I have no experience with the company, just typing my thoughts:
Where did you buy it new from? See if you can take it back to where you bought it and explain the situation. They may have more leverage with the distributor or manufacturer to get a replacement.
If it really is new, I don't see why the manufacturer won't warranty it as it says in their policy below. If it was sold to you as "new", but in reality had a previous owner who never shot it, you may be SOL in technicalities. Not good juju for the company though.
Tell us some more details. Did you buy it directly from them? Did you buy it through a vendor/internet site? Did you buy it at a store? Any pictures of the crack?
This “Limited Lifetime Warranty” has been drafted
to comply with the MAGNUSON-MOSS WARRANTY
FEDERAL TRADE COMMISSION IMPROVEMENT
ACT which is effective for goods manufactured
on or after December 31, 1976. DIAMONDBACK
FIREARMS, LLC. warrants that this firearm was
originally manufactured free from defects in
material and workmanship.
DIAMONDBACK FIREARMS, LLC. agrees to correct
any defect in material and/or workmanship for the
original purchaser by repair or replacement ONLY.
A service or handling charge may be applied at the
DIAMONDBACK FIREARMS, LLC.
LIMITED LIFETIME WARRANTY
DIAMONDBACK FIREARMS, LLC. will not be
responsible for defects resulting from careless
handling, unauthorized adjustments or
modifications, corrosion, neglect, abuse, failure to
provide reasonable and necessary maintenance as
described by this owners manual. Damage caused
by hand loaded and/or reloaded or improper
ammunition or damage caused from firing with any
obstruction in the barrel will not be warranted. This
warranty does not apply to ordinary wear and tear
of any parts, including metal, plastic, or any other
material used to manufacture this product, as well
as all surface finishes.
Under no circumstances shall DIAMONDBACK
FIREARMS,LLC. be responsible for incidental or
consequential damages with respect to economic
loss or injury or property damage, whether as a
result of breech of express or implied warranty,
negligence or otherwise. Some states do not
allow the exclusion or limitation of incidental or
consequential damages, so the above limitation or
exclusion may not apply in certain cases.
If you need to return your gun to DIAMONDBACK
FIREARMS for warranty or service related claims,
please refer to the shipping instructions in the
SERVICE section of this manual.