Fire lapping may or may not help the accuracy of cast bullets in you Guide Gun. Fire lapping might also ruin your barrel. The main thing you have be careful about is to clean as much of the copper fouling out of the barrel, before trying lead-alloy bullets. I personally have never fire-lapped any barrel, and have had good success shooting pretty fast cast loads from centerfire rifles. For example, I have a Ruger #1 45/70 that shoots a Lyman .457122 HP, 342 grain bullet at over 2200 feet per second into about 1.25" @ 100 yards, and will do the same with a hot load of the Barnes 400 grain spitzer. But, I clean well before shooting lead bullets after using the rifle with jacketed loads.

I recommend trying the rifle thoroughly BEFORE doing any fire-lapping!!

Larry