be verified before using them, else a jammed shot tube could result. All this has been covered in various posts that you may not have gotten to yet so my best advice is to keep reading. I know the thread is long but at least this way everything is in one location rather than dozens of separate threads.
ETA- Regarding larger diameter lead shot, to the best of my knowledge all the currently available ".177" lead shot is about the same diameter as most ordinary steel shot. Which is to say there's no advantage in shot-to-barrel fit using lead shot. If you know for sure there's a brand of lead shot that's larger than mainstream steel BBs, let us know.
Also lead shot will obviously be considerably heavier (weights shown below) than steel which is around 5.3 gr. average. The heavier shot will limit range and MV. (As a comparison, the larger diameter Marksman steel BBs weighed 5.74 gr. average and the Daisy Avanti Precision Ground steel Shot are 5.45 gr. average on my scale.)
Then there's cost. The following is from my notes. I'm not sure if this was from the manufacturer's advertising or was measured by an individual- my fault for not noting which it is but I'll check later at Pyramyd to see if they have the same info. But if I saved it, at the time I thought it was accurate .
LEAD SHOT VS DAISY STANDARD ZINC
Gamo .177 Cal, 8.2 Grains, Round Lead Balls, 250ct, ~$4.00 / 1.6¢ per round
13.3 x more than standard Daisy BB
H&N Rundkugel, .177, 4.50mm Dia., 7.70 Grains, Round Lead Ball, 500ct $15.25 / 3¢ per round
25 x more than standard Daisy BB
H&N Rundkugel, .177 Cal, 4.54mm Dia., 7.70 Grains, Round Lead Ball, 500ct $19.95 / 4¢ per round
33 x more than standard Daisy BB
H&N Excite Smart Shot BB, .177 Cal, 7.4 grains, copper plated lead, 150ct to 1500ct / 1.3¢ to 2.7¢ per round, depending on quantity
11 to 23 x more than standard Daisy BB