The warning is from the Accurate Arms load guide. This is the text of the warning in full:
This data is intended for use in firearms with barrels which fully support the cartridge in the chamber. Use of this data in firearms which do not fully support the cartridge may result in bulged cases, ruptured cases, case head separation, or other condition which may result in damage to the firearm and/or result in injury or death of the shooter and/or bystanders.
In recent years it has become very apparent that there exists a situation regarding some pistols chambered for the 40 S&W cartridge. Some of the pistols currently available to shooters may not provide complete support to the case when a cartridge is chambered.
The Sporting Arms and Ammunition Manufacturers Institute (SAAMI) has established a maximum average pressure for the 40 S&W of 35,000 psi. The loading data published by Accurate Arms was developed in our ballistic laboratory in accordance with SAAMI test methods and equipment and does not exceed this figure. This information is safe for use in firearms which provide complete support of the case. Failure to fully support the case with cartridges of such intensity may result in bulged cases, ruptured cases, separated case heads or other consequences which may result in damage to the firearm and/or injury or death to the shooter and/or bystanders, This can happen no matter whose propellant is used.
If you own a firearm chambered for the 40 S&W, we recommend you contact the manufacturer to determine if the case is fully supported, or have a competent gunsmith examine the firearm and determine the amount of support provided the case. If your firearm does not provide complete support for the case, DO NOT USE Accurate Arms Company data or products to reload your 40 S&W ammunition.