What is a "Breech"?
The "breech" is the area of the gun where the pellets are loaded. It is what holds the barrel and bolt-action assembly in place, so is a crucial part of the GRAVmag platform.
"Steel Breech" vs "Plastic Breech"
Some Crosman variants come with a "plastic breech" (2240, 2250b and 1377). Others come with an upgraded "steel breech" (2250XL, Benjamin Discovery, etc).
Guns that come with a plastic breech can easily be upgraded to a steel breech, via a kit, which can be found on places like eBay and GMAC Custom Parts for around £50 GBP.
The steel breech is often prefered as it is more sturdy and allows a scope to be mounted directly onto the gun, thanks to the built in 11mm dovetail.