Here at Case Club we do a lot of gun cases & often get asked which foam to go with. Obviously the answers depend on circumstances, but 90% of the time the best foam for gun cases is polyethylene (abbreviated PE) . Polyethylene is hands down the best for gun cases because unlike polyurethane, polyethylene is closed cell. What does that mean you ask? Closed cell foam prevents moisture from absorbing into it. Have you ever seen those foam noodles kids play with at pool parties? Notice that they don’t absorb water like a sponge would. The same can be true with the oils from your gun or the rain from the field. No matter how wet PE foam gets the liquid stays at the surface so it can be wiped clean.
On the other hand if you use open cell polyurethane it will get extremely gross over time. It will absorb moisture, oils, & cleaning solvents causing it to break down much quicker than PE. Hope that answers the question for all you gun lovers out there!
Learn more here: https://www.caseclub.com/different-types-of-foam-for-cases/