Benjamin multi-pump revisited

[jdupre']

My cousin and I have been experimenting and tweaking a couple of old Bejamin 22 cal. multi-pumpers. My cousin, a dang good machinist, doesn't like all the pumping that goes along with these guns. To that end, he came up with an insert that goes in the compression chamber that decreases the number of pumps for any power level by about half.

He started out with his 30 year old gun that seemed to be an above average specimen. It shoots approx. 30 fps faster than mine at almost all power levels. He's getting an average of about 370 fps at 1 pump, 465 at 2 pumps, and 570 at 3 pumps. He uses 2 pumps for almost all small game and maybe 3 for the odd ornery coon. We're not interested in 700 fps guns that require 14 pumps and arms of steel. Two pumps and you can dispatch a starling or a squirrel.

This evening, I cut the hollow end off of the bolt and drilled and inserted a thin steel probe to push the pellet into the chamber. A little tweaking of the length gave me about 25 fps increase for each pump. Two pumps will drive a pellet into a piece of pine flush with the surface.

I have a new outlook on those venerable old pumpers now that all that pumping has been decreased to an acceptable level.

Joey

 

[Kayak Jack]

Thanks, Joey. As a kid, I had a Benjamin .177. I have no,idea about FPS, but starlings would hear it and jump up. I learned to aim a little high, and catch'em in the air. After a few years, it took 11-12 pumps to do,what 3-4 had been doing. Then, I started shooting a .22 and forgot the old Benji.

 

[Wannabe]

Joey,
Putting a insert in the compression chamber decreases the volume of air, but increases the pressure with each stroke of the pump. Does the loss of volume change the performance in any way or is it all a balancing act (to a point)?
Bob

 

[Kayak Jack]

As I remember, a few drops of 3-In-1 oil helped the leathers a lot in my old Benji. But, eventually, it all succumbed to real gun powder.

 

[jdupre']

Jack, all multi-pumpers suck in air (and debris) with every pump just like your car engine. Eventually, the guts need to be cleaned out and the seals replaced. A little oil now and then helps but the "stuff" has to be cleaned out.

Wannabe, from what we've seen, the max speed really doesn't change much, if any. The advantage is about half the pumping. One pump gives more speed than the better BB guns and quite a bit more than the Red Ryder.......and with a pellet that weighs over twice as much. MUCH more energy on target. Two pumps will get you plenty enough power for pigeon/ squirrel-sized game.

If a person really likes to shoot, you can't beat airguns. My Diana 34 in .177 has a trajectory very much like a .22 LR at 50 yards with as good or better accuracy. It gives a pretty good showing out to 80 yards ,too. I can go out in the back yard after supper and shoot without having to get all the paraphanalia together, get in the car and drive to the nearest range or woods. BIG, BIG advantage!

Joey

 

[tx river rat]

JD

I wonder if your buddy are you would mind giving some specs on what you did and maybe some pics, I have a lath and a milling machine so I think I could build the parts.
Ron

 

[Wannabe]

Does it have to be precision made or just sized small enough to fit in the chamber. What percentage (50% or what) of the chamber is taken up by the plug?
Bob

 

[jdupre']

Not sure of the exact dimensions. The insert takes up approx. 80% of the chamber volume. It has a 1/16" hole down the center from front to back. The exhaust spring is something my cousin got from Harry Smith, an airgun smith over around Lafayette. He cuts the spring to about 3/8" long from the 2" stock length. This arrangement leaves JUST enough room for the exhaust valve to open. He's thinking of maybe making a kit with the insert and the spring to sell.

One unforseen advantage is that the gun is noticeably quieter at all speeds. From what I've read, as much as 50% of the stored air ( you know, the air that you have to pump 8 times to get in there :roll: ) is wasted in a stock Benjamin or Sheridan. High pressure wasted air is noisy.

Oh, the spring is a bit weaker than the stocker so the gun has to be cocked before pumping. No big deal. It soon becomes second nature.

Together, we have shot over a thousand rounds through the two guns. At this point, I see no reason to use a stock Benjamin.

 

[Wannabe]

Makes me wonder. Would the same hold true for the Sheridan?
Bob

 

[jdupre']

Bob, the concept of a small chamber would work. I'm not sure of the size and shape of the innards of a Sheridan though. The insert took some experimenting to get it right. It needs to be fairly precise. My cousin has the idea of building a single stroke, or at most a two stroke pumper with a larger piston and longer stroke. From what we've seen, that should be doable.

The Benjamin and Sheridan platforms are very versatile and pretty much bullet-proof. They work well for a mass produced design for the general public. But, like a custom boat, they can be improved upon to suit different needs and wants.

This is a very good video to ilustrate how the mechanism works. t's a very compact, well thought out design.
In our version, the insert takes up most of the area that the longer spring occupies.

http://www.youtube.com/watch?v=ycrFlCz7I60