Thanks for saying that "not recommended" part, 303. Those seem like desperate measures to overcome a serious chamber problem - not needed or wanted by anyone more fortunate.
Regarding the "pressure signs" of stretched cases and poor groups, both can be caused by over-sized cases. By which I mean cases that have been FL sized way too much. They're actually UNDERsized in dimensions, and they're trying to expand to fill the chamber when fired.
I use an FL die to size my Hornet rounds, but I partially size them. I stop about 3/4 of the way down the neck, leaving a clearly visible unsized line above the shoulder junction. You might try backing your sizer off about one even turn. Then go back to the 12.5 LG load.
Note also that the Hornet likes a very mild primer and a firm crimp to avoid the primer blast from unseating the bullet prematurely. I use either the Rem 6 1/2 small rifle or the CCI 500 small pistol primer in mine, an even 13.0 of LG and always use a Lee Factory Crimp die. I also prefer 40 or 45-gr bullets designed for the Hornet, whether from Nosler, Hornady, Sierra or Speer.
