View Full Version : Uhh why is there a ledge in my GPR Barrel?

February 16, 2012, 10:41 PM
Ok so I was cleaning the GPR and I kept noticing the patch would get really tight about 3 inches from the base (breech) of the barrel. So I'm thinking man this barrel probably has a ton of rust in there. So I peak down it with the bore light and it shines like new penny. So I take a small brass cleaning rod and run it down the barrel keeping in contact with the side of the barrel. And yep about 3 inches from the end I hear a *tink* then I move the rod just a tad toward the center of the barrel and it will travel the rest of the way to the end. What in the world??????? Do I have a defective barrel? I have never seen such a thing. Its clean as a whistle. thoughts? These things should have a removable cap on the base of the barrel.

February 17, 2012, 06:49 AM
GPR barrels have what's called a patent breech, or Nock's breech. It's actually a smaller diameter combustion chamber in the breech end. That's probably what you are feeling. Here's a drawing of the concept (this is not a scale drawing of the detailed GPR design, and it's not my drawing; unfortunately I've lost the source reference):

February 17, 2012, 09:44 AM
That's really strange - is the "step" present all the way around the inner circumference of the barrel? And is it nice and sharp, like it was machined that way?

February 17, 2012, 11:17 AM
Very interesting mykeal,

Thanks for the info.

Wouldn't you say the Nochs was designed to help lessen blow-by?

February 17, 2012, 12:07 PM
I honestly have no idea.

prof marvel
February 18, 2012, 02:05 AM
this link should answer your question:


prof marvel

February 18, 2012, 07:58 AM


February 18, 2012, 08:03 AM
..."improved ignition and increased velocity.

Thanks prof marvel.

February 18, 2012, 11:02 AM
Thank you everyone. I was dumbfounded when I found it. Thats very interesting thank you for posting up that info.

prof marvel
February 18, 2012, 05:17 PM
If one browses about one finds that the Nock Patent breech was highly recommended for fowlers as well as the large bore rifles represented by American Hawkens and British Percussion Sporting Rifles. It seems the larger bores "need" a smaller "chamber" to achieve better consistency and efficiency.

Happy to be of service -

prof marvel

February 20, 2012, 07:14 PM
So in essence you have to put enough powder in to fill up this smaller chamber? That or when the ball is rammed all the way down, there will be an air pocket otherwise. Am I correct in thinking this way. The Lyman manual only gives a "max" load for the .54cal they don't give a starting load.

February 20, 2012, 08:41 PM
I was reading about it here, and it pretty much tells why it's there and what it's supposed to do. It's on page 117 and page 118 of this pdf. It's exactly as Prof Marvel said. I can't find a drawing that actually has a scale on the same picture, but I'm bettin this little chamber only holds a smaller charge than you would ever play with (in reference to worrying about not filling it up). But, that's just me looking at it. I didn't scale it into my cad program. It was interesting, but not that interesting. :D


February 21, 2012, 12:33 PM
You also have to be more careful swabbing the bore with the GPR patent breech since it doesn't clean out particularly well when using a full width jag and patch. I use a regular rifle cleaning rod with a slotted patch holder to clean that portion out. Otherwise rammed down fouling and excess cleaning patch lube/solvent builds up in there and can cause ignition problems. It's handy for keep you from ramming a ball down to the breech w/o powder, but other than that, it's a hassle for cleaning with a normal BP rifle kit.