by Tom Gaylord
Writing as B.B. Pelletier

Pump-Assist Benjamin 392
The Benjamin 392 pump assist is an interesting side street in the hobby.

Part 1

A history of airguns

This report covers:

  • Pump-assist pump effort
  • Sheridan Blue Streak pump effort
  • How it feels
  • Velocity test 1
  • Velocity test 2
  • Velocity test 3
  • Velocity test 4
  • Velocity test 5
  • Trigger pull
  • Conclusion

Today we look at velocity and some other things that relate to the pump mechanism of the pump-assist Benjamin 392. We will start with the effort to pump the gun.

Pump-assist pump effort

In the past I have used the chart supplied by the pump-assist manufacturer, Bob Moss, to show the pump effort of the pump-assist Benjamin 392. Today I actually tested it, by pumping the gun on my bathroom scale. I know an analog spring bathroom scale is not an accurate test instrument, but it should give us a basis for comparison, because I will also measure my recently rebuilt Sheridan Blue Streak.

I laid a thick book on the scale and pressed down on the book with the pump handle for each pump stroke. I did discover that if I went too slow the pump effort remained very low, because air was not being pumped into the reservoir. So I pumped each stroke as fast as normal and them shot the gun to verify that it was filled. I have subtracted the weight of the book (2 lbs.) from these numbers.

Pump stroke…………..Effort in lbs.
1………………………………….11
2………………………………….16
3………………………………….13
4………………………………….15
5………………………………….15
6………………………………….15
7………………………………….15
8………………………………….14

After verifying the gun was full, I then pumped it as fast as I could this way. The scale needle never went above 20 lbs., and 2 lbs. of that still has to be subtracted for the weight of the book.

Sheridan Blue Streak pump effort

Next, I tested my recently rebuilt Sheridan Blue Streak the same way. We know from recent velocity testing that this rifle is on spec. for power.

Pump stroke…………..Effort in lbs.
1………………………………….13
2………………………………….27
3………………………………….33
4………………………………….36
5………………………………….40
6………………………………….40
7………………………………….42
8………………………………….42

There you have it. I would say those numbers accurately reflect the difference in effort for the pump-assist rifle and the Blue Streak. Until you try it the first time it is impossible to imagine — particularly if you are a veteran multi-pump shooter. Fred — you now own Mac’s pump-assist (and mine before he got it). What’s your take?

How it feels

I don’t have that video of my pumping the rifle ready for you today, so I will describe how it feels. Initially the pump effort is very easy, but somewhere in the middle of the stroke the effort goes up to the maximum. It stays there for an instant then drops back to almost nothing as the stroke is completed. The peaks are the efforts listed above.

I watched the pump piston head while I was pumping and discovered that the peak effort comes as the pump head is almost home, which is what you would expect. The pump-assist mechanism changes where in the stroke this happens, because as I reported in Part 1, the fulcrum changes as the number of pumps increases. Now, let’s look at the power of the rifle.

Velocity test 1

First I want to establish the velocity of this rifle on a varying number of pump strokes. The 392 manual says to use a minimum of two pump strokes, and not to exceed eight strokes. From experience I have decided that three strokes is a better minimum. Maybe that was the recommended minimum at some time in the past, or maybe I had a bad experience with fewer pump strokes, but three is the fewest number of strokes I will use. Here is the velocity of the gun with Crosman Premier pellets on a varying number of pump strokes.

Pump stroke…………..Velocity f.p.s.
3…………………………..451
4…………………………..494
5…………………………..535
6…………………………..567
7…………………………..585
8…………………………..608

That’s right where I expected it to be. It seems to be functioning like a new gun. At the maximum velocity this rifle generates 11.74 foot-pounds of muzzle energy. I have seen 392s go from 11 to almost 14 foot-pounds with Crosman Premiers, so this one is on the low end of normal. Just for fun I went back to Part 2 of the test I did on this rifle back in November of 2007. At that time 5 strokes got me 509 f.p.s. and 8 strokes got 589 f.p.s. Things are pretty much where they were back then, if not a little better.

Velocity test 2

In this test, I first oiled the pump piston head. I noted that the pump head was oily when I did this, so I doubted there would be any change, but sometimes people wonder if the gun was given a fair chance to do its best. So I oiled it. Here are the results. I’m still shooting Premiers in this test.

Pump stroke…………..Velocity f.p.s.
5…………………………..508/535
8…………………………..594

Notice that my first shot on 5 pumps was slow. That often happens right after you oil a pneumatic. Then the speed increases as the oil gets distributed around the pump head. That’s why I shot a second time.

Velocity test 3

This test was for the stability of the gun with a certain number of pump strokes. I tried it with 5.

Shot…………..Velocity f.p.s.
1…………………..536
2…………………..537 (fastest)
3…………………..531
4…………………..530
5…………………..534
6…………………..529
7…………………..526 (slowest)
8…………………..533
9…………………..527
10……… …………534

The average for 10 shots was 532 f.p.s. The maximum spread was 11 f.p.s. That means the 392 pump-assist is pretty stable.

Velocity test 4

Now it was time to try the rifle with some different pellets. Because this is a pneumatics, it’s going to become more powerful as the weight of the pellet increases. First up was the H&N Baracuda Match with 5.51mm heads.

Pump stroke…………..Velocity f.p.s.
3…………………………..390
5…………………………..464
8…………………………..523

On 8 pumps the rifle develops 12.82 foot-pounds with this pellet. That’s plenty of power for hunting, and of course the 392 is a .22 caliber airgun, after all.

Velocity test 5

In this test I shot the lighter JSB Exact Jumbo Heavy that still weighs 18.1 grains in .22 caliber. Even though it’s lighter than the Baracudas, it’s still considered a heavy pellet. They gave me these velocities.

Pump stroke…………..Velocity f.p.s.
3…………………………..416
5…………………………..492
8…………………………..558

On 8 pumps this pellet delivers 12.52 foot-pounds at the muzzle. It’s another good performer for hunters.

Trigger pull

This is a new 392 so the trigger is not like the one in my Blue Streak. The 2-stage trigger broke at 5 lbs. 9 oz., which is not exactly light.

Conclusion

I tested the pump-assist Benjamin 392 multi pump several different ways and learned that it is still performing like new. And, this was the first time I tested the actual pump effort. My test was simple, but the results are similar to the chart the maker gave me.

The pump-assist Benjamin 392 was a wonderful idea that was never fully realized. If it had migrated to the hand pump, the world of PCPs would have changed, in my opinion. We may never know, but I know I own a remarkable invention that few airgunners will ever get to appreciate.