Breechplug
From those two replys - I think your original assement may be correct the T7 primer might not be the best fit.
While they are the longest 209 primer that i have they are also the skinniest. The Win W209 that UC talked about is almost the same length. I just tried dropping that one in a Remington hull also. I could seat it by just pushing it in with my finger so it is not much thicker than the T7 (which makes sense since they both are made by Win)
The Remington STS is thicker than both of the Win's, as are Cheddites and Feds.
The next thing is to look into to what FG said...
i backed out the firing pin bushes a few thousandths to push the primer tightly into the BP.
I have no idea what he may be talking about but if that can be done it would certainly help if you could pin the primer against the shelf in the BP.