Stroking the B3-3

In my last post, I mentioned installing a shorter piston in my B3-3 to increase power. I was originally going to use a piston from a B4-2, but that turned out to be too small. Looks like I’m going to have to modify my original piston.

Piston air rifles are similar to car engines. Common techniques to improve power with car engines are boring and stroking. Boring involves making the cylinders a tad larger. With multiple cylinders, this can really add up. Our rifle has a single cylinder and not enough meat to remove much. Stroking is going to be our best bet. More air will be ahead of the piston and the piston will generate more speed and momentum, both increasing power.

A shorter piston gives a longer power stroke. The original stroke length was only 58mm, much too short to produce any real power. In fact, it produces 7.73 fpe at the muzzle with RWS Superpoints. Let’s see if we can shorten the piston, increasing the stroke, and gain some power…

I noticed that the head of the piston was attached to the skirt by means of pins. I drilled those out and expanded the skirt by hammering a wedge down the back seam. With a bit of prying and wiggling, the head popped free from the skirt:

Time for modification
Time for modification

Take a close look at the skirt on the right. You’ll see a collar that fits in the rear groove of the piston. I cut the skirt right behind that collar. The head was then placed back into the skirt. I placed the whole assembly on a small anvil and hammered the skirt carefully around the head. Here’s what it looked like at that point:

Cut down piston by the compression chamber
Cut down piston by the compression chamber

To attach the head to the skirt, I drilled holes in the skirt above the rear groove in the head. I then welded them together, concentrating the heat into the head, and letting the puddle bond with the skirt. The same technique was used on the top and bottom where the seam and cocking slot exposed the head.  In this way, the head and skirt have become one nearly seamless piece, much stronger than the original pinned design.

I then rounded the piston on a belt sander. It fits the compression chamber beautifully. Several holes were drilled along the body of the skirt to further lighten it. The cocking slot was ground into the head to give the necessary clearance for the longer stroke. Here’s what it is looking like:

Nearly completed piston
Nearly completed piston

Ignore the plastic on the rear for now. I cleaned everything up and added a light coat of lube. There was some minor adjustments I had to make to the cocking linkage. To test the fit and function, I installed the spring with almost no preload. The results were immediately impressive. The RWS Superpoints went from averaging 490 fps with the spring nearly coilbound to 525 fps with almost no preload. This is an increase of over a foot pound, with a huge decrease in cocking effort. Stroke length went from 58 mm to 70 mm.

Of course, I’m not content with that. I calculated a potential muzzle velocity of 588 fps and began increasing spring preload in an attempt to get there. I got to 545 fps and then ran into a bit of trouble. There was too much play between the rear of the skirt and the body of the action. Under heavier spring tension, the sear would not stay engaged, and the rifle would attempt to fire as the cocking lever was brought forward. Remember that plastic in the last picture? That was a lame attempt to fix that issue. It fixed that issue, but also acts as a brake.

Moral of the story – always remember these rifles are known as finger chompers! Keep your fingers out of the breech unless you have a firm grip on the cocking arm with your other hand. When returning the lever, make sure your hands are not between the lever and barrel. I observe these rules and my fingers and hands are intact.

To fix this issue, I’m going to attempt heat forming HDPE plastic to fashion buttons. This will provide a low friction surface between the skirt and action wall, while keeping everything tight and centered.

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Been shooting and experimenting

The weather has been nicer lately, so I’ve been doing a good bit of shooting as well as experimenting. I came across my old Hercules B-4-2 air rifle. It is in terrible shape,  but has usable parts. I plan to cut, shorten, and reweld the piston to add some swept volume to my B3-3. Perhaps I can get it up to 550 fps?

Old B-4-2 Piston
Old B-4-2 Piston. Shortened a bit, time to weld for safety.

Of course power is nothing without accuracy. In my never ending quest for spring piston accuracy, I’ve decided to soft bed my B3-3 with Sorbothane. Man, that stuff is expensive! I got a square foot of it, enough to bed all my rifles and then some. Of course, it arrived just before dark, on the night before my workweek starts. Even so, I’ve got the bedding job done and did a little outdoor plinking. The initial results are promising, and I’ll be sure to detail it all soon.

Soft bedding
Soft bedding my B3-3 with Sorbothane disks.

A lot less vibration is felt and heard though the stock. I did some 30 to 60 yard plinking on clay pigeons using a firmer rifle type hold with very pleasing results. I’ll put it on paper this upcoming week and see how it does. I’m hoping that Sorbothane will make my spring pistons less hold sensitive.

In spite of having higher quality guns as well as more powerful guns, it is funny how I always end up back at the old B3-3. I’m very tempted to pick up another in .177 to balance my collection.

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Springers and hold sensitivity

Occasionally you’ll hear someone say that air rifles aren’t accurate. Perhaps that’s what brought you to this page. Most of these complaints are the result of magnum spring or gas piston air rifles being mass marketed without informing the consumer that they cannot be held like traditional rifles and shoot accurately.

Here are two 6 shot groups I shot indoors (it’s cold out) at 8 yards:

Rifle vs. Artillery Hold
Rifle vs. Artillery Hold

The rifle I used was my B3-3. It is nice and quiet for indoor shooting.

For the test, I removed the bipod and shot from a sitting position. The top group was shot using a traditional firm rifle hold. The bottom group was shot using the artillery hold. As you can tell, it is pellet on top of pellet. Keeping in mind this was at 8 yards, imagine what it would look like at 20.

So what is this artillery hold? It is named after the way that artillery piece recoils within its carriage:

Most spring and gas piston air rifles prefer to rest on your open palm at their balance point on their forearm. The buttstock is lightly shouldered. Breathing and trigger control is the same as any shooting discipline. When you squeeze off your shot, remember the artillery piece in the video. You are the carriage. Let the air rifle recoil freely.

Be sure to rest the gun on your hand at the same place each time. Try to apply the same pressure against your shoulder as well. Variations of this technique might be required to find the optimum hold for your rifle. Some will prefer to rest ahead or behind the balance point. Others like more pressure against the shoulder. The key to shooting accurately is finding what your gun likes and being consistent with that hold.

If all this hasn’t turned you away from shooting springers, I have a few more bits of advice for you. Don’t make your purchase based off the advertised speed. Instead, start with a lower powered rifle. High powered rifles tend to recoil violently and are often very hold sensitive.

Start shooting at 10 yards. When your groups start shrinking to your liking, back it up 5 yards. Continue that process until you are shooting good groups at your maximum range.

Don’t set unrealistic expectations. Most springers are rough around the edges, filled with combustible lubricants, and often have faulty seals right out of the box. You aren’t going to be stacking pellets at 100 yards. Shooting squirrel head groups at 30 yards is reasonable.

Be sure your stock screws as well as scope mounting are secure. Don’t be afraid to use some thread adhesive to keep them in place.

Experiment with pellets. This applies to pretty much any kind of air rifle. Some pellets will shoot better than others. It might take a few varieties to find the best ones for your gun.

I hope that this information gets you shooting nice groups with your springers! Springers are not for everyone, so don’t write off airguns if you don’t have the time or patience to master them. PCPs, single stroke, and multistroke pneumatic air rifles might be better options for you.

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