Funny you should mention that...
I just got done opening up the channel in my synthetic forearm about 5 minutes ago.

My forearm for one of my 45-70 barrels was touching all the way down too.
I wrapped sandpaper around one of my 20 ga. barrels (no sights in the way) and worked the forearm back and forth over the sandpaper.
Only took about 10 minutes and it opened up perfectly. I just checked the fit a couple of times during the sanding.
One other thing I did was to put a couple drops of oil between the frame and the forearm (where the spacer would be on a wood forearm). The barrel just falls open now when you push the button.
I hope this helps
Spanky