I'v been working with the 25-20 GB Lee of last year and this is to provide a bit of news on my findings. First let me advise that after 400 bullets cast I had to install a lock screw on the sprue plate pivot screw as it was loosening and letting the plate float too much. Likewise a screw for the sprue plate handle rocker to contact to stop aluminum deformation. And yes, JB in all the handle holes, and a careful countersinking of all the sprue plate holes to increase pour flow and fill quality. A lap job on the plate was required too. She casts beautfully now.
My old loads with the 257420 were reduced and used to start new for the GB. The 257420 weighed 73 grains tumble aloxed with GC and not sized @ .2585". My GB is 83 with GC fully sized and lubed 50/50 and tumbled with thinned alox @ .258". Turns out I could go right back to almost the same load levels as the Lyman to achieve excellent accuracy (.5-.6") @ 50 yards for 5 shots, 1 1/2" @ 100. I'm using 2/3 WW and 1/3 lino with 2 oz. tin per 20 pounds of melt and culling closely.
There is a dimensional difference from the Lyman with a GG and front band showing on the GB loaded cartridge at a length which engraves the front band when levering into the chamber snugly. My carrier accepts and feeds 1.648 cartridges after modification long ago. The GB bullet nose is shorter than the Lyman too.
14. 5 grains of old data AA2200 is my load over a CCI BR primer. No crimp is used. I've also used AA 1680 and 2015 at this level with great results. The AA 2200 load runs close to 1900 on the chrono with a 26 sd. No crimp is used. My gun has factory long headspace so I only neck size a little and try to start with .32-20 cases. Annealing every other firing is my procedure to keep neck tension equal across the cases I use. Its critical in .25-20 for accuracy, and freshly loaded too. (tension changes quickly over time)
Using fired cases, annealed and with flasholes opened a lot, I'm using the tumble lubed GB's without a GC working on a fairly quiet load for pests. So far I'm at 1.1 grains of 700X/ and a hard down tissue wad with no crimp, and get good ignition. I want to go a little lower to the sticking point, before going for an accuracy @ 40 yards/quietness point.
Hope this helpful. BvT