My Benjamin 392

[Bryan Heimann]

You have found THE solution to scoping and pumping!  You are the man!

Best 392 I have ever seen!  I like this better than the Cothran stocked steroids out there.

[rsterne]

"Rather unfamiliar" is putting it mildly.... This was quite an adventure, and like I said in the beginning, I had no idea where it was going to end up.... Here is a bit of trivia for you.... The test I did at 32 pumps with the 25.4 gr. Monsters.... I got 3 shots of 33.4, 24.6 and 10.1 FPE, plus a puff of air left over.... That is over 68 FPE total in a .22 cal pumper from 10.8 cc of air at 1630 psi.... The total amount of air pumped (the total swept volume) was 1856 cc (113 CI).... The crazy part is, that if you dumped all the air on the first shot, you would still only get that 33.4 FPE.... All the remaining energy, over half the total, came from retaining air on each shot.... 

Bob

[Techie]

The crazy part is, that if you dumped all the air on the first shot, you would still only get that 33.4 FPE.... All the remaining energy, over half the total, came from retaining air on each shot

Well, that brings the following question to my mind, even though it is really unrelated to this project:
How many pumps result in the most efficiency in a typical (stock) .22 caliber pumper that dumps all air in one shot?  One pump?  Two pumps?  In other words, when you compare total pumping effort to FPE produced from that effort, where are we most efficient?  When I say "effort" I don't just mean number of pumps, but a combination of number of pumps and pumping effort.

I know the answer can vary based on pellet weight, barrel length, pump lever length, compression tube diameter and probably many other factors, but still would like to hear some kind of guess from someone much more knowledgeable than me.  I find it an interesting question.  Maybe assume a typical stock 392.  If this is a stupid question with no answer, please forgive me.

Edit: I just read the recent "FPE" thread.  Now I don't even want to talk about FPE anymore!

[rsterne]

It depends entirely on the valve volume in relationship to the barrel volume.... The smaller the valve, the more efficient, but the less power possible.... On the other hand, valve volumes of over 50% of the barrel volume are very inefficient in pump and dump guns.... For several of the pumpers I have had, the highest efficiency (measured as FPE in the pellet compared to the volume of air pumped) occurred at about 3 pumps.... Consider these two graphs, both for "pump and dump" guns....



As you can see, the smaller the valve the less power, but the higher the efficiency....



The second graph was for a valve that was 24% of the barrel volume.... The pump swept volume was 33 cc filling a 3.5 cc valve, using an FTP with minimum headspace.... My guess is that the pressure at 20 pumps was about 2300 psi....

Bob

[Techie]

Thank you Bob for answering what must be a rather difficult question.   

Small valve volume (or small volume of air swept by the pump) must be what makes a Daisy 880 so easy to pump.

[JPSAXNC]

Hi Bob, The only disadvantage I found with the two piece valve, was that the piston would push on the front half of the valve rearward, and the pressure would push the front half of the valve forward, causing the screws securing the front half of the valve to loosen due to the back and forth motion.

[rsterne]

I will watch for that.... This is the 3rd pumper I have had with a 2-piece valve, and I have never had a problem before.... On the other hand, the 392 does have a much softer brass tube.... You definitely don't want to set the pump piston too long, I have mine adjusted so that with no pressure it just touches the front part of the valve....

Bob

[Rob M]

Hi Bob, The only disadvantage I found with the two piece valve, was that the piston would push on the front half of the valve rearward, and the pressure would push the front half of the valve forward, causing the screws securing the front half of the valve to loosen due to the back and forth motion.

James , was this 2 piece piece design on the 32 cal sheridan ?

[rsterne]

As in James Perotti?.... Welcome to the GTA, whether that is you or not.... 

Bob

[JPSAXNC]

Hi Rob, Yes I used the two piece design on the .32 Sheridan. On the .32 Bengi, I used a three piece design, A center tube where the check and firing valves screwed into the ends.  I felt it was the best overall way to go, although it was heavier than the two piece valve.  James P.

[JPSAXNC]

Thank's Bob, Yes, it's me.

[rsterne]

Hey, the Pumper-Master himself....    …. I'm sure you will have lots of things to share, your knowledge of all things pumper is legendary.... 

I know that everything I have done you did years ago, but being this is my first 392, it was a fun project.... 

Bob

[JPSAXNC]

I thought Id'e mention another 392 valve mod. I had opened up the porting in the valve, tube and barrel on a .32 cal. Benjamin conversion. I was using a .205 transfer port, I machined a delrin plug to go inside the valve. the plug had a 1/16" hole drilled through the center, and a small pocket at each end to clear a short spring and the check valve and a short spring and the poppet valve, and on 12 pumps from empty, the gun would fire a .310 44 grain ball @ 590-600 fps. The pressure @ 12 pumps was between 2000-2200 psi. I hadn't tried that mod in a smaller caliber but it might make good power with minimal pumps.

[rsterne]

So you reduced the internal volume instead of increasing it.... 35 FPE from 12 pumps is certainly impressive.... I don't think I would like to pump it to 2200 psi though, certainly not with a stock linkage.... I know one of our members here has been playing around with reducing the volume on 13XX pumpers....

Were you using a separate receiver (for the larger caliber) with a TP insert between the two?.... That would certainly eliminate any leaks.... 

Bob

[Rob M]

james , If you need me to post pictures, i believe you have my email so it would be no issue.

[JPSAXNC]

Hi Bob, Yes, I made a very minimal receiver out of round stock, and both the barrel and receiver were soldered to the tube. For the transfer port, I had made a 5/16 or 3/8 counter bore on the transfer port hole on the top of the valve, pressed in a piece of delrin stock with the transfer port hole in it, that protruded from the valve a bit. Then chucked up the valve in the lathe and turned down the delrin piece in place so that it was about .007 above the surface of the valve, then I compressed the valve and delrin piece in a lathe collet, and before the delrin could spring back all the way, I drove the valve into the tube. That made very effective seal. 

[rsterne]

I like that idea for a Delrin transfer port pressed into the valve and then turned to shape and left a few thou high.... I may would have liked to try that in my 392.... Not Pretty sure I can can't hold the small rear section of the valve now, though.... it may will be too short....   

Bob