BNM Breech Reducing Pellet Velocities 50-70 fps
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Topic: BNM Breech Reducing Pellet Velocities 50-70 fps (Read 1240 times))
breakfastchef
Expert
Posts: 1097
Real Name: Larry
BNM Breech Reducing Pellet Velocities 50-70 fps
«
on:
May 08, 2013, 10:35:11 PM »
Looking for a possible explanation of why a BNM multishot breech would reduce pellet velocities of my PC77 carbine build. To make a long story short, this gun will push a 7.0 grain pellet from 700-710 fps at 10 pumps with a Crosman steel breech. When the BNM breech is installed, velocities of the same pellet drop closer to 640-650 fps.
I am guessing that two features of the BNM breech may be reducing the pellet velocities. First, the nature of a riser breech creates additional airspace behind the pellet. The extra airspace needs to be pressurized. Second, the BNM bolt probe is a bit beefy and is probably robbing a few more fps.
Sergio builds fine products and they work as advertised. I have owned this breech for over a year and know how to install and uninstall it. I reinstalled the Crosman breech to verify the gun was working fine and to make sure I did not overlook something simple.
I simply need to figure out why the pellet velocities are lower with the BNM breech installed so I can sleep at night.
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Western NC
rsterne
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Bob and Lloyd
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Real Name: Bob
Re: BNM Breech Reducing Pellet Velocities 50-70 fps
«
Reply #1 on:
May 08, 2013, 10:48:00 PM »
I think you have probably guessed the primary reason, extra volume in the transfer port.... That is especially critical in .177, and the lower the FPE level the more important it would be.... The probe could be an obstacle to the airflow, it depends on which is greater in area, the annular area between the probe and the bore, or the 0.133" diameter of the transfer port.... If the probe is 7/64" (0.109") or larger, it would likely be losing velocity in a .177 barrel....
There is one other possibility you could check out.... Is it possible that the cocking pin is dragging on the slot in the breech and slowing the hammer down?.... If that is the case, it should be accompanied by retained air (fire again without pumping, if you get a "pop" that's retained air).... If it's not dragging enough to cause retained air, it shouldn't matter, but then again.... who knows....
RE the transfer port volume issue.... What is the diameter of the (longer) transfer port in the BNM breech?.... If you are running a stock 0.133" barrel port (and in .177, you should be), there is no reason for the transfer port to be larger than a stock 0.140" ID (9/64").... Adding more volume will just hurt performance.... Possibly you could try sleeving the breech down to that size?....
Bob
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Coalmont, BC, Canada
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breakfastchef
Expert
Posts: 1097
Real Name: Larry
Re: BNM Breech Reducing Pellet Velocities 50-70 fps
«
Reply #2 on:
May 08, 2013, 11:11:29 PM »
Thanks for your thoughtful reply, Bob. Transfer tube diameter appears to be of stock I.D.; the valve and barrel porting is untouched. The potion of the bolt probe that pushes the pellet past the barrel port measures .126". Yea, pretty thick.
I am satisfied that this is the 'nature of the beast'. You get multishot, multi caliber features, but lose some performance, in part, by design. As to sleeving the transfer tube, I am not sure how to accomplish that with crude stone tools.
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Western NC
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BNM Breech Reducing Pellet Velocities 50-70 fps