Credit for this step also goes to Iowegan for the expert knowledge he shares in his IBOK (Iowegan's Book of Knowledge, for the GP-SP series). I also want to acknowledge Fred Z from 4D Products in Wyoming (www.4-dproducts.com) for renting me the 38/357 Special Throating Reamer with 7 Pilots. The quality of that tool was superb, and the ability to select the exact sized pilot needed (hint: I used 4 of the 7) helped ensure an accurate job. The rental price was also great, versus buying a new tool and set of pilots for 1 job, so I'd encourage you to check the website if you decide to do this task.
Again, I will skip the step-by-step details, but just cover the highlights. The picture shows the 4D reamer with 1 pilot mounted, extra C clips, the other pilots and crude drawings showing the size pilots I needed for the cylinders. Note there were 3 pilots needed for the GP100, ranging from 20 thou's to 30 thou's undersized for the 0.358" bullets. The reamer cuts to exactly 0.358", as you'll see later. The l'il SP101 had 5 throats all the same -- which was a surprise compared to what I found on the GP -- but even those were 10 thou's undersized. As you'd expect, using the same smooth cutting action and liberal cutting oil, the effort on the SP was less than that to ream the GP.
Here are the throats before, with the 4D tools:
By the way, the T-handle on the reamer came from the forcing cone tool set, so make sure you have access to one as it's not included with the reamer & pilots rental. Fred at 4D liked this picture so much when I sent a copy of it to him with the reamer return, that he asked me for a jpg copy for his website, so you may see it there some time in the future. Fred was great to deal with and if I ever need to rent another reamer, 4D will get the nod from me.
Throats after:
As you can see above, after the reaming all 11 cylinder throats accepted the 0.358" SWC lead bullets I am using, requiring the same slight effort to push them through. I was really pleased with the consistency of the sizing and the smoothness of the cuts. But here again, you must be diligent to clear all chips from the reamer before you ream another and NEVER back up a reamer as it could chip the cutter (which you'd be responsible for) or damage the work (which you'd regret for a long time). Once you start the reamer, you slowly twist it clockwise and smoothly all the way through, using liberal cutting oil.
OK, it's time for the rubber to meet the road!
Before, using the same load and rest, my GP100 was giving me 2" to 3" groups at 50 feet. Check out the "after" target below, and I take full credit for the lone flyer which ruined an otherwise exceptional group.
GP target after:
The SP101 was shooting 3" to 4" groups before, due mostly I think to the shorter sight radius and barrel, a heavier trigger pull and combat sights. Still there's a sizeable improvement, with yet another flyer provided by me. (That's the NEXT problem I need to work on -- the loose nut behind the trigger!)
SP target after:
There are no photo's to back it up, but the leading in the cylinder throats is all but eliminated and the forcing cones look great after 100 rounds each as well. Overall, I'd have to say this project was a huge success! And now that I've corrected the leading issues, the clean-up won't be such a chore, so I hope to shoot these a lot more and improve my skills and technique. Gotta fix those flyers next...
Thanks again to Iowegan, Brownell's and 4D Fred.
Tight groups to all!
Old No7
Nice report !! 
