I did just that.
Jameso,
I did just that. I built an 870P Max from my 18-1/2" Express. Like a fool I keep track of the costs (taxes, shipping and duties included) of all my builds. Here goes:
2005 M870 18.5" Express Syn 2Rd Ext----------------400
Speedfeed IV-S Stock Set---------------------------137
Surefire 618FA Forend Light--------------------------298
Wilson Combat Track-Lock II (Front H3)--------------195
Wilson Combat High Visibility Follower-----------------17
Wilson Combat Jumbo Head Safety--------------------30
Wolff Magazine Tube Spring--------------------------13
Total----------------------------------------------1090
In addition, I also added:
GG&G Left Sling Loop---------------------------------17
GG&G Single Point Sling Loop -------------------------40
Tac Star Six Round Sidesaddle------------------------40
Speedfeed SWAT Plate (reduces LOP by another 1")---20
Suppliers:
Ellwood Epps (870), MD Charlton (stock), eBay, (surefire forend, GG&G parts), P&D Ent (Wilson Combat parts ), Brownells (Wolff spring), Smith Tac (Tac Star Sidesaddle)
Ok, so it's not
precisely an 870P Max, but
functionally it is. The biggest exceptions are the plastic trigger plate, the 18-1/2" barrel (c'mon, a 2 rd ext on an 18" barrel looks silly), cylinder choke as opposed to improved cylinder, and I used the Wilson Combat front sight instead of an XS one. My shotgun isn't parkerized either. Not yet, at least.
In the end I saved about five hundred dollars. Arguably, I don't have an 870P Max, but I enjoyed the build and it beats shelling-out all that money up front.
Here's my two (mostly) finished 870s.