Do you mean 25 Rim Fire? If so how can you have flattened primers?

What caliber are shooting?

Sure it's 25 RF, if so what case are you using? 
What action are you firing it in?

In something like a Favorite you might see damage to the rifle before you see a true pressure sign in a converted case like a 17 WSM.
 

You're not going to see any kind of a "flat primer" in a rimfire. 
If the headspace is loose you might get a rupture at the rim. Most likely the first pressure sign you would get is hard extraction and by then you have gone way too far.

You might be able to predict that by keeping measurements of the base as you would for case head expansion in a centerfire case, but I have never tried that.

Never tried Herco. I have been using 2.5 grains Trail Boss. With reasonable accuracy. I think I stopped at 2 grains of Unique, but that's memory.

I never thought of installing a .25 RF barrel on a centerfire action. I have a .25 RF Walnut Hill barrel that I may just try on one of my centerfire 44 rifles.