It is a somewhat easy install. Hardest part will be dremeling the delta ring and the handgaurd cap. Once you snip those two peices, everything will go on like butter. No problems.
If you so choose, you can remove the front sight base and then remove the handgaurd cap, delta ringe, snap cap and weld spring, but I found cutting the dleta ring and handgaurd cap much simpler. They are inexpensive parts.
If you would rather not do any of that work, just get any C model SIR like the #45, #46 or #50. The C models will justdrop right on any flat top. ARMS has said all SIR models will eventually be available in both M and C models, but they just dont know when this #58mod will be available as a C model.
I do love the #58mod though, it is clearly my favorite SIR. Its great!
[img]http://www.imagestation.com/picture/sraid86/pd6e80ef63545af9a4f59212ed289b9d4/fab11890.jpg[/img]