I guess I'll chime in a little here and echo the notes of caution espoused by others. That said, I extrapolate some loads, reduce shot, and even use some 20 gauge loads in my many 16 gauge guns. I also have found that in order to come close to published ballistics with slow powders (longshot, bluue dot) I must use magnum primers, even though it probably raises the pressures a little (500 psi from the charts). I just chrono'd some "1350 fps" (powder mfr recipes) loads with blue dot, that actually made 1000-1050fps with the prescribed primers, same loads with CCI mags produced 1310 fps. Some guns with slightly larger bores or long forcing cones may give totally different (& unacceptable) results. I never substitute when a load is near pressure max, but when the velocity is way lower than it should be, I sometimes do.
When I shot a lot of skeet (500 targets every week) we all saved a buck wherever we could. This included picking up used wads of any type on the range, using any hulls we could get our hands on, from Fed papers to AA's to remingtons.. The powders were whatever target type (700X, red dot etc,) we could get at the best price in 12# kegs and we used whatever primers were on sale.. We were probably loading 10Kpsi loads and no one ever had a problem with high pressures. No one ever weighed a charge, we just used a MEC charge bar from their charts. Some bloopers from blow-by but never a damaged gun. We were pretty unscientific, and got away with it. Though today I wouldn't recommend those loading techniques
).
There is a huge safety factor built into shotgun barrels. Probably 3X plus. In over 40 years of gunsmithing and shooting shotgun sports, I've ever seen a shotgun blow up from pressure (& only a few rifles). Plenty from obstructions, and even poor workmanship or materials, zipper cracks, etc. I've replaced dozens of defective Reminton 1100 barrels, before the courts ordered them to make them stronger.
I don't want to downplay the importance of meticulously following proper reloading procedures (including following good recipes), but I also hate to alienate potential reloaders, by overstating the dangers of "rolling your own". Like flying or diving, reloading is not inherently dangerous. But it can be unforgiving of inattentiveness, neglect or stupidity.
Greg
