i do not consider the Grizzly barrels 'compatible' with an 870.
there is a slight difference in barrel indexing between the 870 and the Grizzly. if you install a Grizzly barrel on an 870, the front sight will be cockeyed and the barrel/magazine tube will look 'twisted'. safe to shoot? i guess - the bolt will still lock into the shank of the barrel. but its not going to be accurate.
there were many people buying Grizzlies just for the short barrel, then selling the barrell-less gun on the EE. i tried a Grizzly barrel on both my Wingmasters and Express and i do not consider the result acceptable at all. i really dont understand how people can actually use an 870 with a Grizzly barrel on it cockeyed.
that said, if you are looking for a 12.5" barrel then rather than spending $300 on a Dlask i recommend you spend $300 on a Grizzly with a 12.5" barrel. its at least as good as an 870 Express (imo better), so its a much better value this way. the Dlask barrel is nice, but it has some small issues too. the chamber looks like it was cut by a 5 year old with a hand file, and the extra-thick barrel is a bit heavy - it weighs more than an 18.5" barrel. this can be a plus/minus depending on whether you are actually carrying the gun more than a few feet. on the plus side, i never need to worry about a dented barrel.