Someone on THRrecently finished their Lyman GPR kit and this was posted about how to solveits poor ignition problem:
1st time out, main charge fired on first attempt about 3 times out of 10. Took rifle home, cleaned and drilled out vent to 1/16". 2nd time out, main charge fired on first attempt about 8 times out of 10. Took it home and drilled the vent to 0.070" (#50 bit). 3rd time out will be in a couple of days. I expect that ignition will be 100% then....
This wasin response:
You can safely take it out to 5/64" (0.0781), but if that #50 works, stop there. Most people are stopping at #51.
The GPR has a patent breech, so be sure you thoroughly clean each time; I remove the vent liner each time I clean just to make sure the patent breech crud accumulation has someplace to exit. If, over time, you start having ignition problems again, it's likely because the patent breech needs to be cleaned out. Remove the vent liner and use a small (approx .32 cal) copper/nylon brush and your favorite cleaning solvent; you can see if the brush is entering the patent breech by looking in the touch hole when the vent liner is removed. If you don't see the brush tip, use a smaller diameter brush.
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http://www.thehighroad.org/showthread.php?t=367339&highlight=kit%2A