Posts Tagged ‘RWS Basic pellets’

Gas attack

by Tom Gaylord, a.k.a. B.B. Pelletier

Today, blog reader Vince continues the saga of converting a steel spring rifle to use a gas spring. We last read about this project in Part 2 of I’ve got gas, where he showed us the pitfalls of making such a conversion to a Gamo breakbarrel. Let’s see how he does the second time around with a Crosman rifle.

If you’d like to write a guest post for this blog, please email us.

by Vince

Back when I tried reworking the Crosman gas spring retainer, I discovered that drilling a straight and properly located hole on a round surface is a bit, well… tedious. And hard to do, at least without the proper drilling jig.

Of course, it would be very expedient to use the spring tube itself as a jig. After all, it’s perfect — as long as I can keep from damaging it, that is — because all the holes are obviously already in the right places. Put the retainer in place, pop in the pin and go at it through the existing bolt hole.

Two minor problems became apparent. First, the hole is too large to properly guide a 1/4-inch bit. Second, the edges of the bit might damage the existing hole in the spring tube. But both problems have an obvious, simple and cheap solution: a bushing.

Bushing for under $1.00 from McMaster-Carr.

I got mine from McMaster-Carr (part #2868T44) for less than 70 cents. If you’re feeling rich, you can probably get an equivalent at Home Depot for about $3.00. The important thing is that it has a 1/4-inch inside diameter, a 5/16-inch outside diameter, and that it be made from brass, bronze or steel. Plastic probably isn’t a good idea.

The process is simple — and THIS time I’m doing it on a Crosman rifle instead of a Gamo. No particular reason, I just wanted to show that it works on the Crosman platform as well. Specifically, this is a Crosman Sierra Pro, but mechanically it’s the same as the other Quest variants.

Crosman action
It looks like a Gamo, talks like a Gamo…

I ran into a bit of a problem sliding out the piston — it seems that the scope stop screw that I identified in this picture was binding the piston. Backing it out one turn solves the problem. As expected, the threaded hole in the Crosman gas spring retainer doesn’t align with the one in the spring tube — just like the Gamo.

The Nitro-Piston gas spring retainer…

…and why it doesn’t work.

So, what we’re gonna do is turn it 90 degrees and drill a hole on the other side.

This is where we have to drill.

See that little ledge sticking into the hole? I’m going to grind it out of the way:

after grinding
Ground a flat spot, just in case.

In retrospect, though, this step may have been unnecessary.

Setting up the jig is about as straightforward as it gets. After installing the gas spring retainer and securing it with the retaining pin, I place the bushing in the hole in the spring tube and start drilling. The steel is pretty soft, so it’s not that difficult.

The bushing sits in the hole and is the jig for drilling. Simple!

But I only drill about half way and for a very good reason. If I keep going like this, I’ll hit something I don’t want to hit. Not a water or gas main, but that retaining pin is very definitely in the path of that drill bit. The solution is to slide the pin almost all the way out (but still engaging the retainer on one side); so when the bit breaks through, the pin won’t be damaged. Drilling the rest of the way thus proves uneventful.

retaining pin out
Don’t want to drill through that pin.

Next comes the tapping — M8x1.25 inches, which is very close to 5/16-inch NC. If you don’t have a metric tap, get one. Do NOT try to make the SAE size work. You’ll regret it if you do! But my old and worn tap steadfastly refuses to start because it wants something a tiny bit bigger than 1/4 inch, so I have to bore out the hole to 17/64 inches. That makes all the difference, and a few minutes later I have a properly tapped hole.

Just a smidgen bigger…

…before I can tap the hole.

A quick test-fit shows that everything goes together just as it should.

test fit

As for the rest of the work, it’s a simple matter of cleaning everything out, lubrication and assembly. If you recall, the gas spring got scratched up from rubbing on the piston in my Gamo 220, so I colored those scratches with a Sharpie. That way, if I wind up with more rubbing in the same place, it’ll be readily apparent.

After a good cleaning, everything goes back together just as I described for the Gamo. But don’t forget that little disc that goes into the retainer.

plate in retainer
I suspect this may be important.

One thing I sort of glossed over last time is how to get that pin installed. Since the gas spring has all of about 1/8 inch of preload, the pin can be started using a screwdriver to pry the retainer into place.

pin install1
Prying the retainer to start the pin.

That’s good for getting the pin started. But you won’t be able to get it the rest of the way through because that spring is still pushing the retainer rearward, and the itty-bit of slop in the whole thing means that the holes won’t quite line up on the other side of the tube.

The solution is easy enough. Once it’s started, tap the pin in until it gets to that point. Then, lay the action on its side with the protruding pin downward, and push down on the spring tube while tapping the retainer with a hammer or mallet. The impact of the hammer will make the retainer jump forward just enough to momentarily line up the holes and allow the pin to start coming through. Three or four taps ought to be enough.

pin install2
Tapping the retainer allows the pin to slide home.

The only minor difference between this Crosman gun and the Gamo is the endcap, which on the Crosman slides inside the tube. It’s a little different from the one that comes installed on the springer:

end caps
The gas spring endcap is on the left, the original on the right.

The gas spring version just slides into the rear after it’s all together, and we’re done!

all done
Don’t forget to tighten the scope stop screw.

The action reinstalls in the stock with no mismatched screw holes.

Shooting it demonstrates the same sort of changes in behavior as with the Gamo I converted, only more so — and less so — all at the same time. For one thing, it runs a little hotter than it did in the Gamo. With the same RWS Basic pellets, it did the following:

991 f.p.s.
997 f.p.s.
1014 f.p.s.
1005 f.p.s.
990 f.p.s.
1007 f.p.s.
992 f.p.s.
1002 f.p.s.
1004 f.p.s.
994 f.p.s.

That’s an average of just about 1000 f.p.s., or 15.5 foot-pounds of energy. This represents an improvement of just about 100 f.p.s. over the original Crosman powerplant.

Firing behavior and feel, however, wasn’t as vastly different as it was in the Gamo. The Crosman “sproings” a fair bit less than its Spanish forebearer (the rear guide tends to be a tighter fit); and with a tarred spring, the smoothness of the firing cycle is pretty close to that of the gas spring.

After I returned the Crosman to its original configuration, I was able to examine the gas spring for any damage. Oddly enough, there was some scratching again but nowhere near as bad as the last time and only on the front 1 inch of the cylinder. So, it’s not related to the centering of the gas spring at all. I suspect the end of the cocking link may have been rubbing it.

I remember a while ago a reader asked about the specs of the gas spring, in particular its pressure. I decided to measure that using my high-precision bathroom scale (!) and a Chinese hydraulic press. This was a quick and dirty way to get a ballpark figure. The pressure was almost constant as it was compressed but not quite. It did creep up just a bit, starting at about 130 lbs. and ending in the vicinity of 150. Overall length of the spring is 10.25 inches with a cylinder diameter of 0.715 inches.

And that pretty much wraps up my gas attack. Exactly where does that leave us?

Well, we’ve shown that the gas spring conversion is certainly doable. It’s not as straightforward as I would have liked — buy a few parts and stick ‘em in — but it’s not beyond the realm of the average handyman. The gas spring itself pretty much lives up to its reputation… smoother, somewhat harder to cock for a somewhat elevated power level. The big mechanical advantages — no coils to break, no degradation from being cocked for long periods of time — are already well-known. The main subjective advantage, the smoothness of the firing cycle, all depends on how bad was it to start with. In a 10-year-old Gamo, the improvement is likely to be rather spectacular (especially in an untuned gun), but if the rifle is already a smooth shooter, less is going to be gained. I guess it just comes down to personal preference — whether it’s worth $50 and a couple hours of your time is up to you.

I’ve got gas: Part 2

by Tom Gaylord, a.k.a. B.B. Pelletier

Part 1

Today, we’ll finish the conversion of a Gamo 220 from steel spring to gas spring, and blog reader Vince gives us a report on the outcome.

If you’d like to write a guest post for this blog, please email us.

Take it away, Vince!

by Vince

When we last saw the Gamo 220, I’d disassembled the powerplant and compared the old parts to the parts I ordered from Crosman. Today, I’ll install those new parts and test the gun for you.

The gun is laying on the bench, ready for assembly. The new piston slides in, followed by the gas spring. Be careful when sliding the piston seal past the end of the cocking slot and tuck the soft seal material away from the sharp edges of the cocking slot so the seal isn’t damaged. A flat-bladed screwdriver works well for this.

Gas spring new piston in
The new piston that works with the gas spring is slid into the spring tube. Notice that I’ve lubricated both ends of the new piston with moly grease.

Gas spring new spring in
The new gas spring (Nitro Piston) slides in after the piston. The small end of the spring fits into the socket inside the new piston I mentioned in Part 1. No lubrication is required.

The trigger and cocking link go back in (reverse order of removal), and the plain plate gets dropped into the rear spring retainer.

Gas spring spring retainer
I’m dropping the plain plate into the rear spring retainer.

Now, I’m starting to sweat a bit. You see, I KNOW that the gas spring has a TON of pressure on it even when fully extended (very much unlike a coil spring) — so, how on earth am I gonna compress it enough to reassemble the gun? Oh, well, I’ll cross that bridge when I come to it –which is, well, right about now. After I install the rear retainer, I notice something.

Gas spring no preload
There’s almost no preload on the gas spring

Almost no preload at all! THAT’S right. Because the gas spring is ALWAYS at or near full pressure, there’s plenty of preload pressure as soon as the piston comes off its stop, so very little preload travel is required.

What is preload?
When a conventional coiled steel mainspring is installed in a spring gun, it’s usually longer than the space into which it must fit. It is, therefore, necessary to compress the spring by some amount to get it to fit inside the spring tube. This compression causes the spring to be under pressure even when at rest — this is called preload. If you’ve ever seen a long, empty flatbed trailer on the interstate that looked bowed up in the center because there’s no weight on it, you’ve seen what no preload looks like. It takes several tons of weight just to get that trailer flat again — and much more to make it bow the other way.

Airgun tuners can add spacers that preload the mainspring even more when it’s resting, which causes it to develop greater power when compressed because it’s closer to its maximum potential that exists at the point when all the coils are touching. But gas springs don’t work that way. They’re under full compression (internal gas pressure) when they’re at rest. All cocking the gun does is move the internal piston against the already-compressed gas that’s ready to blast it back when the sear releases it. There’s a very small amount of additional compression of the gas, but it isn’t what makes the gas spring work as well as it does. The gas spring unit is always at full potential — even at rest.

So, this gas spring unit has very little farther to go at this point…under a quarter-inch, in fact. THIS sure makes things easy for me. Pry the retainer forward on one side while starting the pin through the other. [Note: If I used a mainspring compressor, I wouldn't need to pry anything. I would just tighten the compressor until the assembly pin holes lined up, then insert the large crosspin.]

Gas spring pin install
The crosspin will go in, but the hole for the rear spring retainer bolt (that large-headed bolt I removed when I disassembled the powerplant in Part 1) doesn’t line up with the hole in the spring tube. This is a problem.

Immediately, a problem becomes apparent. Look at the hole where the rear spring retainer bolt goes. It’s not lined up with the hole in the tube. There’s approximately a .080″ misalignment here. This ain’t gonna work. My first inclination is to simply elongate the hole. But when I reinstall it, there’s another problem.

Gas spring trigger relocated
There’s a gap between the plate on the spring retaining bolt and the trigger assembly. It won’t support the trigger this way!

The trigger isn’t properly supported by the plate that’s attached to the bolt. Worse, this changes the spacing between the front and rear stock screws and doesn’t allow the action to be reinstalled.

Hmmm. I’m wondering if this is exactly what Crosman (or BAM) had in mind — preclude an easy conversion with existing parts (since the same problem would exist on a normal Quest). That leaves me thinking: Can I just butt the gas spring against the original Gamo spring retainer?

If you look at the picture of the new rear spring retainer above, you’ll see that there’s a small plate that drops into the cup that retains the gas spring cylinder. The cylinder wants to butt up against a flat surface, and the Gamo retainer has a large (approx. 1/2″) hole in it. I need a metal plate to go over it. Wait a minute! I’ve got one right here in my pocket!

Gas spring quarter
A perfect spacer for the new gas spring and it costs — well, about a quarter!

And, so, it gets reassembled. Believe it or not, the whole thing works.

Time to test!
I’ll run through this pretty quickly — the velocity is now up to about 964 f.p.s., which represents a muzzle energy of about 14.5 ft-lbs. Not killer, but obviously a lot better than the detuned gun. Accuracy shouldn’t be changed — or should it? Oftentimes, guys will detune their guns to make them more accurate — or to simply make them easier to shoot. That might have some merit, as I now couldn’t break 0.37 inches at the same range. Not a big difference, and I’m certainly not gonna suggest that the gas spring decreased accuracy. But I don’t think it helped.

So what’s it like to shoot?

First of all, everything anyone ever said about “thunk” vs. “sproing” is absolutely correct. The gun “wumps” with a gas spring, and you can actually feel a kick back into your shoulder. Nothing like a typical centerfire gun, although maybe something like an 1894 shooting low-velocity .38 specials might be comparable. But that’s just a guess.

Cocking the gun is another matter. Effort peaks at about 33 lbs., which isn’t all that high — except for the fact the effort before that peak is certainly a lot higher than with a normal coil spring. This is what we’d expect, of course, with the relatively constant pressure of the gas spring. It isn’t unbearable, but it does take some getting used to.

Back to a coiled steel mainspring
After about 40-50 rounds, I decided it’s time to restore the gun back to original spec. I rummage around my spring box and find a REAL low-mileage Gamo spring, and put it all back together the way God intended it. NOW, I can really get a back-to-back series of impressions.

First, the velocity did drop a smidgen. It’s now down to an average of about 943 f.p.s., or a little under 14 ft-lbs. Second, and despite the tar on the spring and rear guide, we DEFINITELY are sproinging ourselves rather energetically. Lastly, the cocking effort is predictably much milder. Peak effort is down by 5 lbs., and the effort before that peak is even easier. Accuracy is unchanged from the gas spring.

How did my quarter, er, my impromptu gas spring backing plate pan out? Not too well.

Gas spring quarter 2
The pressure of the gas spring punched a deep divot into the quarter.

Gas spring quarter 3
The flip side doesn’t look any better.

I flattened it back out with a hammer, and I’m really hoping it’s still legal tender. Anyway, as I sort of expected, the relatively soft quarter didn’t do well. The backing plate really ought to be steel, 0.060 inches (1.5mm or 1/16″), just like the original.

But the bigger problem wasn’t the quarter.

Gas spring scrape
There’s a serious indication of metal-to-metal galling.

There was some serious metal-to-metal contact going on here between the cylinder of the gas spring and the inside of the piston. If you look at the above pictures of the quarter, you’ll see that the indent isn’t centered. The pocket in the original rear spring retainer keeps the spring cylinder right in the middle, and apparently that’s real important because it won’t center itself.

And that’s about it for now. If this is going to work, we need a simple and cost-effective way of keeping the gas spring centered properly without permanently altering the original parts…and do it in a way that the average tinkerer can accomplish on his own. The first thing that comes to mind is to drill and tap a new hole in the new rear spring retainer, opposite of and slightly forward of the existing hole. I tried that, and found (predictably) that getting the hole in just the right spot is a bit difficult without a custom drilling jig.

For now, I’m just going to give it some thought.


I’ve got gas! Part 1

by Tom Gaylord, a.k.a. B.B. Pelletier

Today, we’ll have the first part of a¬†guest blog from reader Vince. For those who don’t know him yet, Vince is our “go-to” guy for fixing all sorts of strange vintage airguns. In this post, he tells us the tale of a wild idea he just had to try.

If you’d like to write a guest post for this blog, please email us.

Over to you, Vince!

by Vince

“Nitro” is da bomb, right? I mean, in current usage, “Nitro” anything means hot, fast, powerful and overall bad. This normally benign element sure shows its alter-ego when combined in properly mischievous proportions with oxygen to form nitrous oxide. More fun can be had by mingling it with oxygen, carbon, and hydrogen in various arrangements to come up with nitromethane, nitroglycerin, nitrocellulose or TNT (trinitrotoluene). So, yeah, “Nitro” IS da bomb — in every sense of the word.

Now, when Crosman started producing the Nitro Piston series of air rifles — well, it sends the imagination reeling, doesn’t it? That is, at least, until we come back to reality and realize all they’re doing is using something called a gas spring, which uses pressurized nitrogen to exert pressure on a piston-rod and cylinder assembly. Push the rod in, and it pops right out again, with different degrees of enthusiasm, per the individual design of the particular gas spring.

The advantages of the gas spring in an airgun application are numerous. Because there’s no metal coil spring inside, the gun doesn’t FEEL like there’s a metal coil spring inside. No twang, sproing, buzz or anything else of that nature — and no twisting or torquing reaction as the spring extends and slightly unwinds. Because gas doesn’t fatigue, a properly functioning gas spring will never take a set or get weaker with time. This also means you can leave it cocked for as long as you want with absolutely NO effect on spring life.

What’s not to love?

Well, specifically, those two little words…”properly functioning.” Yes, the gas will never fatigue, but sometimes it leaves home and never comes back. And if the gas spring DOES leak, there’s nothing left to do except go get another one. And almost certainly, it means going back to the manufacturer — and praying that they still have them available.

You see, gas springs are something of a specialty item. They kinda have to be designed around a specific application. There are universal gas springs out there, but the chances of finding one with parameters comparable to your airgun is gonna be difficult. Unlike a coil spring you can’t just get a longer one and cut it to length. You can’t tell what sort of rate a gas spring has (or had) just by measuring things, like you can with a coil spring. I believe you can still get custom coil springs made on a case-by-case basis, but making a gas spring is a bit more involved. Heck, you can make a crude coil spring yourself using a paper clip and a pencil. Sure, it won’t be good for much — but it shows that the basic process for making one is, well, pretty basic.

That’s why I’m not terribly tempted by all these Nitro Piston (of any sort) air rifles that are out there. Will you be able to get it working 50 or 75 years from now? I certainly won’t, cause I’ll probably be gone by then. But that’s beside the point. I’ve got future generations in mind here! I don’t want little Billie- or Betsy-Bob cussin’ out their great granddaddy simply because he bought a gas spring rifle they can’t fix. Heaven knows they’ll probably have enough reason to do that anyway. Why add fuel to the fire?

That’s when I got to thinking. How about gas-springing a spring gun? When the gas stuff craps out, I (or whoever) will easily be able to cram in all the old conventional spring stuff and the gun will be back in business. After examining the Crosman gas guns (for some reason this makes me think of the “fart gun” from Despicable Me), it seemed obvious that they’re based on the ubiquitous Quest platform. That rifle, as we all know (except for those of us who don’t) goes back to the old BAM B18/B19 air rifle. Which, in turn, was a near-clone of the old steel-barreled Gamo series of the time (Shadow, 220, 440, 890, etc.).

Seems to me that a proper pickin’ of replacement parts ought to let ANYONE with a Quest variant (which are as numerous as the stars in the sky) or a Gamo 220 variant to gas ‘em up without giving up long-term serviceability. The best part is that Crosman is generally the most tinkerer-friendly airgun company out there. Not only do they sell parts — ALL parts — for many of their springers, but they’ll sell them to anyone. And they’ll let you keep your first-born, your arms and your legs, and your very soul because their prices are so reasonable.

So it is that I started combing through the parts list for the Storm and the Titan GP air rifles. My suspicion of the close relations was confirmed by Crosman’s designations for these guns — both start with C1K77. I believe C designates the gun family, the 1K means 1000fps, and the 77 points to .177 cal. In any event, the commonality of parts (especially the cocking link, trigger assembly and piston seal) tells me I’m on the right track. Things can get a little tricky, as some parts that are virtually identical might have different part numbers depending on cosmetic details (like lettering). Trusting my own judgment, I came up with a list of 5 parts that I THINK will fill the bill:

Gas spring……………BT9M22-00-5
Plain plate…………….BT9M22-00-7
Back spring guide…..BT9M22-00-2
Tube cover…………..BT9M22-00-3

I ordered the parts and waited. As a side note, I told the customer service rep at Crosman what I was trying to do, and he seemed rather interested in my results. I think I’m gonna just send him a link to this blog. As another side note, there’s some commonality between Crosman’s part numbers (which were revamped a couple of years ago) and Stoeger’s. Of course, both guns are made by BAM in China.

A week or so later, the package arrived!

Gamo gas spring parts
The parts received from Crosman. The camera perspective distorts the gas spring at the hottom. It’s really straight.

Picking a guinea pig is easy. I immediately turn to a tried-and-true, long-time member of my collection: a Gamo 220.

Gamo 220
My Gamo 220 was the guinea pig.

The Gamo 220 is a bit of a mule, frankly. All the 1000 fps Gamos of that period used essentially the same powerplant with a 25mm bore, a 100mm stroke, and the same 29 lbs./in. spring. The Shadow, which was the first decent airgun I ever bought, had a nice-to-hold, if utilitarian, synthetic stock and the low-grade rear sight. The significantly more expensive 440 had a nice-looking wood stock with Gamo’s “better” micro-metric rear sight (which, incidentally, is actually inferior to the lower grade one). The upscale 890 was a sightless 440 that came with a scope.

The 220, price-wise, sat in the middle — with a completely unadorned, mud-brown and slippery wood stock with about as much aesthetic appeal as an old shoe. It lacks the utilitarian friendliness of the Shadow and the visual appeal of the 440/890. It really was the least-appealing of the Gamo magnum breakbarrel lineup. Why do I have one?


As a result, it periodically becomes a test-bed for projects when I don’t want to mar up a NICE gun. Projects like this one.

The first thing I do is baseline the rifle for accuracy and power. In the accuracy department, it didn’t disappoint — a 0.30-inch 5-shot group at 36 feet. That’s about what I remember for the gun. Power-wise, however, was a different kettle of fish. I used RWS Basic pellets for velocity and averaged only 814 fps with a spread of 22 fps. The spread isn’t too bad, but the velocity stinks and represents a measly 10 ft-lbs of power. Oh, well. Frankly, I don’t even remember what guts are in this thing.

The gun comes apart in pretty much standard Gamo/Quest fashion, starting with the three stock-to-action screws. Once the action is out of the stock, you go to the rear of the spring tube, where the big bolt comes out first.

Gamo 220 disassembly
The big bolt comes out first. Then, the pin is drifted out, but only after the end cap is restrained.

The pin is next, but the rear retainer has to be suitably restrained in order to contain the approximately 2 inches and 60 lbs. of mainspring preload typical for these Gamo’s. Or not, as the preload in this gun turned out to be less than an inch. What happened? We’ll find out soon enough.

Next step is removing the trigger, which means the barrel has to be broken open so the trigger can slide backwards a bit.

Gamo 220 cocking linkage
Break the barrel to put slack in the cocking linkage. The slotted bar that runs back to the trigger is the link for the anti-beartrap device.

Then, the cocking link gets pulled down, freeing it from the piston and allowing you to disengage the anti-beartrap link.

Gamo 220 disassembly trigger out
The cocking link can be disengaged from the piston, then the anti-beartrap link disconnects from the cocking linkage.

At this point, the piston just slides out. So — what’s up with that low power and lack of preload?

Gamo 220 powerplant parts

Wow! That mainspring sure is short. And that rear guide AIN’T Gamo.

Oh, yeah — that’s right. I detuned this thing with a Crosman 500X spring. Incidentally, that spring is part #B12-1-00-4A, for anyone who wants a real pleasant detune on a similar Gamo or a Quest variant. That’s why velocity was so low and why I could cock it with my pinkie! That also explains the lack of preload.

But the guide? Near as I can tell, it was a custom guide I picked up somewhere. It’s dimensionally close enough to the normal Gamo parts that I can be sure it isn’t affecting power.

Gamo 220 mainspring guides
The spring guide in my 220 (top) and a regular Gamo spring guide.

As a matter of curiosity, I looked at the two pistons side-by-side.

Gamo gas spring piston
The new piston that goes with the gas spring (bottom) has a slightly shorter stroke than the old piston.

The upper one is the original — and it’s a little shorter, but it appears that, for whatever reason, the gas spring wants a slightly shorter stroke than the coil spring. Also — although it’s hard to tell from the pictures — the inside of the old piston has a flat surface for the top hat, while the newer one has a shallow hole meant to locate the rod end of the gas spring. You will see why this is needed in the next installment.

That’s where we are going to leave this story for now. Vince has the old parts out, and the new parts ready to install. We’ll see what happened in the next installment.

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