A little advice, please?

[nosrettap1958]

What the heck kind of rifle is this?  I’m thinking, God forbid, that you may have been lucky to have that Green Mountain barrel given all that back pressure.   

[bgf]

I'm troubled by the need to grind off the nipple -- that may very well be part of the problem.  If I understand correctly, the threaded portion of the nipple is so long that it hits the opposite side of the drum before it is shouldered when inserted full length?  Or is the hole in the drum just not threaded far enough to shoulder the nipple -- maybe you didn't use a proper tap to get full threads for the length of the threaded shaft on the nipple?  Still trying to figure out exactly what is going on, but I'm pretty sure the size of the orifice is the cause of the blowback.  It really isn't safe, as someone else pointed out -- those caps can hurt/do damage when they fly off the nipple, not to mention accuracy is a $#@* shoot.

[docone]

I have shortened the nipple on my rifle. No worries, if the drum is thick enough to have threads. I found, when the nipple exited close to the drum wall on the other side, I got misfires. Another issue was the nipple getting peened over with firing. I chucked the nipple in my foredom handpiece, and sanded the edge off. Dry firing is especially hard on nipples. Another place to look is inside the hammer. I have found pieces of cap, residue on the inside corners making nipple contact harder. Those are other issues with hangfires.
Get yourself a new nipple, make sure it is not too long. File the burrs off the base, and I like to counter sink the flash hole in the drum.
Should be good to go.

[aflo]

The nipple had been drilled all the way through at the .076” size, eliminating the smaller hole at the bottom. Apparently when I working off the bottom of the nipple to properly seat it into the drum, I thought the hole was buggered up by the grinding and “cleaned” it up by running a drill through it…. Wrong.
With a Google search, I happened across a patent drawing of a nipple cross section, showing the necked down area. I did my best to duplicate it by silver soldering a short piece of brass rod in the bottom ¼” and drilling the little hole with the smallest drill I had, about .035”. After cleaning everything up, I reinstalled it in the rifle and took it for a test.
The bottom line is the blowback to half cock problem is gone, not a single one in 16 shots – most of which were at 75 grains of load. There is still a little residue around the area to clean up but nothing like before. I will buy a couple of new nipples soon because this one may not last.
Thanks again to everyone.

[Steve-In]

I get my nipples from Pecotonica River.  They are hardened Stainless, I think 17-4.  These have very good life, no battering and are sized proper.  To test for leakage around that drum put in a 1/4-28 bolt in the nipple hole and put some water in the barrel.  Run your ramrod down and smack the end with a mallet.  Should be no water come out at the barrel/drum.  The drum concerns me more than the blowback unless the fouling is a result of powder flash being directed down by the hammer cup.  Post some pics of the fouling.
BTW I shoot 50 to 65 grains 2f Goex and a .445 round ball with .020 denim patches in my GM .45
http://www.longrifles-pr.com/  <- NIPPLES