Donald,
I would suggest trying the SRH first and see if your accuracy improves. It could be that you just had a bad day. Try shooting the big Magnum first on your next couple of range trips. That way you can eliminate the fatigue issue.
Hope this helps,
Carl
TNR,
I did basically the same mod and I used a 2-56 tap with the appropriate drill in my min-mill. I did mine a little differently. I put a hole and a set screw in both sides. I just flipped the housing over in the vice and drilled the other side. My setup has a set screw for over travel and...