Lapping a barrel IS a way to good accuracy along with tuning/bedding the rifle and bench prepping the ammo. "Accuracy" also depends on WHAT you mean by accuracy and what you think "good accuracy" actually is...1/2" groups at 200 yds or 2" groups at 50 yds.
Lapping a barrel just knocks off the roughness left after machining on a NEW barrel . It is no more than a quick way to "wear" a barrel down a bit and polish the throat. If the barrel cruds up after a few shots then starts spraying bullets everywhere including behind you. then maybe a lap job would help greatly.
You can do lapping in several ways, fire lapping, lead lapping or with some 320-600 grit on a tight fitting patch wrapped around a nylon cleaning brush and stroked in the throat area about twice as many times as you stroke down the barrel...do half a dozen strokes, clean, put on a new oiled patch and push it down the barrel slowly...you can feel where the tight/rough spots are...mark them and do a few more strokes. The 320-400 grit cuts the metal, but the 600 is more of a polish.
I also use Flitz polish the same way...25 strokes, clean, check, another 25 strokes...etc...fire off a few lead bullets and check the bore, clean to see how quickly the crud comes off...crank off a few jacketed bullets...same as firelapping without the grit or normal shooting...it isn't rocket science. I stop the Flitz at 200 strokes or before...100 to 200 is usually plenty no matter how rought the barrel/chamber is.
Remember lapping is "WEARING OUT" the barrel...so to speak...you don't want to overdo it.
This is for CUT RIFLING...you don't have to bother so much with with button or hammer forged barrels as they are usually smooth as a baby's butt, EXCEPT for the chamber and throat where machining takes place.
There is lots of ways to lap barrels and many ways to do it and the information is all over the net.
Slathering on some 600 grit in oil all over all over the working parts and cycling the action a dozen times then clean wel. The will also help slick up the action and the shiney spots or where the bluing is worn off show you where the wear points are so you can polish those points...makes ANY action work slick as glass. ;D
H and IMR 4198, AA2015, RL-7, RL-10X, RL-15 H322, H335, IMR 3031, Benchmark, and Varget are all good powders for different weights in the 444 including the 290 gr. Hodgdons online load data is chocked full of good load data, and so is many of the other powder makers reloading sites.
Start low and work up slow...words to live by. ;D