What is a top hat? Exploring the guts of a spring gun

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

Last week, I made reference to a heavy top hat affecting performance in a spring gun, and blog reader Joe asked this question:

“You wrote ‘…weighted top hat inside the piston, or the piston itself is heavy. Either way, the rifle should shoot medium and heavyweight pellets better than lightweight pellets.’

What is a ‘top hat’? Why would a heavy piston or top hat shoot medium to heavy pellets better?”

Joe, thank you for asking this question. This blog is now in its 10th year, and I forget that the readership has changed over that time. If one person asks a question, it means that many other readers are wondering the same thing and not writing in. Today, I’d like to review the main parts of a spring-piston airgun powerplant and discuss how they affect performance.

Piston
The piston is the most recognizable part of the powerplant. When the gun fires, it’s propelled forward by some kind of spring. It may be a coiled steel spring or it could be some kind of gas under pressure — whatever the source of energy is, it pushes the piston. And before anyone asks — yes, there have been guns with springs that pull the piston.

The piston compresses air in front of it as it goes forward. There is some kind of seal on the piston that keeps most of the air contained, so it compresses air very well.

spring piston seal detail
The piston seal (this one is a parachute type, whose sides flare out and seal the chamber better) seals the air in front of the piston.

Tuning tip
Removing excess tolerance makes a spring gun shoot smoother. The top tuners put Teflon or nylon bearings (called buttons) around the rear of the piston skirt to hold it steady and away from the spring tube walls. The piston seal does the same thing for the front of the piston. In a well-built airgun, the metal piston should never touch the walls of the spring tube.

Nitro Piston 2 buttons
These small white nylon bearings on the Benjamin Nitro Piston 2 are part of the reason that rifle is so smooth when it fires!

Compression chamber
Let’s talk about something that’s dirt-simple, yet confuses a lot of people — the compression chamber. It’s the end of the spring tube, which has been closed off by a metal cap. A hole through the cap, called the air transfer port, allows the compressed air to move from the compression chamber to the barrel. If there’s a pellet in the barrel, the air cannot get past it and has to shove the pellet out of the way. This doesn’t happen until the air reaches a high pressure, which is at the heart of today’s discussion. Essentially, the pellet stays put until the piston has come to almost a full stop, then the compressed air blows it forward, like the cork from a champagne bottle.

spring piston powerplant
The spring-piston powerplant is so simple that it confuses people.

Spring
We all know what the spring does. It pushes the piston, which compresses the air. A spring can be either coiled steel or compressed gas; but for the remainder of this report, I’ll be talking about a coiled steel spring because it’s the thing that relates to the top hat.

spring inside piston
Thanks to a generous cocking slot, we can see the mainspring inside this piston’s body.

Spring guide
The spring guide does what the name says — it guides the spring. What it actually does is keep the coiled spring from twisting or bending too much as it’s compressed.

spring guide inside spring
This is the spring guide. It’s hollow to allow the piston rod to pass through when the gun is cocked. The metal one is an older Beeman R1 guide, the white plastic (probably nylon) one is what they use more often these days. A lot of vibration can be eliminated by very closely fitting this guide to the inside of the spring.

When the gun is cocked, the piston comes back and compresses the spring. The spring fits up inside the piston, so it’s contained by the piston skirt as the piston comes back over it. The spring guide is a hollow tube at the rear of the spring that the spring fits over. As the spring is compressed, it tries to bend to the side, but the spring guide prevents it. The piston skirt keeps it from bending at the front.

Tuning tips
When a tuner wants to smooth the performance (the shot cycle) of a spring gun, getting rid of excess clearance is a great way to eliminate vibration. The outside of the mainspring is fitted to the inside of the piston skirt very closely. Here’s something you may not have guessed. When the spring is compressed, it also expands just a little (measured in thousandths of an inch), so the internal fit to the piston skirt has to take that into account.

The spring guide is also fitted to the inside of the mainspring. In the best-tuned airguns, the guide cannot be put on the spring unless the spring is twisted against the guide counterclockwise. Airgunsmith Jim Maccari calls that a guide that’s “nailed on.” The mainspring grows in diameter as it’s compressed. When the gun is cocked, the spring loosens a little and slips down the guide.

Top hat (forward spring guide)
Okay — here it is. You had to wade through lot of blather to get to this point, but I hope it’ll be worth it. A top hat is a spring guide that is on the front side (the piston side) of the spring. It fits inside the piston and slides on the central piston rod. Obviously, the mainspring fits over this guide, too, just like it fits the guide in the rear.

top hats and spring
Two top hats (forward spring guides). The heavier one at the bottom was made by airgunsmith Jim Maccari for a special Beeman R1 tune that worked for heavier pellets. I wrote about it in the “Beeman R1” book.

Tuning tip
Joe — this is the answer to your question. A top hat adds weight to the piston. A heavy top hat adds a lot of weight.

When the piston compresses the air in the chamber, it can reach very high pressure before the pellet starts moving. When the pellet begins to move, the air pressure drops rapidly. Although the pressure is very high, there’s not a lot of it — pressurized air.

If the pressure in the chamber is too high and the pellet hasn’t started moving, either because of its size, weight or both, the piston can rebound off the compressed air cushion and travel backwards a few hundredths of an inch before the pellet starts to move. Heavier pellets will resist moving longer than lighter pellets — I hope that’s obvious.

By adding a lot more weight to the piston, it can resist rebounding to a greater degree (because of its greater inertia). Heavier pistons usually do better with heavier pellets. And top hats are one way to make pistons heavier. That’s why I said what I did.

Whew!
That was a quick and dirty look at the spring-piston powerplant and some of its subtleties. Don’t think this makes you an expert — there’s a whole world of things like this that I haven’t addressed.


Air Arms TX200 Mk III air rifle: Part 1

by B.B. Pelletier


Air Arms TX200 Mark III air rifle is impressive in its optional walnut stock.

I’ve reviewed this rifle before, but it’s been a long time and many of you are asking about it again, plus I’m going to look at the Benjamin MAV 77 later this year, and I promised a comparison with this rifle. So, for those reasons, I decided that it’s time to look at the Air Arms TX200 Mark III, again.

Some of you may know that Bill Sanders, the managing director of Air Arms, passed away recently. Bill was very uncharacteristically enthusiastic about all the guns he made. I say that because most principals in this industry are not shooters, nor do they own the guns they make. But Bill did, and he also knew how to use them. Maybe that’s why, in the more than 20 years the TX has been around, the quality has only gone up.

History
The TX200 came about in the late 1980s as an improvement on the design of the HW77, which was considered the best spring rifle around at the time. The first model was simply called the TX200. But after several years, Air Arms added a ratcheting catch to hold the sliding compression chamber from slamming closed during loading. That rifle was called the Mark II. I bought one and competed with it in field target for a couple of years, until I switched over to a PCP. My rifle was tuned first by Jim Maccari and then by Ken Reeves so I could write about each of the tunes. In truth, the TX was pretty smooth right out of the box, but the Reeves tune did make it just a bit smoother.

When the TX200 Mark III came out, I bought one to test for The Airgun Letter. I found that rifle to be just as smooth as the Reeves-tuned Mark II, plus it had a shrouded 9-inch barrel, which made it very quiet, to boot. I didn’t need two perfect guns, and the Mark II was sold. I still have the Mark III, which is the gun I’m testing for you here.

I recently had the opportunity to shoot a brand-new Mark III, and I see that the performance and looks of the gun are unchanged, except for better checkering on the new model. Instead of diamonds, they now have a fish-scale pattern that usually comes only on very costly guns.

Hump-backed look for high-tech design
When Beeman Precision Airguns started selling TX200 air rifles in the U.S., the first thing I noticed was that the rifle had a definite hump-backed profile. Why? Remember I said the barrel is 9 inches long? Guess what? All the science you have been reading about on this blog really works. And Air Arms applied it to its maximum in the TX200.

They put the center of the barrel in the center of the compression chamber, so the air transfer port aligns with the bore. That gives the most efficient airflow, but it also means the barrel, which is a smaller diameter than the spring tube, has to be mounted lower than the top of the spring tube. Hence, the hump-backed profile. Study the first photo, and you’ll see what I mean. Look at the place where the barrel connects to the spring tube. On most other guns, they’re level.

A 9-inch barrel prevents friction from slowing the pellet after it’s accelerated to maximum velocity. A spring-piston gun develops maximum velocity in the first 6-9 inches of barrel. After that, the pellet is just coasting. The baffled shroud that houses the short barrel is much longer and gives the appearance of a bull barrel, hence the barrel length is often listed as longer than it really is.

Air Arms has used everything that’s known about spring-piston guns to wring the maximum performance from a relatively short stroke and small piston bore. They do it without fanfare, but anyone who works on spring guns knows what they’ve done.

The rifle
The TX200 Mark III is an underlever spring-piston rifle that has a sliding compression chamber. The chamber slides back, giving access to the rear of the barrel for loading. Then it slides forward again, once the anti-beartrap latch is held down. The old Mark II has many stops in the ratchet, causing it to click loudly when cocked. Shooters objected to that noise. The Mark III has just three notches and is much quieter.

All metal parts, except the trigger and safety button, are highly polished and deeply blued, resulting in one of the finest finishes in the airgun world. The standard stock is beech, but the wood is shaped very sharply for either a right- or left-handed shooters. No compromise here. Fish-scale checkering roughens both grip panels and the forearm. The optional walnut stock is a good choice because it subtracts weight from the gun as well as adding interest. Blog reader Jerry got a walnut stock on his TX, and it looks very similar to the rifle pictured above.


The stock is sharply profiled with crisp edges and a classic shape. The standard beech stock is shown.


The long lever, located behind the silver sliding compression chamber, is the beartrap release. After cocking, this lever is held down to close the sliding chamber, as the cocking lever returns to the stored position.

The trigger
The TX trigger is not just an improved Rekord, it’s a new design that offers greater flexibility when adjusting, so you can get the pull weight and release down to a finer, lighter value than with a standard Rekord.


The TX trigger closely resembles the Rekord, but it’s simpler and more adjustable. The larger parts are machined.

Years ago when Ivan Hancock was still building airguns, I bought one of his Mach II trigger, which are handmade copies of the TX trigger to replace the Rekord unit in my Beeman R1. That trigger cost half as much as the entire rifle, but it was very finely adjustable. The current trigger in my TX200 is the standard one that comes with the rifle, yet it’s just as fine. When I report on its performance, I think you’ll be surprised.

Several “truths” negated
The success of the TX200 reminds me of a friend who built engines for Formula Vee racing. Those cars look like Formula 1 racers, but they’re much slower. However, this builder’s engines were always in demand because they out-performed the others. Everybody was always looking for his “secret.” The secret, of course, was that there was no secret! What he did was pay scrupulous attention to detail when building his engines. All parts were balanced to the last gram, and all tolerances and torque specifications were followed. The engines were what racers refer to as blueprinted, and that, alone, gave them their edge.

Well, you may think of a TX200 as an air rifle that’s been blueprinted. The piston isn’t wide and the stroke isn’t long, yet the rifle develops remarkable velocity. The trigger appears dirt-simple, yet it can out-perform so-called “target” triggers in much more expensive guns. The mainspring isn’t under much pre-tension, yet the rifle doesn’t buzz when it shoots. Everything is just right.

Rolls Royce is the standard by which all cars are compared, and the TX200 is the standard for spring-piston air rifles. Even when the Whiscombe was being made, I used to say that the TX200 was its equal for accuracy.

It’s heavy!
Yes, this air rifle is heavy. Especially the model with the beech stock. But for its purpose, which is field target first and hunting second, the weight is ideal because it promotes stability.

It’s hard to cock!
It will seem hard to cock a TX if you’re used to a smaller rifle like a Diana 27. But compared to the current magnums, the TX cocks easily. How it feels depends on your experience. I’ll publish the cocking effort of mine in Part 2.

Accuracy
It has been several years since I shot my TX, so this is a chance to get behind the trigger, again. I expect to find a pellet that will give around 1/4- to 1/3-inch 10-shot groups at 25 yards. That’s a tall order for any spring gun, but we shall see!


The HW 55CM target rifle: Part 4

by B.B. Pelletier

Part 1
Part 2
Part 3


Is this Custom Match the best HW 55 ever made? Read the report to find out.

In the last report, I tuned the rifle and got rid of the objectionable firing cycle. It’s always a great pleasure to return to a classic air rifle like this one after testing so many modern airguns, because these oldies are so reserved and well-behaved. I know it’s not going to kick, roar and fight me at every turn. It may only be suited to shoot 10-meter target, but sometimes — and by that I really mean often — that’s exactly what I need.

Sight-in
I had to remove the sights during the tuneup, so the rifle needed to be sighted-in again. It wasn’t that far off, but the indices are so dark on a 55 rear sight and my eyes are so bad that I had to play around until I discovered which way to adjust the sight to go right. In this respect, a modern 10-meter rifle has it all over a vintage one.

The first pellet I tried was the H&N Finale Match Rifle pellet. I haven’t had a lot of recent success with this pellet in target rifles, but in the past this was one of two to contend with — the RWS R-10 Match Pistol pellet being the other. This time was different, though. Although the first group wasn’t what I wanted, it showed enough promise that I shot a second and a third. By the third group, I could tell this pellet likes this rifle.


Not bad! Five H&N Finale Match Rifle pellets went into this group at 10 meters. Any one of them could be a 10 if the sights were adjusted.

Next, I tried the RWS R-10 Match Pistol pellet, and I gave it the same number of chances, but it never showed me anything. That was a surprise, because I think this pellet is one of the more accurate pellets in several of my other 10-meter rifles.


Five RWS R-10 Match Pistol pellets made this nice round group. This is impressive to anyone but a veteran target shooter, who would see that it’s about twice the size it needs to be.

Following the R-10, I tried the RWS Hobby pellet, because in my HW 55 SF — the 55 that has no barrel lock — Hobbys do surprisingly well. Again, there was no joy this time. I’m showing the group to contrast with the others in this report.


RWS Hobby pellets are just not right for the HW 55CM.

At this point, I was satisfied that this rifle is accurate, though it won’t give an FWB 300S any competition. But why stop there?

I next mistakenly loaded some obsolete and nondescript European diabolos that I mistook for JSB S-100 competition pellets. Boy! If you ever wanted to see a comparison between good pellets and cheap ones in a good gun, this was it! How about a three-quarter-inch five-shot group?

Back to serious ammo. The next pellet I tried was the H&N Match Pistol. This is not a Finale Match pellet, and I find that these sometimes vary in weight a lot more than Finales tend to, but there can be surprises. Not this time, though. The best group looked like Hobbys. Oh, well!

After that, I tried H&N Match Rifle pellets. They’re the same as Match Pistol, only heavier. But for some reason that nobody understands but everybody believes, they shot great! These are the pellets for this rifle — until I find something better.


Now, this is what we’re after! Five H&N Match Rifle pellets are obviously working very well in this rifle. This would be the pellet to stick to until a better one comes along.

Overall evaluation
The rifle is shooting fine with the new tune. I could live with less power, but what I have isn’t bad. The trigger is a joy, because it breaks at just 7 oz., and that’s as light as I need it to be. Shooting from a bench in the rested position doesn’t give you the full feeling of the rifle. All it shows is the potential for accuracy, and this one’s got it.


The HW 55CM target rifle: Part 3

by B.B. Pelletier


Is this Custom Match the best HW 55 ever made? Read the report to find out.

Part 2
Part 1

This is a special Part 3 for the HW 55 Custom Match target rifle. It will be a retest of velocity, following a strip-down and lubrication with black tar to get rid of some uncomfortable vibration when the gun fires. When I tested it for velocity in Part 2, I discovered the rifle was shooting way too fast for an HW 55. Probably the Beeman Company replaced the mainspring when it went back to them for an overhaul. At any rate, when RWS Hobby pellets average 694 f.p.s., as they did for this rifle, you know something is wrong. I’ll try to remove as much of the harshness as I can with this tune, and I really don’t care how much velocity is lost.

A word about the Part 2 report is needed here. I combined it with Part 2 of the report on Mac’s Diana model 60 target rifle because I don’t want to crowd the blog with too many reports about vintage air rifles. Since the velocity report goes pretty quick, I just put the two of them together. But, today, the 55 CM gets its own report, because as well as testing velocity I’ll be disassembling the rifle and applying a tune. There will be some observations for that, as well as the velocity results afterward, so this work rates a report of its own. Of course, there will still be a Part 4 accuracy test to come.

Disassembly
There were no real surprises when disassembling the rifle, except to find a very canted mainspring. That was where the vibration came from — of that there can be no doubt. I rooted around in my collection of replacement mainsprings and found one that Jim Maccari made for a TX 200! Talk about inappropriate for an HW 55 — but the dimensions of this spring were so great that I had to try it.

I discovered that the trigger was still coated with a drying, tacky layer of factory “tractor grease.” I kidded Hans Weichrauch, Jr., about this years ago and he had no comeback. From his perspective, the grease is always fresh and new. I removed everything I could from the trigger but expected no change in performance. This was more of a conservation step than a restorative one.

The new spring was very liberally buttered with black tar, and the rifle was assembled once more. However, when I cocked it the first time, I knew that wasn’t the solution. The cocking effort started out light but quickly stacked until I was pulling back around 30 lbs. of effort. That’s way too much for a 55 target rifle.

On the plus side, I probably added at least another 50-75 f.p.s. to the velocity. But, with a target rifle, who needs velocity?

So, once more, the action came apart. This time I used a spring that had very similar dimensions to the one that came from the gun — only this one is straight. It got buttered with tar, too and then everything went back together.

How does the rifle feel?
The rifle now requires 26 lbs. of force to cock, compared to the 20 lbs. before — so that part isn’t good. The firing cycle, however, feels lighter and much quicker. Gone is the objectionable vibration that came from the canted mainspring. This rifle will now be easier to shoot accurately.

Velocity
The first pellet tested was the RWS Hobby that was such a speed demon with the old tune. Back then, the rifle averaged a blistering 694 f.p.s. with this pellet. That’s way too fast and does nothing for the potential accuracy. The extreme spread was 17 foot-seconds with that pellet and tune.

With the new tune, Hobbys average 603 f.p.s., ranging from 602 to 610 f.p.s. for an 8 foot-second spread. That’s more like what I wanted, and maybe even on the low side of what I was looking for. But with that tight spread, I know the rifle is doing well with this tune.

Next, I tried H&N Finale Match Pistol pellets. The old tune gave an average 632 f.p.s. velocity with a 14 foot-second spread.

The new tune gives an average of 573 f.p.s. with a 12 foot-second spread that runs from 567 to 579 f.p.s. The spread is pretty close to what we had before, but the velocity is now down where I expect it to be.

The last pellet I tested was the RWS R-10 Match Pistol pellet. With the old tune, the pellet averaged 632 f.p.s. with an 18 foot-second spread that went from 619 to 637 f.p.s.

The new tune sends this pellet downrange at an average 565 f.p.s. with the total velocity spread that runs from 560 to 567. Only 7 f.p.s. separates the slowest shot from the fastest.

Firing behavior
The rifle now seems much more calm and settled when it fires. I can’t be sure until I shoot for the record, but I think I’ve tamed the beast.

Am I satisfied with this tune? Yes, except for the extra cocking effort. An HW 55 should cock at around 15 lbs. of force, and this one takes 26 lbs. That’s heavy, even though it doesn’t set off any alarms. I would still like to get it back under 20 lbs., but I’m not going to hold up the show just for that.

Accuracy testing is next, and then we’ll have complete tests for five popular 10-meter spring-piston target rifles: the HW 55 SF, FWB 150Diana model 60Walther LGV Olympia and this HW 55 CM. Guns I haven’t yet tested (that I own and have access to) are the FWB 300S and the Haenel 311.