Fiberglass stock flexing upwards

You may have a GRIP like Samson or Hercules. I still think that 1/4" is a lot of flex. "Not Good" meaning extreme flex. I would contact McMIllan.
We had a synthetic stock on one of our Winchester Model 70 Shadow that had flex like that. Winchester put a new stock on it.
Put the old stock back on , give the stock back too where you got it and buy a good one from McMullen
 
Something is not right with your setup. HTG stocks were used by the USMC for decades. There pretty bomb proof. As stated above unless you're an exceptionally strong person you're not going to flex a HTG stock 1/4 inch.
The action is not fitted, someone hollowed out the fore end, something else is wrong.

Good Luck
 
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