Carphunter,
I shoot a Lyman GPR and am assuming that they would be set up the same as your Deerstalker. The thread is a metric that is about the equivalent of a #8 screw. Mine was threaded straight through into the snail and bottomed out against the nipple. One time I removed the nipple for cleaning and neglected to check the tightness of the clean-out screw. This resulted in the clean-out screw blowing out and being lost. The fix was a trip to the hardware store where I bought a #10 x 32 tap and a allen set screw. The total cost was about $2.75! I threaded the clean-out hole for the allen set, but not tapping completely through, only deep enough for the allen to bottom out before it comes in contact with the nipple, but the face of the allen is flush with the outside of the snail. It works great. It stays tight when the nipple is removed and is easier to take out without stripping out stripping out the little slot on the original clean-out screws. This is more than what you asked for, but that's what works for me.
