When I order a pad, what measurement do I use - the butt with existing LOP or after the allowance for the thickness of the pad?
You mention in your first post that you were to use a grind to fit as the LOP is good. Obviously if the LOP is good you are going to have to cut the thickness of the pad (regardless of thickness) off the stock. Using a grind to fit or other is not changed by LOP. You need a grind to fit as there is no factory option that I know of to fit the SKB shotguns.
The SKB 500 (or mine at least and my 100) is crescent shaped at the butt plate. It needed to be cut anyway just to square it up for the flat recoil pad. NOT for the weak at heart as this need be done on a table saw with some attention to detail. I used a fine tooth blade designed for Malamine and plywood. I taped the area to be cut to avoid splintering, used the top as square, measured twice, had a coffee and filled my mind with negative images, went for a walk, measured twice more and cut. The scary part obviously is that there is no turning back.
My 500 is near mint and I too shoot it very well, however at 13 5/8" LOP I knew it was too short in spite of my shooting it well. I shoot other shotguns on a regular basis and all of them are at or about 14 1/4" LOP which is correct for my arms, build and face structure. The drop is pretty equal or within 1/16" on all of them.
Switching between shotguns is where the troubles would arise, especially on the first or second shots of the day or even shooting a bunch of guns when out for clays as I often do. I grab my Beretta or FAIR and it would get caught up on my jacket or shoulder awkwardly after shooting the SKBs. It was either shorten all my other (read much more valuable) or the SKBs.
What I am trying to get at is that you should be absolutely sure of your proper LOP before chopping a shotgun and just because you have adapted to a certain LOP and shoot it well does not mean it is correct. If the LOP is wrong you may experience the exact same uncomfortable recoil that you experienced before the procedure. I had a vintage 250 Savage that was a way too short for me and I would rather shoot a well fitted magnum rifle all day than that little beast.
What I had the luxury of doing, was to take exact measurements of other shotguns that I shoot as good or better and try different thicknesses by taping spacers to the end of the SKB and trying them. Once I was able to switch between shotguns without any adjustment, I knew I had it perfect.
You probably already know all that so please don't take the above as condescending. I have screwed up more than my share of pretty simple projects and just want to save you some possible frustration.
Once you have the stock cut to the right length, it is from that you would make a template or take measurements. The specs for the actual pads are on line but I just took my plate with me as there is two stores here that carry a good selection.
I am 99.9% sure it was the small Old English that fits the SKBs but I don't know the others.
Another thing that worked very well for me was using the silver tape that HVAC guys use for masking off the stock before hitting the bench grinder. This stuff is expensive but if you walk into a furnace place, they many give or sell you all you need. I am in the industry so a freebee. It sticks really good (very hard to remove) but allows for a little touch or two without damaging the stock. I used lemon oil as lubricant and it too worked much better (smelled much much better) than petroleum type lubricants that can in fact damage the pad. Once you get it very close, the final work need be done by hand. I wrapped fine emery paper over a file for the finish work.
Good luck.
