Clean out screw size??

[Scota4570]

I am working on a Lyman Great plains rife.  The owner complains it is not reliable, it fires only about 50% with real BP. 

I pulled the plug.  The main powder chamber is plenty big.  The clean out screw hold connects the nipple fire to the main powder channel.  The hole is about 1/8" diameter.  The screw appears to be a M4x.7.  The nipple is not blocking the channel. 

I could drill it out and re-tap it to 10x32.  That would make the "fire hole"  0.159".  I'd think that would help getting powder under the nipple. 

OR......give it back and tell him to store it muzzle down? 

 

[EC121]

 A tap drill would have to be the size drilled.  A 4mm tap drill is .138".  Not very big.  10-32 can't hurt.

[Scota4570]

I thought about it for awhile and decided to go at it differently.  I used a diamond bur that was about 1/8 egg shaped on a 3/32 shaft.  I worked over the flash channel and made it larger.  I only opened up the are between the nipple and the powder chamber. 

I also told him to store ML rifles muzzle down. 

Hopefully that will help

[D. Taylor Sapergia]

If he stores his rifle on its butt, and over oils his barrel, he'll have no end of issues with misfires.  Be careful opening the flash channel that you don't come too close to a thin area...think about what you are cutting away.

[Mad Monk]

Also take a look at how far the nipple goes down into the flash channel when it is installed.  I had ignition problems with my Lyman percussion ignition Trade Rifle. It took me awhile to figure out that when seated the nipple extended below the threaded portion of the hole.  When I added another washer to the nipple it lifted it in the flash channel.  If the nipple almost touched the bottom of the flash channel the flame has trouble making the turn and going over into the min charge.  I then started to look at the length of the threaded portion of the spare nipples and selected short ones for that rifle.  I had at first thought the nipples were all a standard length.  Not so.

[Darkhorse]

I had a GPR in Flintlock and it only fired about 75% of the time. I had a time trying to figure out what was happening. I started using a vent pick 100% of the time and noticed that at times I could feel no powder. The only way to fire it was to get as much 4fg down into the TH as possible. Even then it might take a few tries to get it to fire.
The GPR  has a patent or nock breech. Not really sure of the term. There is a chamber for the powder that is a smaller diameter than the bore. If I just dumped the charge down the barrel it would often bridge at the point where the diameter narrowed. No powder would be in the chamber at all and the rifle would not fire. What I did was simply to stop just dumping the powder and really slow down the powder charging, I also leaned the rifle more when charging and bumped the rifle barrel with my palm as I charged the weapon.
This stopped all the misfires. I don't see why a GPR in percussion would not do the same thing.
I would check into this before I cut or drilled any metal.

[Scota4570]

The powder chambber is relatively large diameter.  Maybe 5/16".  I have a Hatfield that you could clean with a 22 cal bore brush.  That rifle gave problems until I opened it up.  This one is as large as I would make one myself.  The sideways part of the fire pathway is/was small.  I did open it up as I described, not much but some.  The nipple is positioned OK. 

After I worked on it, I asked him what type of caps he was using.  He is using CCI musket caps.  He installed a musket nipple hoping to fix this.  I see that CCI only makes "Reinactor" caps now.  I bet that is the problem.  Those caps are junk.