If there's no missing pieces, the break can be separated and thinned-out Accra-Glas, tinted to match the stock color, worked into every part of it, then bound tightly with banding until it cures.
On some rifles, that I've done like that, I've also furrowed out a 1/4" wide trench in the broken sidewall, on the inside of the magazine well, running from front to back about midway in depth.
I make the trench deep enough to glass in a length of 1/4" rod that spans the break, reinforcing the magazine well sidewall.
If the sidewall's too thin for 1/4" stock, I use thinner rod stock, and make the trench appropriately shallower, etc.
The stock will need to be refinished, of course - and the inside of the magazine well dressed flush/smooth.
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