So after reading the mother of all Magazine threads http://www.450bushmaster.net/viewtopic.php?f=2&t=23 I successfully modded 2 20Rd PMags for my .450 following Siringo's awesome advice and directions for the PMag mods.
I did this because I wanted a non-metal (shortest possible) mag for hunting. However I now needed to limit the mags capacity (because they hold 7) to it's legal hunting capacity (in Maine) to 5 rds total.
Magpul has recently release new mag limiters for their 20rd PMags. They come in a 5 & 10 rd variety. For the .450, the 10rd limiter is what we are going to use http://store.magpul.com/product/MAG281/101
Pre Mod Limiter:
As seen in these pics (not mine, but used/modded for demonstration purposes). If you cut the top off the limiter, the 20rd will now hold a max of 5 .450 rds and still allow the use of the dust cover. The best part is this limiter is made for the PMag system so it works flawlessly.
Here is the place you need to cut:
What the finished limiter will look like for a max capacity of 5 .450 rds in the mag:
Very simple and easier then making a custom block.