I can attest that they fit nicely on a standard A2 birdcage flash hider, but are a bit larger than my CAI CETME's flash hider so the cover loosely sits over it. Curious to see how many other types of muzzle devices these will fit. These are exactly as advertised and make a great $2 add-on (for five covers!).

These fit snugly over your flash hider and keep rain, mud and general debris out of your barrel.
Good item to have on any rifle with a flash hider.
They can be shot off without removing them first without damaging your rifle or affecting accuracy.
Great item to keep a pile of handy. Be glad when they get back in stock so I can pick up a few more packs.
Handy to have on hand.
P.S. These also work on hunting rifles to keep accuracy effecting debris out of your barrel while trooping through the woods.
Works a lot better than condoms for that purpose, I try to always keep a pile on hand.
RemMax

I use these on AR's too... They fit well and aren't too difficult to remove.. Great price/value..

HK muzzle breaks are longer than standard bird cage breaks. These caps cover the entire flash hider, can be shot off the rifle without damage to the rifle, and are excellent protection in the field to keep debris out of the barrel should a mishap (such as a slip in the mud with the muzzle down) happen.