Posts Tagged ‘Slavia 631’

New BKL mount adjusts for barrel droop: Part 2

by B.B. Pelletier

Part 1

It’s been half a year since I did Part 1 of this report. I always meant to do today’s test, but other things seemed to crop up every time I was ready. I did make an excursion in another direction to test BSA’s 2×20 pistol scope using another mount on the Beeman P1 air pistol. Such is the tangled life of the airgun blogger!

New territory!
Today’s report takes me into fresh territory with my Slavia 631 breakbarrel rifle. I had earmarked it as a testbed rifle for testing the accuracy of lead-free pellets a long time ago, but the lack of a scope mount caused me to substitute the Whiscombe rifle at the last minute. You see, the Slavia air rifles all share a common problem when it comes to mounting scopes. They have dovetails that are among the very widest on the market. Most 11mm scope mounts will not expand wide enough to fit the 14mm dovetails (they are still called 11mm, which creates a world of confusion among buyers who try to scope their rifles) that are standard on all Slavia breakbarrels. Even for me — with a drawerful of specialized airgun mounts and prototypes — the Slavia remained a gun I could not scope until this new BKL mount hit the market.

Those dovetail grooves may be called 11mm, but they’re really 14mm apart. And that makes a huge difference. Almost no scope mounts will open that wide. Those three scalloped notches are for a specific type of scope stop that no longer exists in the U.S.

I’ve owned this 631 since back in the 1990s when I was still writing The Airgun Letter. I got it from Compasseco (now owned by Pyramyd Air) for a test and liked it so much I decided to keep it. Over the years, I’ve used it for other tests, such as testing the accuracy and penetration of round lead balls; but these tests were done with open sights. Today, I get to discover for the first time how the rifle shoots when a scope is mounted.

Just fit!
The BKL adjustable mount is a one-piece mount that just fits the length of the scope grooves on the 631. There isn’t a millimeter to spare on either end. As for the width, the fit is much easier, though I did have to spread the clamping rails to get it on the gun. For those who are unfamiliar with BKL mounts, they hold onto the airgun by clamping pressure, alone — there are no mechanical scope stops on any BKL mount. It’s often necessary to spread the mount base a little to get it onto the dovetails of the rifle. BKL has designed an ingenious way of doing this with the base screws applying reverse pressure to spread the base “jaws” just the right amount. It’s easy to do and takes only a minute or two extra. Once the mount is on the gun and the base screws are tightened, you have a scope mount that’s not going to move under recoil, no matter how severe.

The second great thing about this new mount is that the rear scope ring elevates to compensate for barrel droop. Newer readers may wonder what droop is, so allow me to explain

The BKL is mounted on my Slavia 631 rifle. The mount is silver because it’s an unfinished preproduction model, not because it’s finished that way.

Barrel droop
Breakbarrel springers are notorious for having barrels that are angled downward from the sight plane. Because the manufacturers mount both the front and rear sight on the barrel, they remain in a fixed relationship that masks the droop or downward slant of the barrel. When you install a scope, it goes on the spring tube and the barrel droop becomes painfully obvious. You adjust the scope up as far as it will go to bring the strike of the round back up to the intersection of the crosshairs. Sometimes, you just barely get there, but other times you can’t even get that high before running out of adjustment. Either way, when a scope is adjusted all the way up as high as it will go, the internal springs relax and the point of aim starts moving all over the place. New shooters blame this on scope shift, but it’s really a different problem that’s completely correctable

You want to mount the scope in such a way that its vertical adjustment is about in the middle of the range or even closer to the low end. That’s where the droop-compensation scope mount, or “drooper” as it’s called, comes into play. With a droop-compensation scope mount you can slant the scope downward so it follows the line of the bore more closely.

And this new BKL is a drooper mount! But until I tried to sight in my Slavia 631, I had no way of knowing that it’s a breakbarrel with a droop problem. Once I confirmed that it is, I adjusted the rear of the BKL mount upward and got the scope dead-on at 25 yards! It took only one adjustment, and I had the scope back into the middle of its adjustment range again. Now, it was time to see how this rifle shot.

This is going to be a longer report, so I’m cutting to the chase right away. When I started shooting the 631 at 25 yards, I discovered that this rifle is twitchy. What does that mean? Well, if a breakbarrel is very powerful, it’s usually extremely difficult to hold for accuracy. It wants to spray its pellets all over the place — that’s what I call twitchy.

But lower-powered breakbarrel springers like this 631 aren’t usually twitchy. Usually, they lob all their shots to the same place. They’re also very tolerant of different types of pellets. But my Slavia 631 is none of those things. It’s twitchy. Allow me to show you what I mean. The first group I tried to shoot was with the Air Arms Falcon pellet.

This first target shot with Falcon pellets revealed a lot about the gun. Do you see that two pellets are close together in each of the three groups, but the point of impact moves? That’s due to very small changes in the hold. Four of the 10 pellets missed the target altogether!

The first group I attempted told me this rifle is twitchy. But sometimes that’s only with a couple pellets, so I pressed on.

Next, I tried shooting RWS Hobby pellets. They did better and were less twitchy but were not really that good.

Ten Hobbys went into a real group at 25 yards. It looks like only 6 shots landed because several went through the same holes. This is a better group, measuring 0.73 inches between centers, but it’s still not great.

I had to use every bit of technique, short of a scope level, to get that group. The differing points of impact were obviously the result of very subtle changes in the hold. This was obvious to me as I shot, because I was able to feel where the pellets wanted to go. But in spite of that, I did my best to shoot the tightest group I could.

I tried Crosman Premier lites next, but they were all over the place. Then, I tried the JSB Exact RS pellet that often proves best in rifles of this power level. This time, though, they were too hold-sensitive to do well.

Finally, I tried the BSA Wolverine pellet that’s also a medium weight JSB but is subtly different from the others of the same weight (8.44 grains). Like the Hobbys, I got a group of 10; but like the others, it’s interesting for being more of a cluster of several smaller groups.

Ten BSA Wolverine pellets gave this group, which measures 0.75 inches across. There’s a cluster of 6 in one hole, then 4 others below. The fourth shot lies between the two that are stacked vertically.

Bottom line
The BKL adjustable scope mount works as advertised. It’s easy to install and to adjust. And it has jaws that are wide enough for the widest 11mm air rifle dovetails. Just don’t try to use it on a Weaver base, because it isn’t that wide, nor is it configured for the proprietary shape of a Weaver dovetail. This mount is one elegant solution for a drooper.

The Slavia 631 is a twitchy breakbarrel that shoots at a mild level of power. If I hadn’t done this test, I never would have guessed that from the muzzle velocity, alone. That made me think of another report I can write — and probably should: What to do with a twitchy breakbarrel. It would be a collection of the tricks and techniques I would use when I encounter a twitchy breakbarrel. In my role as an airgun tester, I see a lot of them over time, so I’ve built up a bag of techniques I employ to deal with them when one comes along.

The 631 is also a great potential testbed for an adjustable muzzle weight to be used for tuning the harmonics of a spring gun. I’ll look into that.

Testing the Slavia 631: Part 2

by B.B. Pelletier

Part 1

This is going to get a little confusing, because I’m changing things in midstream. Part 1 of this report was titled Testing the Slavia 631 with non-lead pellets, because that was what I thought I was going to do. Instead, though, my Slavia 631 needed attention, and, while trying to fix it, I broke it. I tested the non-lead pellets with my FWB 150 and found them to be so accurate that a whole other test was born. I haven’t done that test yet, but today I’m getting back to the Slavia, which has recently been repaired and returned to service. There’s enough of a story in just fixing the gun, that I thought I would make a report out of it, plus I want to use the Slavia as a testbed for other things in the future and I needed to establish it as a working airgun again.

Confused? I know I am.

What happened
When I used the 631 to test the velocities of the various pellets that would be used in the test, I noticed the rifle was very hard to cock. I believe it took 35 lbs. to cock the gun, which is way out of profile for that rifle. I thought I’d do a quick lube job and see if things would return to normal. Well, they did, but not entirely.

After lubrication, the cocking effort dropped to 21 lbs., which is about where I expected it to be; but while assembling the gun, I had difficulty getting the trigger to work right. Then, during the final disassembly, I lost the small coiled spring that fits between the trigger blade and the sear. The sear is held on a pin inside the end cap, and this spring that fits into a hole in the end of the part helps hold it in place for the trigger to act on it at the right time.

The spring I lost is about half the diameter of a ballpoint pen spring, so it’s really tiny. Nothing in my small collection of spare parts was close. I inquired of several places, but nobody had a replacement. Then, while reading a blog about the disassembly of the 631 on Another Airgun Blog, I discovered there is a second spring that acts as the trigger return spring that I had never even seen. So I thought I had lost that one as well. I didn’t, but the way the blog was written, there was no clue as to where this spring fit or where I should look for it.

So, I switched test guns over to the FWB 150, which proved very serendipitous, because I discovered that non-lead pellets can indeed be accurate under just the right circumstances. But that left me with this broken classic 631 on my hands.

I remembered that I’d purchased the rifle from Compasseco, so I contacted Eric Munson, the son of the former owner, to see if he still had any Slavia parts. He didn’t, because Compasseco had gotten rid of them years ago when they stopped carrying the Slavia line. Then, I asked the right question. Did he have any old broken 630 or 631 rifles laying around? He did, because airgun dealers frequently have piles of guns they never fixed for one reason or another.

To cut to the chase, Eric sent me an old broken 631 that had been cocked for many years, and I was glad to get it. It had the tiny spring I needed, but it had something even more important. In that rifle, I spotted the trigger return spring that was not completely described in the blog I’d read. It’s a permanent part of the plastic triggerguard and stays inside the stock when the action is removed. Sure enough, when I looked inside my good rifle’s stock, there was the spring I thought I’d lost.

Trigger adjustment screw (second from left) is key to the assembly of the rifle.

Looking down through the trigger slot in the stock, you can see the trigger return spring that’s captive in the triggerguard. What appears as a square hole in this photo is actually a slot the trigger blade must fit through. There are two flat steel parts that together look like one in this view.

This view shows the trigger return spring better, and you can see how the trigger blade passes through the slot in the spring. Note the screw threads on the left that are part of the trigger-pull adjustment system. These are the key parts that must be assembled correctly or the rifle will not function.

Now, I’ll describe how a Slavia 630/631 is disassembled so the owners who have never done it will be able to follow my description and not make the same mistakes I did. This will not be a astep-by-step set of instructions because, in my opinion, the Slavia 630/631 is not a rifle for the beginner to work on. If you can understand what I’m showing you and telling you, you’ll be able to work on this rifle. If not, please don’t try to take it apart!

The action comes out of the stock by removing three long bolts — two in the underside of the forearm and the rear triggerguard screw. After the action is out of the stock, look down in the stock at the underside of the triggerguard to see the trigger return spring that Slavia also uses as a trigger adjustment. This is the spring that foiled me, and it will foil many of you unless you know where it is. The two photos I’ve already shown will reveal how these parts fit together.

You don’t have to do anything with this spring except to know that it’s there and how it functions. When the gun goes back together, the placement of this spring will determine whether your trigger works or not, and it’s very easy to get it in the wrong place.

The rest of the disassembly couldn’t be much easier. The first step is to drive out the one and only pin that holds the trigger blade. Once that’s done, the trigger blade can be pushed forward and up and will clear the sear, making it possible to remove the trigger blade and coiled spring from the action. There’s no easy way to explain this, but a few careful moments of fiddling with the trigger blade will do it. Be careful not to lose the tiny coiled spring that’s in the front of the trigger blade at the top and in the end of the sear.

The trigger pin is out. The tiny coiled spring that fits between the trigger blade and sear is not shown. The sear is still inside the end cap, held by a similar pin on the left side of the end cap. The long slot in the top of the trigger blade is for the automatic safety button. The piston pushes the trigger out of the way when it comes back during cocking. It pushes the automatic safety button back until a ridge on it cannot allow the trigger to move. The knurled knob at the right of the end cap is the automatic safety.

Once the trigger blade and sear spring are out of the action, thread one of the long stock bolts into the hole at the bottom of the end cap and use it as a handle to turn the threaded end cap out of the spring tube. The barreled action should be installed in a mainspring compressor as this is done, and you’ll need to use a small socket to fit over the safety knob to put pressure on the end cap as you go.

The end cap is slowly being unscrewed from the spring tube. The threaded hole in the knurled section of the cap is where you thread in one of the stock bolts to start the end cap turning. After it gets going, you don’t need that bolt any longer.

The rifle I received for spare parts had been cocked for years, so the mainspring was under full tension all that time. I was extra careful when removing the end cap, but the spring was so collapsed from being compressed for so long that the end cap only came out of the gun by about an inch.

The cocking link on this rifle is a two-piece articulated one that doesn’t use a cocking shoe. Once the action is out of the stock, the cocking link will fall free from the spring tube as soon as there’s clearance. Since this rifle was cocked, it fell out immediately.

The tune
You may recall that I said my rifle was somewhat buzzy as well as being dry. Once it was apart, I could see it had never been lubricated from the factory. That was what was causing the cocking effort to be as hard as it was. I lubricated the mainspring with a product that’s no longer obtainable — Beeman’s Spring Gel. It was less aggressive than their Mainspring Damping Compound, which will subtract significant velocity from any spring gun to which it is applied. Spring Gel never worked well for me in the past, but I thought I’d give it another try, so I slathered it on the mainspring and on the outside of the spring guide. And that was the full extent of my lube job. I could see that the piston seal is a synthetic one, but I didn’t rub anything on it. It seemed oily enough from an earlier application of silicone chamber oil, and I thought I’d leave it that way.

The piston is a solid steel part that’s well-made. The piston seal is synthetic, and on this spare parts rifle the edge of the seal is chipped.

Then the rifle was assembled in reverse of disassembly, until we got to the trigger. I installed the small coiled spring and eased the trigger back into its proper place by inserting the other end of the coiled spring into the front of the sear. Then I aligned the holes and drove in the pin that holds the trigger in place.

Next, I installed the barreled action into the stock, taking extra care to “thread” the trigger blade through the trigger return spring that was still installed in the stock. This time, I knew about the spring; and even then I got it wrong a couple times before getting the spring into the right relationship with the return spring. Once that was done, the rifle cocked and functioned perfectly.

The trigger adjustment screw should not be tuned with the rifle disassembled; because if it’s turned too far out of adjustment, the rifle will not cock. That’s a safety measure in the design of the gun, but it also makes it difficult to assemble the rifle if you don’t know if the problem is where the adjustment screw is set or if you’re missing the correct positioning relationship of the trigger blade to the return spring.

Since I’d already chronographed this rifle before lubricating it, I had a good baseline against which to compare the now-completed rifle. I knew that the cocking effort had dropped from 35 lbs. to 21 lbs., which is a good indicator that the lube was doing its job, but what about performance out the muzzle?

Before the lube, Crosman Premier 7.9-grain pellets were averaging 589 f.p.s., with a spread from 586 to 593 f.p.s. After the lube, they averaged about the same 589 f.p.s., but the variation was much broader, going from a low of 577 f.p.s to a high of 614 f.p.s. That’s what you get following a lube tune, and it will soon settle back to where it was before. I don’t think there’s been any change in velocity at all.

The buzzy nature of the gun seems not to have changed at all, so I’m still having no luck with the now-obsolete Beeman Spring Gel. However, it did accomplish one thing — the reduction of excessive friction during cocking. That was the goal of the tune to begin with.

The results
Now, I have a rifle that I can rely on, and I’ve scheduled at least one test for this rifle in the near future, so it’s good to have it back. I wrote this report because nowhere have I been able to read about the trigger return spring, and I wanted to document it for all who decide to tune this rifle in the future.

Testing the Slavia 631 with non-lead pellets: Part 1

by B.B. Pelletier

Announcement: Here’s this week’s winner of Pyramyd Air’s Big Shot of the Week on their facebook page. He’ll receive a $50 gift card.

Choon Weng Chua submitted this photo from an airsoft skirmish. He’s this week’s Big Shot winner.

It’s a little crude but also elegant. The Slavia 631 is the testbed for this report.

I reviewed the Slavia 631 breakbarrel air rifle one time back in March 2006. At that time, I wasn’t reviewing airguns in the format you see today, so it got a quickie once-over and we moved on. Perhaps, if the rifle had been available here in the U.S., I might have done more with it, but since it wasn’t being sold here, and still isn’t, I didn’t think there would be that much interest among the readers.

I also used the rifle for a testbed back in 2003 when I wrote an article about the performance of pellets versus round balls. That’s an interesting article you may wish to review, and I find it a little interesting that here I go again, using the 631 to test the performance of non-lead pellets against lead pellets. Apparently, there’s something about this rifle that makes me think of it as a standard airgun.

This test isn’t really about the air rifle as much as it is about the pellets. We’re just using the 631 because of its accuracy and mild manners.

The Slavia or CZ 631 and 630 are breakbarrel spring-piston rifles made in the Czech Republic. The 631 Lux model I have was purchased new about 12 years ago when they were still being imported into the U.S. It has a rather plain stock that’s too blocky and square for my taste, but the barreled action is where the performance lives so I can put the looks aside. The 630 and 631 are the same rifle in different stocks, just like the Diana 48 and 52 sidelevers. The 631 is the dressier one.

The stock is a medium brown beech with a thin, non-reflective finish that’s very topical. Mine has several scratches that cut through the thin top layer of wood. Because it’s blocky, it doesn’t fit me very well. I do like the slender forearm, but the pistol grip just doesn’t fit right. There’s impressed reverse checkering on the forearm but not the pistol grip, and the overall appearance is one of cheapness. The metal parts are not polished beyond tumbling and are finished with a thin black color that comes through as matte.

This is a low-to-moderate air rifle power-wise. I expect to see the lighter-weight pellets shooting in the low- to mid-600s somewhere. We’re going to find that out in today’s report.

One of the reasons I’m using this rifle is because a number of our readers own them. Canadians can still buy them, as can Europeans and almost anyone else except Americans. But I’m also writing about it because one of our readers suggested that I test the accuracy of non-lead pellets in this rifle. He felt the power was not so high that it would scatter the pellets, and also because the 631 is a very accurate air rifle in its own right. I agreed and here we are.

As I describe the rifle, bear in mind I’m describing the one I own. It’s at least 12 years old and possibly more, and there may have been some changes to the design since mine was made. Generally, cosmetics change faster than fundamental design items, so my rifle is probably still a good representative of the type.

A couple features before I test velocity for you. First, is the barrel lock. Before you can open the barrel to cock the gun, you must push forward on the spring-loaded barrel lock that’s located under the barrel just forward of the baseblock. It retracts the chisel detent and allows the barrel to flop open a little. From there, you can cock the rifle with 35 lbs. of effort. My rifle is in need of a tuneup, and I can feel some metal rubbing when I cock it. I think it would drop back to a 28-lb. cocking effort with proper lubrication. I did oil the piston seal for this test but not the rest of the powerplant.

The barrel-lock latch (left) is pulled forward, retracting the chisel detent (right) and allowing the barrel to open.

Another feature is the automatic safety. It’s a knurled knob located at the rear of the spring tube; and when the rifle’s cocked, it pops out. You must press it back in before the rifle can be fired.

Although the safety is automatic, there’s no anti-beartrap device and the rifle can be uncocked by taking the safety off and pulling the trigger while restraining the barrel.

The two-stage adjustable trigger is very light. I don’t think too many owners will complain about the crisp l-lb. trigger-pull. Oh, and it has a metal blade! A screw in the plastic triggerguard adjusts the length of the stage-one pull, and it can be eliminated entirely.

Trigger adjustment screw (second from left) controls the length of the first stage, only.

The sights are things of beauty! The all-metal front sight is a hooded post that appears as a sharply defined rectangle through the rear sight. The rear sight is a study in elegant design. It’s all-metal, fully adjustable and yet simple and probably inexpensive to build. It ought to be a required study for any engineer wanting to design airguns.

The front sight is hooded and looks a lot like the front sight of an SKS.

The rear sight is all metal and an elegant design. It shows what some thought can do.

The firing behavior of my rifle is very buzzy. It takes me back to the 1970s, when all spring rifles buzzed, and I guess it’s the one thing I like least about the gun. But I’ve left it as it is all these years so I’d have something unaltered to reference.

The rifle does have dovetail cuts for a scope mount, but this is the very rifle that caused B-Square owner Dan Bechtel to enlist my aid in the 1990s to measure the width of as many airgun scope rails as possible. We found that 11mm airgun scope rails range from about 9.5mm separation to over 14mm! And, the Slavia 631 is the biggest one of them all. So, those “11mm” scope mounts you buy will probably not fit this rifle unless the package specifically names this model or the 14mm width. To my knowledge, only BKL now makes scope mounts specifically designed for this air rifle.

There they are. The widest set of “11mm” air rifle dovetails in existence. They measure just over 14mm, so they require purpose-built mounts. To keep the scope mount from moving, a half-round transverse pin in the bottom of the mount has to fit one of the grooves or you’ll need BKL mounts. That knurled knob on the bottom is the automatic safety.

Pellets selected
Because this report is really about the accuracy and performance of lead versus non-lead pellets, I’m not going to spend a lot of time on the performance of the gun other than to document the accuracy with lead pellets of known quality. Today, I’ll show the pellets that will be tested and chronograph each of them for you. In the next test, we’ll finish with accuracy at 10 meters.

The pellets I’ve selected to test are the following:


Crosman Premier 7.9-grain
JSB Exact RS
Air Arms Falcon


Gamo Raptor
Skenco Hyper-Velocity Type 1
Crosman SSP

The lead pellets should show the relative accuracy of this rifle under the best conditions. What the non-lead pellets will do is what we’re going to learn.

Premier lites averaged 589 f.p.s. The range was 586 to 593 f.p.s. They generated an average 6.09 foot-pounds.

JSB Exact RS pellets averaged 593 f.p.s. with a range from 586 to 596 f.p.s. The average energy was 5.7 foot-pounds. These pellets fit the bore very loosely.

Air Arms Falcon pellets averaged 592 f.p.s. They ranged from 591 to 594 f.p.s. The muzzle energy was 5.68 foot-pounds. They fit the bore tighter than the JSB pellets, but not as snug as the Premier lites.

Gamo Raptor pellets fit the bore variably — from snug to so loose they fell out. They averaged 741 f.p.s., ranging from 727 to 754 f.p.s. The average energy was 6.59 foot-pounds.

Skenco Hyper Velocity Type 1 pellets averaged 719 f.p.s. The range went from 689 to 749 f.p.s. Their average energy was 6.2 foot-pounds.

RWS HyperMAX pellets averaged 743 f.p.s. They ranged from 639 to 750 f.p.s. The average muzzle energy was 6.38 foot-pounds.

Crosman SSP pellets averaged 738 f.p.s.. The range went from 658 to 750 f.p.s. The average muzzle energy was 4.84 foot-pounds.

So these are the pellets I’ll be testing for accuracy, and we now know how well they perform in this rifle. I’ll save my conclusions of the test for the finish, at the end of Part 2.

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