I acquired a CZ 410 O/U in mid July and have since run 750 shells through it. Last week I picked up a used MEC 600 and I purchased 500 once fired Rem STS hulls. I have been reloading the hulls that I shot as "new" ammo (Win-AA, Fed-GM, Fiocchi, & RIO) all w/o any problems what-so-ever. Enter the Rem STS's. Using Win 209 primers it takes so much pressure to get the primer seated properly that the priming ram gets jammed down in-between the primer cup and the base wad. Causing me to have to grab the hull with pliers to pull it down off of the ram. This also causes a buldge in the brass head that measures from .oo5" up to .0085" on about 35 hulls out of the 50 I started.The .005 shells will fit into the chamber but the gun doesn't close and the larger ones don't even fit into the chamber. I have loaded thousands of 12ga STS hulls w/o any problems so what gives on these STS's ????

The hulls were probably fired in a 45/410 judge. I purchased 500 hulls about 6 months ago and had the same problem. the judge has a .45 cal chmber so the 410 brass blows way out. I had to buy a special sizer ring just to get them to chamber in my mossberg bolt action.

If you would use STS primers they would work alot better. You wont like the price if STS primers. Where i live they are double the price of Winchesters.
After you use the Winchesters one time they will fit in the STS hulls fine the next time. Just a pain the first time you use them.

A MEC 600 does not re-size the hulls.

If it's a Mec 600 JR, they resize the hulland the brass head of it at the first station.
There's pre & post 1985 models, but I don't recall what they changed back then.

You can adjust that first station, the one that deprimes and resizes so that it resizes the hull right down to the rim,,or not.
If it's not adjusted right and the base wad inside is a bit higher than the other hulls you've been sizing, the depriming ram damage the basewad inside. That may be what is causeing the resistance to re-priming.

If the re-sizer isn't adjusted to size the 'brass' all the way to the rim, it'll leave a bulge there from what ever other gun it was fired in,,and it may not fit into your chamber.

Here's a link to MEC's instruction's for the 600JR.

As far as the primers are concerned,,Winchester, Federal, Remington 209's are just that,,size 209.
They've been that way since the 60's.
A very slight difference,,a couple .0000" maybe,,but that's about it. Anything else is a bad batch. Some of the early Cheddite primers were oversize but even they seated w/no extra effort.

This is a little late probably but I'll say it anyways.im more of a shotgun loader and shooter than any other firearm I've loaded every type of hull I think there is even paper hulls in .410 and I'm suprised to hear about your problem because the rem. sts hulls are my absolute favorite hulls I think they load amazing. The nitro golds are great too and old AA's are fine. My only thought would be the primers maybe, I've used fiocci with no issues, thousands of rounds. I have had a few stick on the .410 Mec maybe the primer punch is slightly bent I'm not sure if this is any help I'm sorry for the bad luck I alway thought they were good hulls

Rem hulls suffer from this problem due to their molding. The Rem 209 STS primer has a smaller profile due to it being more rounded where as the Win 209 has a more squared off profile. When you see the hulls hanging up on the deprime punch or on your ram, run a #1 drill through the primer pocket to clean out the excess plastic. The primers will seat easily after cleaning up the pocket and it will not harm hull life. This problem can be encountered in all Rem 1 piece hulls (12, 20, 28 & .410) from time to time I suspect it is a QC issue or perhaps related to the molding.

I believe that Fiocci primers are slightly bigger than Winchester primers. I only used them in certain (12 ga.) hulls that required them. I only reload Win AA hulls in 410 however.
Peter.