Why does my basketweave stamping wander off line once I get past the flat part?

Stamp first, mold second. That's how pretty much every holster maker does it, and there's a good reason: once the leather is formed around the frame, the surface is curved and stretched, and a stamp that sits flat on scrap will rock on you and walk off line. Stamping on flat cased leather, then molding after, keeps the pattern true. Basketweave holds up fine through molding if the leather is still damp but not soaked when you form it. If you case it too wet the stamping softens and loses its edges, so let it return to near-normal color before you wrap it around the gun.

The drifting itself is normal, even on flat work, and the curve makes it worse. Trick that fixed it for me: scribe a single spine line down the middle of the panel following the contour, not the edge. Then use a pair of dividers or an edge crease to step off guide lines working outward from that spine, one row width at a time. Because you're measuring from the center, the lines stay symmetrical and any curve error gets split evenly between the two sides instead of piling up at the bottom corner. If you scribe off the edge on a curved panel, every little variation in your edge stacks up and by the time you come around the muzzle side you're way off. When you stamp, keep each stamp lined up with the spine line rather than the row below it. On a curve the rows have to fan out slightly, so trying to keep them visually parallel to each other is what throws the whole pattern crooked. Sound weird but the pattern looks parallel when the stamps stay square to the centerline, not to each other. Also stamp before you cut out the pattern if the shape allows. Easier to keep rows true on a bigger flat piece, and then your outline cuts give you a clean edge to trim to.

The thing that finally cleaned up my stamping was switching to a granite slab under the work. I used to stamp on a piece of plywood over my bench and every hit was bouncing a little, so the stamp would creep a hair each time and by row ten I was a full width off. Hard flat surface plus a rawhide mallet with light even taps fixed most of my drift without me changing anything else. Might be worth trying on your scrap first to see if that's part of it. Also curious what weight leather you're using, I found the heavier stuff drifts faster because it compresses unevenly under the stamp.

One thing to expect even if you stamp flat first: molding around the frame will compress the rows on the tight side of the curve and stretch them slightly on the outside, so a perfectly stamped panel never comes off the mold looking as even as it went in. I noticed this on a knife sheath I did over a finger guard, where the weave looked tight near the bend and airy toward the flat. Also, I let the stamped piece dry completley before wetting it again for molding, because I convinced myself once that a half-dry impression blurred less if I formed it early, and it absolutely did not.

I fought the same crooked-row problem on a belt slide a while back and what saved me was just practicing the pattern on the actual scrap from the same hide before touching the good piece. Same temper, same moisture, so the stamp behaves the same. I'd do a full practice panel every session before starting, kind of like a warm up, and by the third day my spacing stopped wandering. Also don't overthink how it looks mid-job. Mine always looked wonky halfway through and then the finished rows read fine once the whole panel was filled in. Half done basketweave always looks worse than it is.

One thing that helped me was rotating the work instead of reaching around it. I used to twist my wrist to keep stamping along a line and that's when my rows went sideways. Now I turn the leather so every hit comes straight down from the same comfortable position. Feels awkward at first but the consistency's night and day.

Also, after each strike I'll drag a fingernail along the stamp's edge to feel if it's sitting square before the next hit. Sounds tedious but it takes a second and you catch drift after one row instead of ten.