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All Springer/NP/PCP Air Gun Discussion General => Machine Shop Talk & AG Parts Machining => Show us your Custom Airgun Parts (TRICKS-N-TIPS) => Topic started by: OneGear on January 05, 2019, 10:50:44 PM
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Moral of the story: Likely best to leave well enough alone :P
Picked up a P17 at Fleet Farm for indoor 7 yard range shooting. It was good, not bad, but not particularly good as weeks went by and the nice trigger was betrayed by randomness of the pellet.
Close inspection with loupe showed an apparent casting flaw just behind barrel crown. Maybe 25-30 degrees long roughly parallel to end of barrel, easily found with a toothpick as a sanity check of what was seen through loupe. Hard to tell exactly how deep behind crown, as a loupe has a very short depth of field and the feature is tough to see otherwise.
I decided to play. Worst happens I ruined a $40 gun barrel that wasn't impressive to start with.
Yeah, that's what happened a couple weeks ago. I began with an attempt to simply chamfer the crown and checking progress with a loupe revealed the casting flaw.*
Initial crowning plan was to use a masonry bit as a form turning 400 grit then 1000 grit wet-dry sandpaper into the crown to simply polish a consistent taper and clean up the factory edge. After shocking discovery of flaw (LOL) plan became chaotic. Result was a pistol that shot wherever it wanted. It wasn't very good to start with but now it was idiotically random - I would waste an 8x10 sheet of typing paper just testing a pellet. I'm not that good a shot but seriously.
So tonight I tried to fix it. Used a step drill bit, an Irwin Unibit 1/8 - 1/2 in.
Tried the same bit the first time (turned by hand) but did nothing except scar the crown. This time I cleaned up the crown with 1000 grit wet-dry rolled into a tight cone with toothpick slid in for a little tension. Then used the step bit in hand to slowly cut a square crown. Just enough pressure to cause friction, special care taken to let the bit self-align in the existing bore. Really just went at it slowly, removing and clearing the minimal swarf every few turns. I would have filled the flute with wax to catch the shavings if I thought this was going to work at all. I was just killing time in the sun on a nice afternoon :)
Anyway, I finished by using the base of a drill bit with light rubbing compound to knock down any burr. I used the straight shaft of a drill bit that barely fit the bore to avoid any kind of taper on the crown.
Polished the leade with the same 1000 grit wet-dry tight-rolled cone and toothpick core with very minimal pressure. Just to knock off the sharp leading edge of the rifling (easily visible in a loupe).
Regarding the leade before and after: I can now load wadcutters. They go in pretty easy until the skirt is flush and they don't fall out. Before leade work I could only load pointed or domed pellets and it took significant attention and effort to seat them flush to close the action. IOW, loading is WAY less fiddly.
I ran ten or so pellets through the barrel in the course of the work today. There is a spot 1/4 way through from the breech end which feels rough. However, seating a pellet is smooth, the rough spot feels rough on all sides, and the exit feels tight but releases the pellet suddenly on all sides at once.
I ran a patch through 20-30 or so times with light rubbing compound to attempt to knock down the roughness and the only roughness left was the aforementioned breech end section.
I'm writing this half so I don't forget to jump in right away with the step drill on a crown and address the leade when it needs it, and half to encourage others to have at it. I prefer hand tools because I can feel the changes taking place vs. a lathe, drill press or electric rotary tool. I have power tools, they just seem dangerous overkill for a few thousandths here and a few tens of thousandths there. At the rate you remove material manually you can do everything with both left hands and still come out ahead :D
The gun now puts pellets pretty much where I think they should land. Even using the pellets I slugged during the work and using a second aim point for fresh pellets, every hole on the 8x10 typing paper was covered by a 4" paper beer coaster. Offhand shooting. Four inches at 7 yards is hardly braggable but we started with a gun that peppered an 8x10 sheet pretty evenly over the entirety.
*I call it a casting flaw because it was like a 30 degree circumferential groove featuring rounded, almost bead-like forms inside, vs a deep scratch. Very small, to be sure, but deeper than the rifling by far. Probably inconsequential in the scheme of things, but certainly not a herald of detail-oriented quality manufacture. I couldn't measure it given the location but a round wooden toothpick would pick it up every time. It does appear my step drill crowning job cut far enough to eliminate the flaw. Which should reinforce we are not talking about a large flaw.
Sorry for the novel. Moby Dick is my favorite book, however...
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Hi Brent,
The flaw in yout P17 barrel is probably a weld seam. Some cheap barrels are made from flat stock formed and welded around a mandrel.
It would be great if you attached images of your barrel. You may not link to outside images before some post count, but you can attached images. Just click the "Attachment and other options" you should see directly below the posting window on the left.