  We consider two SU(3) breaking parameters, $R_1(m_B)$ and $R_2(m_B)$,
appearing in a relation between $B^+\to K\pi$ and $B^+\to\pi\pi$ amplitudes,
which plays an important role in determining the weak phase $\gamma$. We
identify an isospin-related quantity $R_2(m_D)$ measured in $D$ decays,
exhibiting large SU(3) breaking which is likely due to nonfactorizable effects.
With a cautious remark about possible nonfactorizable SU(3) breaking in $B$
decays, we proceed to calculate factorizable SU(3) breaking corrections.
Applying heavy quark symmetry to semileptonic $D$ and B decay form factors, we
find that SU(3) breaking in $R_2(m_B)/R_1(m_B)$ may be significantly larger
than estimated from certain model calculations of form factors.
