Reducing OD of BNM Breech Bolt probe. I need advise!

[rsterne]

I have never found flow-through bolt designs to be the answer on high-powered guns.... I have always had better success (ie more power) with an extended probe that is 50% of the bore diameter (maximum).... On a .22 cal I use a 3/32" (0.094") probe which is long enough to load the skirt of the pellet past the front of the barrel port.... and the bolt face is set flush with the back of the barrel port.... Here is the supporting math, assuming a barrel port of 75% of the bore diameter (the widest you can use without pellet loading problems)....

Bore: 0.22".... Area = -0.0380 sq.in.
Probe: 0.094"....  Area = 0.0069 sq.in.
Difference: 0.0380 - 0.0069 = 0.0311 sq.in.
Barrel Port: 0.165".... Area = 0.0214 sq.in.
Flow Through Bolt (0.025" wall, 0.21" OD): 0.160".... Area = 0.020 sq.in.

The largest flow-through bolt I have seen in .22 cal had a 5/32" hole (0.157").... You need some clearance on the outside so that the probe doesn't drag on the chamber wall, and you need sufficient wall thickness for strength.... That is where I got the above dimensions.... It MIGHT be possible to get the hole in the flow-through bolt up to the same size as the barrel port.... but you could never get as much area for flow as with an extended probe of 3/32"....

Bob

[Rdsail]

Thanks Bob!

It is always good to get some supporting numbers to what I think was true from practical experience.

Daniel

[Rdsail]

I got tire of waiting for bnm to send me a longer t.port so I made my own. I was going to hold on this but I started getting an air leak at the port.

Here i the picture. the top one is the bnm breech.



The left side is the part that slides in the breech. I did make one mod. I made the part that goes in the gasket and valve longer. I opened the valve port to .17o so the tops fits snug into the valve. this was there are less changes for air to scape.

Some measurements:

from the gasket to the barrel port
BNM: 0.438 in
Mine: 0.465 in

The bottom part where the gasket goes in to the valve.
BNM: .050
Mine: .150

ID
BNM: .138
Mine: .141

OD its the same on both at 1/4 in

Now you feel the pressure of the gasket when the tube and breech come togehter. hopefully this will seal everything better

If I have time I'll put it together tomorrow and test

Forgot to mention. I made it using a drill press to spin it. For tools I used my dremel with a metal grinding wheel to remove material in junction with small files. then use wet/dry sand paper

[Rdsail]

Success!

New Tport works great. It does not leak. I gained about 15 fps. not much. i not looking for more power This allowed me to reduce the preload some (about 1/2 a turn). One thing that I was not expecting is that the gun is quieter now.

[ninsei]

Hi.  What is the original piece you started with to make this longer TP for the BNM breech?