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"Fire lapping" an airgun barrel....

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beerthief said:
nobody has said how long this cast should be ? 1/2 inch , one inch ? , 1,2375 ?
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I like making laps about an inch long to have a decent amount of surface area in contact with the bore. More surface area equals more scrubbing action. Of course that means friction too, so for .177 cal if you make the lap too long, it may become so hard to push that the cleaning rod begins buckling. If that happens, no big deal, just cast a new shorter one.

Consider also that you need to be able to push it proud of the muzzle to load more compound from time to time, so the lap needs to be long enough to let you do that comfortably without accidentally pushing it all the way out and losing your indexing.
 
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If you plan on casting a lap…you should really make a fixture to hold the barrel and guide the rod. If you don’t, you will not be able to help rubbing the rod, that will likely be full of compound, against the barrel. If you don’t make a fixture…at least make a simple bore guide.

Also….you will have to heat the end of the barrel to get the lead to flow enough when pouring the lap.

Mike
 
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thomasair said:
If you plan on casting a lap…you should really make a fixture to hold the barrel and guide the rod. If you don’t, you will not be able to help rubbing the rod, that will likely be full of compound, against the barrel. If you don’t make a fixture…at least make a simple bore guide.

Also….you will have to heat the end of the barrel to get the lead to flow enough when pouring the lap.

Mike
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So we're talking about a 28" barrel in this example......

Not that I'm going to do the cast lap method but just curious,....is is standard practice to plug the bore somehow? maybe hammer in an oversized dowel (which will of course catch fire and burn at the aformentioned 700 degrees)? or pour lead down the entire 28" of barrel, getting it nice and adhered to the entire length of the bore? Seems like keeping the entire 28inch monster heated would be tough too for the lead to flow down to the bottom better, as you mention.

Just seems like shooting abrasive laden projectiles is less of a hassle....
 
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I wrap string around a cleaning rod to increase its diameter so it’s a tight fit in the bore. Leave the threaded end of the rod exposed past the string….insert the rod into the bore and slide it up to the muzzle leaving the amount of the desired lap length. The string makes a satisfactory seal and centers the rod. A 1-2” lap for a .22 cal barrel is enough.
 
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I may be misunderstanding what everyone is doing here, if so just ignore this ,if your making a piece of lead fit tight in your barrel to lap with, just remove barrel then set mussel on piece of wood , drop in a slug followed by cleaning rod and lightly tap a couple times with hammer, that slug will form nicely to bore and rifling to lap a barrel.
 
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OldSpook said:
Cole, how did this work out for you? Did you get it sorted?
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Improved, but still not the type of accuracy I'm looking for.

I combined a few different recommendations but ultimately used some of the Seneca 28.5grain .22 pellets, dipped in Clover. They're essentially the same design as the old Eun Jins, having 3 "lube grooves" in the head of the pellet. Per @thomasair s suggestion I also turned down the power to the point that I could watch them fly downrange, and actually even had a couple get stuck at the muzzle. The majority would hit the ground about 35 yards away, with the barrel horizontal, so they couldn't have been going very fast. I shot 20-25 through the barrel, using the 280grit Clover. then I pulled the barrel and cleaned up the mess. After that I polished the bore with JB bore paste and a single piece rod. Sure is shiny! Pellets feel better when pushed through now, but still less than ideal. Those tight and loose spots are still present, but nothing as "grabby" nor "loose" as it was prior. Less pronounced tight and loose sections would be the best way to describe it. Replaced the probe oring, assuming all that abrasive did it no favors.

After all that I assessed accuracy at 55 and 100 yards with .22 JSB Monster RDs. It would shoot 10 shot groups into about 3/4" at 55 yards, and 10 shot groups into about 2.5inches at 100 yards. Which is better than it did before, but not great. It was interesting to see accuracy go to complete crap after about 100 shots. And by deteriorate, I mean 1.5-2inch groups at 55 yards when it had been doing 3/4" just prior.

I inspected the barrel and am finding the last roughly 1/8 of an inch of each land (at the muzzle end) to be pretty chattered up. It appears that whatever tool was used by the "machinist" to put a crown on this barrel also damaged the lands. At this point I think that is the biggest issue negatively affecting accuracy. It probably needs to be given a recessed crown (down past the damage to the lands), or perhaps even better, chopped back some for a fresh attempt at a decent crown.

I haven't given up on it, just got lots of irons in the fire.
 
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I totally agree with what was posted here by @thomasair and @nervoustrig. We have very similar opinions on many barrel related topics.

Here is an example of a bore lap which I used in my .22 poly barrel.

IMG_20231021_204833~2.jpg
 
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