The reason sticky extraction is a revolver pressure sign is that steel is more elastic than brass. That means steel can return to shape from a larger amount of stretch than brass can. During firing, the steel chamber and brass case expand together, but if the pressure is high enough they will expand past the point the brass can fully return to shape. That makes the brass too wide to fit in the unpressurized chamber, so when the steel snaps back to size, it squeezes the brass. That squeeze is what makes extraction sticky when pressure is too high.
The general rule of thumb is to work up a load toward a published maximum in small steps (2% per step; see last paragraph), but back the charge weight down 5% if you experience sticky extraction at any point along the way and limit your charge weight to that.
Hardworker,
Your posted load is lower than the load Alliant posts for the 158 grain Speer Gold Dot seated to 1.575". However, you need to be aware that changing component brands can change pressure. If you use a different brand of bullet or primer or case, the pressure can change. This is due to differences in the lengths of the bullets occupying different amounts of powder space when seated, or due to bullets being harder or softer than the listed load was developed with, as well as due to different brand case capacity and primer vivacity.
Lead bullets, despite being softer than typical jacketed bullets, have been known to exhibit higher pressure with a revolver load than their same-weight counterparts. This was reported by Keith and Skelton, among others. It's possibly because these bullets are soft enough to be upset out into the forcing cone of a revolver, effectively making the back end of the bullet wider. But that's speculation on my part, and not the result of a careful analysis.
There are other causes of high pressure and sticky extraction. One you don't have results from light loads which expand the brass poorly and allow powder grains to blow back in around the cases, jamming inbetween the brass and steel. This is mainly observed with small charges of slower fine grain spherical propellants.
One other cause you might have, since you fired .38 Special loads in the gun prior to firing the magnum loads and if those were lead bullets, is lead build-up in a ring in front of the chamber throat that acts as a collar around the longer .357 cases, causing them to be unable to expand to release the bullet normally. But you usually feel at least slight resistance over about the last 1/8" of inserting the .357 case when this is the problem. If it's severe enough, you can't get the .357's all the way in at all. If the .357's fell into the chambers easily, though, it is not likely to be the problem. It certainly won't hurt to clean the cylinder well, either way. Search this forum and the gunsmithing forum for past threads on removing lead. There are several good methods.
Bottom line here is that you work loads up incrementally and watch for pressure signs and back off the load 5% if one appears. Alliant lists 14.8 grains of 2400 with the 158 grain Gold Dot bullet, but you can't count on that to apply if you used a different case, different primer, or different bullet.
Accurate recommends a starting load for a workup should be 10% below listed maximum for rifles and 15% below listed maximum for handgun cartridges. I think, given the wide range of guns in various condition, that's not a bad rule to follow (except where a powder maker lists a not-to-be reduced minimum, as is the case with 296/H110). 85% of 14.8 grains is 12.6 grains, so that would be the starting load I would recommend.
Richard Lee's rule of thumb is to work up in 2% steps while watching for pressure signs. (Many use smaller steps when searching for rifle accuracy loads, where 1% is about as big as you want the steps to be.) I would add that unless you've had the chambers reamed to uniform maximum (a standard revolver accurizing practice) the pressure may not be exactly the same in each chamber, so I would run six rounds at each load level, just to be sure. Clean the gun and don't shoot anything else in it until the load workup is done.
