Avenger .25 barrel polishing - inclusions

[Tim F]

I spent yesterday polishing the barrel. Used JB bore paste tight fitting patches, muzzle against board, 20 strokes in each of 5 sections. Worked from breech to muzzle. Tried pushing a slug through and found several restrictions.3 where the grub screws hold the barrel, 1 where the grub screw holds the plastic centering bushing and one just before the choke. Continued polishing with JB bore paste and even tried some polishing paper. It improved some but the restrictions are still there.
My question is do I need to remove all of the restrictions,  if so is there a better process. I have some compound for fire lapping PB's, but I think it is to aggressive for a airgun barrel.
At this time I have reinstalled the barrel, as soon as the snow quits I will test for any improvement, any insight would be appreciated.
Thanks

[nervoustrigger]

The proven way to bring an inconsistent bore into uniformity is with a cast lap (poured lead) and start with an abrasive more like 180 or even 120 grit.  Loctite Clover (silicon carbide) if you want something off the shelf, or better still mix your own with aluminum oxide grit and grease.   

J-B is more like 1000 grit, suited only for removing microscopic surface fretting.  And felts or cotton patches expand and contract with the bore, so cannot perform any sort of surface leveling the way a rigid lap does.

For the longest time I worried using something more coarse would be detrimental so I worked my way into ever more aggressive approaches gradually.  And I’m still bewildered by how much lapping effort is required to remove a typical restriction that’s maybe only a couple or three ten-thousandths of an inch.  Bottom line, fixing a hinky barrel takes some serious scrubbing. 

[Tim F]

Jason, thanks for the information. I don't have a way to pour lead.  I was concerned by using something more aggressive I might mess up the bore. I may try the product I have for fire lapping. You add on lead slugs, roll it between 2 still plates and then fire it through the barrel. May prep 3 of the slugs send through the tube and pull the barrel and see what I have.
Thanks again.

[nervoustrigger]

Tim, that sounds sensible.  I think I'd give the fire lapping a try if I were in your shoes.  It works on the same principle as a cast lap as far as preferentially hitting the high spots, but will remove material much more slowly so you can check your progress.  Seeing as how the tight spots are the result of grub screws and not just the typical bore variability, my guess is fire lapping will be slow to produce a noticeable improvement.  But you'll know soon enough how it's going and can make adjustments as needed.  If it turns out that a cast lap is needed...

I don't have a way to pour lead.

In a pinch, a soup can will do.  Cut it to about half height with a hacksaw or tin snips, and make a small U-shaped cut for pouring.  Lead melts at such a low temperature that a propane torch or even a kitchen stove is sufficient.  Heat the barrel before pouring to help the lead remain liquid a bit longer to produce a good fill.

[Tim F]

Jason thanks again, I will give the fire lapping a try when I get a break in the weather, snow right now. If that doesn't work out I will look at melting lead as you describe.