Hmm, hard to believe those huge pressure increases. I have always used Win 209 but now have a supply of Cheddites. I guess there is only one way to know for sure.

I reloaded with W209 and cheddite primers with same recipes and did not notice any difference in recoil loaded 20 gauge Winchester AA with unique powder, shot one box with the W209 and a box of cheddites and seen no difference.

I was just reading a thread on another forum where someone managed to get 14k psi with their normal load because they had over compressed the load, perhaps that could have happened in this case?

You want to see something sketchy look at primer swapping using a Fed 209A I think it was .

Here are the testing data sheets on the 20 gauge loads in question that had pressures roughly 3500 psi higher with Cheddite primers than the same loading using Win 209 primers. They also increased velocity by about 100 fps.

For reference Alliant data for Win209 primer is:

The first load with a Win 209 and 15.7 gr of 20/28 is listed on the Alliant Reloading Guide as being 1200 fps and 9370 psi.

The second load with a Win 209 and 16.7 gr of 20/28 is listed on the Alliant Reloading Guide as being 1250 fps and 10,490 psi.

Will dropping the powder help with pressure ?

It absolutely should. One could use a different powder too. No need to shoot 1300 FPS loads. They won't make any bird more dead or make an XX on the scoresheet better than an 1160 FPS load will.

Unless you test the undocumented load, you have no idea of what the pressures are. Pending the results from Precision Reloading, I will reduce my powder charge until I get acceptable pressures. I will take whatever velocity I get. Billy

Identical loads, one in once fired Cheddite hulls primed with Fioche 616, one in new, primed Cheddite hulls. Unfortunately I cannot say which Cheddite primers were in the hulls as I bought them that way. Spit for difference.

Got curious and dug out examples of a number of different gauge, new, primed, factory Cheddite hulls. 20 ga. paper and plastic, 16 ga. plastic, 12 ga. plastic, 12 ga. paper (no pic), 10 ga. plastic. I find a variety of sealer colors on the primers (no clue if that means something) and the 20 paper appears to have no sealer. All except the 20 paper have copper color primer bodies, the 20 paper is brass colored. Referring to Kevin's info, this would indicate all have the same primer but the paper 20.
In loading various "tries" that I had tested the 20 paper ran considerably higher pressure than identical loads in plastic with what I know to be Cheddite 2000 primers. Now, is this primer difference or case volume? As I haven't accurately measures case volume plastic vs. paper, can't say. That's on my "to do" list.

Different powder , different hulls , and different payloads , they all vary nothing really nominal or lineal . Faster powders tend to spike more so than slower powders .

As stated crimp depth can make a big difference also.