No, they won't break if your lugs or staples are installed correctly.
The pin should only put (pressure) on the bottom of the channel in the lug or staple - it should be pretty much centered in that channel, not touching either the front or back end.
That way it simply keeps the stock and the barrel snugged up to each other but should allow forward of rearward movement either as the result of recoil, heating up/cooling down of the barrel, seasonal changes to the wood (stock) as the humidity rises or falls.
That is why, if you use "solid" lugs it is always recommended that you make oblong holes to allow some back/forth movement potential.
Always remember, THE BARREL supports the forestock, the forestock is NOT there to tightly grip the barrel and provide support for it.