Benjamin 347 multi-pump pneumatic: Part 2

by B.B. Pelletier

Heads up! Before you read today’s blog, I wanted to alert you to a special scope deal Pyramyd Air is running through For one day only, they’ve slashed the price on a Leapers 4×32 compact scope with rings. The scope goes on sale Wednesday morning (9/21/11) at 3:00 A.M. Eastern. I can’t say for sure the exact minute that evening when it’ll go back to the regular price, so be sure to order early if you want it. Click on the Gear Hog link to get yours. There’s also an order limit of 2 per person. Now, on to today’s blog.

Part 1

Benjamin’s 347 multi-pump was sold between 1969 and 1992.

Today will be a “Grasshopper” day, as in basic learning. We will transition from “Wax on. Wax off” to learning a few basic offensive karate moves — metaphorically speaking.

I’m going to demonstrate today how I assess the firing condition of a new (to me) multi-pump airgun. This is a drill you probably should be using with all your multi-pumps when you first get them — new and used, alike.

Today’s lesson requires the use of a chronograph. My choice is the popular Alpha model Shooting Chrony that costs right at a hundred dollars.

The first thing I do is cock the gun and shoot it to release any air that might still be inside. Hopefully, there will be some; but with a used gun, the chances of that happening are less than 10 percent. From this point on, though, you should always store the gun with one pump of air in it if the mechanism allows you to do that. Some guns, like the Daisy 22SG, are designed so they cannot be stored this way, but the Benjamins and Sheridans still can; they should always be stored with air in them.

The next step is to pump the gun to the maximum, which with most modern Benjamins is eight pump strokes. Then, load a pellet and fire it through the chronograph. I always use the Crosman Premier pellets for this; and with the .177 guns, I use Premier lites. With the 347 in this test, I got a velocity of 646 f.p.s. on eight pumps.

According to Crosman literature, a new Benjamin 397 should get up to 800 f.p.s. with the maximum number of pumps. That would be with the lightest pellets, so figure a max with Premier lites of around 750 f.p.s. I happen to have a Benjamin 397 in great condition, and it gets 748 f.p.s. with Premier lites on eight pumps.

So, the 347 I’m testing is a bit weak. However, it’s not as bad as it sounds, because the bolt on the 347 doesn’t have an o-ring sealing it like the 397 does. It might never have been quite as fast when it was fresh, due to a small air loss at the breech upon firing. Not that metal-to-metal seals can’t be absolutely airtight, because they can. But on a high-rate production gun like the 347, the time needed to assure a good seal cannot be taken; while it’s good, it isn’t perfect.

The Benjamin 397 bolt has an o-ring to seal the breech.

The Benjamin 347 bolt seals with an angled metal-to-metal contact. It’s less airtight.

The comprehensive test
Now we’re ready to comprehensively test the subject rifle. There are several different ways to do this, but the one I’ll show you is pretty quick and also very thorough. I’m going to pump the rifle to a different number of pump strokes and record three shots at each level. After I finish the maximum number of strokes, which is eight, I’ll pump the rifle additional times and shoot through the chronograph. After every one of those shots, I’ll cock the gun and fire it to see if any air remained in the gun. When I get to the point that air remains, I’m finished with the test.

2            404, 416, 408
3            488, 489, 485
4            546, 540, 545
5            582, 578, 578
6            609, 614, 616
7            644, 639, 641
8            666, 656, 668
9            678 No air remaining in gun
10          700 Air remaining in gun

The test was stopped at this point, because the gun’s valve cannot handle 10 pump-strokes worth of air. That doesn’t mean I’ll be filling the gun to nine strokes, either. It simply means the gun is a little tired and the valve can handle more air than the eight strokes I’m currently putting in it. But pumping to a higher number of strokes puts more stress on the pump mechanism; so if you want your air rifle to last for decades, don’t exceed the maximum recommendation. However, if you absolutely must have the last foot-second from the gun, then this one needs an overhaul.

Personally, I’ll keep on shooting it as it is, because I don’t need this gun to be a magnum. I have other airguns for that.

Shot string analysis
Let’s look at the shot string and see what we can learn. First, notice what huge jumps in velocity you get when advancing to pump strokes three, four and five. Those large increases start tapering off after five strokes, and the additional strokes only boost the velocity a little. The jump from four strokes to five is about 34 f.p.s.; but from seven strokes to eight, it’s just about 22 f.p.s. We’re stressing the system more for a smaller boost in velocity.

Next, notice how the rifle stabilized and gave very tight velocity variances on pumps three through seven. Apparently, it likes that range of pressures.

After the test
After the shooting was finished, I went back and shot one more shot at each number of pumps to see if the results still agreed with what I got when shooting the strings. What I’m doing here is canceling any bias from the gun heating up.

2            411
3            483
4            539
5            581
6            610
7            642
8            659

Comparing these numbers with what was seen in each of the strings, I’d say the rifle is shooting in the groove and there’s been no heating up from use. However, the first shot on eight pumps differed from the string on eight pumps, so the gun does need a couple shots to warm up in the beginning.

One other test
One final test, and I’m finished testing this rifle. When I bought it from a pawn shop several weeks ago, of course there was no air in it. I immediately put in one pump and have stored it that way ever since. When I started this test, the first thing I did was cock the rifle and shoot it, to see if there was still compressed air inside. And there was! That means it held air for over a week.

For the rest of the time I own this rifle, I’ll test it from time to time to see if it ‘s still holding that pump of air. My Sheridan Blue Streak, which was new in 1978, has been stored with a pump of air in it since new and it still holds air indefinitely. I’ve shot it after storing it for over a year, and there’s still air inside. It’s lost about 75-100 f.p.s. velocity since new, but it still holds air; and that means the valve is still tight. My Crosman model 101 .22-caliber multi-pump rifle, which was built in the 1940s and was overhauled about seven years ago, has held one pump of air for as long as two years, which is as long as I’ve tested it so far.

Do you notice that I only used a single pellet for today’s test, and that I ran the test differently than usual? Multi-pumps are different guns and have different things to watch, so this kind of a velocity test is better-suited to their design quirks.

14 thoughts on “Benjamin 347 multi-pump pneumatic: Part 2

  1. How could a MSP ever heat up enough to affect velocity from pumping? I have always thought that that was another air gun myth. The gun has enough metal to dissipate any heat from pumping ,given the small amount of air taken in at a pump stroke , and the normal cadence used to fill them by pumping by hand with the few storkes it takes to fill them. In freezing cold weather ( like a normal 10-20 degree day here in western NY in mid- winter) I have found that the pump cup may get stiff, but proper lubrication cancels out any effect on practical velocity. When you see hard felt wipers and dried out pump cups it is not from over pumping or pumping fast ,it is because the gun is abused by firing without lube, and lack of maintenance.

    • Robert,

      The gun does become warm. As it does, the seals expand and seal better. You see, they are made from tough synthetics that are rigid at cooler temperatures but become more pliable whyen they get warm. That’s how it works.


      • Although that is partially true ( depends on the seals and their vintage), it is also true that it takes more effort to un-seat a valve stem seal from it’s seat if it is more pliable. Here is what I do before when I get ready to shoot one of my ( 16 at last count)MSPs. I’ll use my oldest a 1959 Sheidan as an example. That gun often sits in my shop and it is very cold if the stoves not going, and it is winter. I first shoot out any air like you do, then I stick my finger tip into the 5 gallon pail of Valvoline hydralic oil that’s always there, and wipe a little on the pump head, and piston/lever connection. I use that oil in equipment that has hydralic drive pumps in them that cost about what a used four barrel set Whiscombe does ,so I don’t worry about it ruining my seals. If I’m inside near my shooting stuff ,I will use pell gun oil or non -detergent 30W. Then I pump it up, and shoot it once without loading it. I think that the first shot will be lower because if you pump it up with a dry pump tube and cup ,as the gun takes in less air. So what I’m saying is, that if you pump up and dry fire the gun once or twice before shooting , it is uniformly lubed and you condition it to the enviroment you will be using it in. I take issue with the idea that a MSP will be less uniform in velocity because they heat up and shoot to different POA , if allowed to let sit for awhile. I maintain that for practical purposes , a velocity drop of 3-15 fps (as I have found in my own velocity testing of shooting MSP in cold weather, or after they have been just pumped up or allowed to sit) doesn’t mean squat. This is the myth I refer to. The idea that a MSP generates enough heat by pumping to cause swings in velocity that make a MSP inferior to a PCP or a break barrel springer in practical accuracy.

        • I remember my Crosman 760 getting warm when I was a kid.

          I think multi-pumps were great in the 1930s, today give me a sprinter, single stroke pneumatic, CO2, PCP, any day >:-)

  2. This is kinda off and on topic. Back in April of 2010 you mentioned that the Benjamin multi-pumps would be your choice for a dependable (survival) gun. You mentioned that there was a seal that could easily be replaced if necessary.

    I know you mentioned that storing the guns pumped is a good way to maintain the seals indefinitely.

    What specific parts/seals would you keep on hand, just in case? I have an order planned with Crosman’s parts department, and want to add some “handy parts to have around” for my 392.


  3. 392er
    Pyramyd has the parts kit you need for $15.95. Just look under accessories and then parts. If your 392 is late model 92 or newer you will use the less expensive kit. I found it very easy to install.

    • One thing to consider when storing spare parts is that they do have a shelf life. My wife is a CNC machinist who builds complex valve assemblies. They throw out their O-rings after a year ,and they are not cheap O-rings either. I also learned from her that plain old petroleum jelly is what they smear on the O-rings to condition them and to preserve them. The spare parts that I have on hand are per served in this matter if not in a sealed package.

  4. B.B., I don’t know about Karate moves, but it sounds like you are getting a workout from pumping that lever. To motivate you in your daily workouts, how about a bit of Anna Kournikova :-)

    Thanks to all for the advice about hearing protection. However, hearing conversations is not an issue with me. I usually go to the range alone. (Clubber Lang: “I live alone; I train alone.”) and I would be glad to resort to sign language. The only concern is to protect the hearing as much as possible. So, I’m all for the biggest muffs out there, and I’m concerned about the second line of defense, the plugs that go inside the ears. I’m thinking that I should try out the densest material out there in contrast to cotton which expands. The related question is whether hearing protection wears out. There is an anecdote of a guy saying that he knew just when his hearing protection gave out while firing a .50 BMG. I’m willing to believe that earplugs can give out from this kind of punishment, but why? My vision is of the fibers in the foam absorbing the sound energy and dissipating it through oscillation. But after a certain point, the elasticity of the fibers gives way, and then the energy gets transmitted to your ear. How does this sound? And what of muffs which have a heck of a lot more foam than earplugs, but work on the same principle? One gunstore employee told me that muffs never wear out unless they are left out in the sun. The sun damaging the fibers and removing elasticity would seem to support my theory. But, hey, it is a gunstore employee and not a particularly credible one at that.

    Duskwight, thanks for the reassurance about the AK74. The more I read about the Arsenal AKs, the more that my mouth waters. It isn’t really a possibility in California. But IF I did, it’s nice to know that my hearing wouldn’t be affected adversely any more than with another high caliber gun. On the range with protection, my hearing is safe. In the military, I guess a combat soldier’s hearing in the long-run is just another casualty of war. But as a related point, the trend towards cut-down full-power .30 caliber rifles with 16 inch barrels I think represents a new assault on one’s hearing. I’m thinking of the Socom II rifle by Springfield and the FN SCAR heavy. Ow. So, Chuck, you motorcycle riders have earplugs. That makes sense judging by the sound of some of those engines. :-) A very good move I think.

    Mike, interesting comments about the Lee-Enfield. So, the regular cartridges, provide more resistance to closing the bolt than the snap caps? That’s odd, since the snap caps have a surprising amount of resistance. But I wonder too if that’s partly what makes the Enfield such a fast action. Knowing the force you need to close the bolt, you really slam it in there with the arm and shoulder. And this is interesting in combination with the tiny front sight blade. At first I found it annoyingly short. But now I’ve gotten used to it, and I’m developing a mania for nailing the front blade on the target and working that bolt as fast as I can. You guys should all get a Lee-Enfield to complete your shooting experience.

    The problem, I’m having now, though, is not with the force needed to close the bolt but with rim jams. The rifle works fine when loaded by hand, but with the charger clip, more often than not, the top round will be jammed behind the one below. For those not familiar with the Lee-Enfield charging system, it is quite a mystery in its own right. The .303 round has a rim, so when you load the charger, the rims alternate overlapping high and low. So, 1, 3, and 5 are flat against the charger and 2 and 4 sit on the rims of the others. But when you push the rounds into the magazine, somehow the overlapping disappears and each round has its rim seated in front of the one below as it must be. How this is done is mechanical genius and beyond my comprehension. However, in my experience the top round often does not make this transition so that its rim is trapped and you cannot work the bolt. In combat, I would think this would be disastrous. I’m hoping the problem is with the snap caps and that with real ammo everything will work. I’ve experienced this difference with my Savage FP which will often jam with the snap caps but is absolutely flawless with real ammo. I don’t understand why this is since the A-Zoom snap caps I use are supposed to be dimensionally exact. The only other factor is the weight. They are much lighter, so maybe this throws off the feeding mechanism. Wulfraed, I got my .303 A Zooms at Midway USA but they are very expensive as are the 7.62X54R.

    Regarding the Weaver hold, I can see how the push-pull might stabilize the backwards push of the recoil, but mechanically it doesn’t seem to correct for muzzle rise which is perhaps the major problem with pistols, and it would also seem to introduce some rotation to the left. One other paradoxical element that has been mentioned is that even though it is described as an “action” stance, under the stress of combat, it is hard to remember all of the nuances without a lot of training, and many find the isosceles more natural. For myself, I just tried them both and did much better with the isosceles.

    On the subject of the air accident in Reno, NV, those pilots aren’t kidding when they say that they are only a millisecond from crashing at any moment. I can’t believe the pilot who crashed made it as far as 74 in that business. I had a similar experience flying my rc Corsair the other day. After countless successful launches, I opened the throttle in the launch position, so that the airplane was straining out of my hand, and launched. But I must have gotten the angle wrong because the plane began heading downwards and I got my launch hand down to the elevator control in the barest instant to save my $300 plane from nosediving into the ground and totaling itself. If it had crashed, $300 is reason to weep, but at least I would have walked away. I think a profound advantage of shooting and rc flying is that you can send projectiles out with great velocity while keeping your feet flat on the ground.


    • Sorry, I didn’t know you were using snap caps in your Enfield. They should approximate the live rounds.
      As to the clips, I have found that even if a rim does get behind the rim of the cartridge under it, a push on the bolt will override the rim of that round. You just have to push a little harder than normal.


    • Matt,

      The kournikova link is hilarious!! Seriously, I never pass an opportunity to see her on video, however, the narrator did nail it when he said “maybe if she put as much into her tennis as she did drilling those Sr. Cit’s, maybe she’d have some titles in her former career! That was , however the first ugly video of her I’ve seen!


      ps, spell check doesn’t even have Kournikova in it. Keeps wanting to spell Kalashnikov!! Ha!

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