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The delta L problem (ΔL problem) refers to certain firearm chambers and the incompatibility of some ammunition made for that chamber. ΔL is a Commission Internationale Permanente (C.I.P.) geometric dimensioning and tolerancing definition for cartridge cases that are longer than the chamber they have to fit in. "Delta L" essentially means "difference in length".
If the chamber in a C.I.P. conforming firearm (a firearm with chamber dimensions specified by the Commission Internationale Permanente) is shorter than a non-C.I.P. conforming maximum sized cartridge (a cartridge with external dimensions larger than specified by the Commission Internationale Permanente), use of that ammunition in the firearm may lead to chambering and/or feeding problems in the shoulder area of the chamber, or other headspace issues.
There is normally no cause for alarm regarding any cartridge designated as a ΔL problem cartridge, as the ΔL problem does not create a dangerous condition in the firearm in the rare event that ΔL problems are actually present.
Firearms users that have to rely on their weapon under adverse conditions, such as dangerous game hunters, obviously have to check the correct functioning of the firearm and ammunition they intend to use before exposing themselves to potentially dangerous situations.