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 GearHog.com. 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.
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 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.
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.