BSA Scorpion PCP air rifle: Part 3
by B.B. Pelletier
Announcement: The blog’s server went down on Thursday, April 21, 2011. It came back online Sunday, April 24. This blog was published early Monday, April 25, and is dated Friday, April 22. Monday’s regular blog will be published in the afternoon of Monday, April 25.
This is a good, long report, so grab your coffee and perhaps another Danish. Today, we’ll learn something about accuracy and group sizes.
I’m testing the accuracy of the .22 caliber BSA Scorpion PCP air rifle, and it’s quite nice! Helping quite a bit was the weather at the range, which was perfect for long-range airgunning, as there wasn’t a breath of wind to be felt. The day was overcast and misting slightly and with every shot you could see vapor at the muzzle when the compressed air emerged.
I scoped the Scorpion with a CenterPoint 8-32x56AO scope that was sharp and clear. Even though I shot at the small 50-foot rimfire bulls like I usually do, they appeared quite large and sharp through the scope. I was easily able to bisect the center ring with the crosshairs.
I had filled my carbon fiber air tank since the velocity test, so I was able to fill the rifle up to its maximum of 232 bar. I couldn’t do that in Part 2 because my tank was below 232 bar, so I’m going to give you a second look at velocity today, with some of the pellets I used for accuracy.
The gun was not sighted-in because I had just mounted the scope the evening before. Normally, I like to shoot the first few rounds at 10 feet to adjust the scope on target at 20 yards, but today I did something different. I stapled a huge silhouette target to the target frame, then put my targets on that. That way, I had two feet by four feet of paper for the pellet to hit. You could also use a large piece of butcher paper or even cardboard to do the same thing. So long as there are no holes in the paper when you start, just attach your real target to the center of the larger paper and start shooting. For those of you who shoot at public ranges where you cannot put your own target on the range at different distances, this is a handy trick to avoid boresighting.
The first pellet I tried was the 18.1-grain JSB Exact Jumbo Heavy pellet that everyone raves about for powerful PCPs. Since this rifle is capable of almost 40 foot-pounds, it seemed perfect for that pellet.
The first five shots were barely on paper, but they did strike the intended target paper of 11 bulls. Though they all landed in the upper right corner of the paper, I scanned the group to show you how the gun did. Bear in mind that this is only a five-shot group, and it not representative of the true accuracy this rifle can offer, but it’s in the ballpark.
The first five shots from the BSA Scorpion at 50 yards made this 0.175-inch group! Now, that’s a screamer, folks! But look to the upper right of that hole and you’ll see another partial hole. That was shot No. 6. And THAT is the reason that 5-shot groups are not enough. Read the text for the explanation.
Okay, I hope you’re as excited as I am by this first group. All five pellets wanted to go to the same place. I’ve tested accurized Ruger 10-22 target rifles that did not give results as good as this. However, as I indicate in the caption, shot No. 6 landed apart from the group. Since I have a little experience with these high-pressure, powerful BSA rifles, I suspected what was happening. I quit shooting this group and adjusted the scope. Because this rifle is advertised as having 20 powerful shots on a fill, I continued shooting after adjusting the scope.
This is the telling target. There are 10 shots here, but they represent shots number 7 through 17 on the first fill. The same JSB Exact Jumbo 18.1-grain pellet was used. Notice the two definite groups we have. The upper group contains the first six pellets and the lower group the final four. This rifle is losing pressure and changing the point of impact as it does! That is why the sixth shot in the first group was apart from the other five. This whole group measures 1.013 inches but the top group of six measures 0.533 inches.
I’ve seen performance like this before in these 232-bar BSA rifles. They have a large number of shots for the relatively small reservoir on the gun, but the valve cannot perform with stability across the entire fill. You do get 20 powerful shots, but the POI will change slightly as the pressure diminishes. Hunters may not notice it, but it shows up clearly on paper at 50 yards. I decided that with the 18.1-grain JSB Exact Jumbo pellet the Scorpion had only five good 50-yard shots on a fresh fill before it started to shift POI. I refilled the rifle and shot another 10-shot group to see if that was correct.
Here’s the proof of what I’m saying. The first five JSB Exact Jumbos stayed in a very tight group, but starting with shot No. 6 the group spread laterally to the right. The last shot went to the far left, so the POI was still shifting. The group measures 1.49 inches between the two widest centers.
What does this mean?
All is not lost, nor has the sky fallen. You have a choice when presented with this type of performance. Either refill the rifle after every five shots and get bragging-sized groups at 50 yards, or accept the 10-shot groups that you do get and continue to shoot this pellet, or find a different pellet.
The weight of the pellet, plus how it fits in the bore, determines the dwell time that the pellet remains in the barrel. For most of that time, the back-pressure of air inside the barrel behind the pellet holds the firing valve open and air escapes. Changing to either a lighter pellet or one that fits looser — or both — will change this relationship and give you different results, as we shall see.
These JSB Jumbos fit the bore so tight that I could feel them pop into the breech as the bolt was closed. So, they’re both heavy and tight.
Next, I tried the Beeman Kodiak copper-plated pellet. They weigh the same as the all-lead Kodiaks, and past testing has shown that they’re equally accurate, so let’s see what they did in the Scorpion.
A beautiful nine-shot group! If only that tenth shot wasn’t in the center of the bull. I don’t know which shot it was, but I do know it was not in the first five, nor was it the last shot. Group measures 1.227 inches between centers, with nine shots in 0.537 inches.
Kodiaks weigh 21+ grains; and like the JSB Jumbos, they also popped into the breech. Well, I decided to try it again because that nine-shot group was a good one. So, the rifle was topped off, and I shot the copper Kodiaks a second time.
This 10-shot group shows more spreading than the first one did. Again, I got a flyer through the center of the bull, but this time there were also two that went to the right. Group measures 1.253 inches between centers, and you can see that the first seven are in a tight bunch in the center.
Well, at this point I reckoned that I had the Scorpion pretty well figured out. Heavy pellets were going to use a lot of air, which resulted in a change of POI in the middle of a 10-shot group attempt, so they were probably not the best pellets to use for what I was doing. However, if you could top off after five shots, then they would be ideal.
I tried one more pellet just to prove my point about the heavy pellet POI shift after five shots. This last one was the 28.4-grain Eun Jin pellet. If ever a pellet was going to hold open a valve, this is the one that would do it.
Well, it doesn’t get much more obvious than this. The first five are in a tight group that then drops to the right. The final three pellets are on the extreme left. The group measures 1.774 inches between extreme centers and is the largest group of the test. No sense shooting another one.
The results were as clear and unambiguous as they could be. The tendency to spread after five shots continued in a most aggressive way. The central group of five measures 0.526 inches between centers. So, shooting the first five shots from a fill is still a viable option, even with this heavy pellet.
Now where do we go?
Isn’t it obvious? If the rifle can’t shoot heavy, tight pellets more than five times on a fill, what will it do with lighter pellets that are loose? Luckily, I happened to have a tin of the old standby 15.8-grain JSB Exacts on hand. They’re the lightest pellet I tested, and they were loose in the bore.
The first target was good, so I shot a second one. It measured so close to the first one that I cannot discern a difference.
I said I would do some more velocity testing for you because I wasn’t able to completely fill the rifle in Part 2 of this report. I’ll test two pellets — the 18.1-grain JSB Exact Jumbo heavy and the 15.8-grain JSB Exact. They should show us what the performance curve looks like, and we can extrapolate to other pellets whether they’re heavy and tight-fitting or light and loose.
The 18.1-grain Exact Jumbos averaged 871 f.p.s. for 20 shots, but the spread went from 818 up to 904. Starting with shot No. 6, the velocity fell off on every successive shot. Shot 15 was still going 848 f.p.s., so for hunting purposes that might be your last shot on a fill. But shot five is the last one for a super-tight group at 50 yards. The first five shots averaged 899 f.p.s., with a muzzle energy for just those five of 32.49 foot-pounds.
The 15.8-grain Exacts averaged 891 f.p.s. for 20 shots with a spread from 810 to 947. Every shot except shot five was a decrease from the shot before, and the rifle dropped velocity faster with this pellet than with the heavier one. Velocity alone doesn’t explain why I got such great 10-shot groups, because by shot 10 the rifle had dropped 45 f.p.s., where the heavier pellet lost only 18 f.p.s. Still, the groups don’t lie. This is the more accurate pellet. The first five shots averaged 936 f.p.s., for a muzzle energy of 30.74 foot-pounds.
I seem to be pushing the 10-shot group a lot for accuracy testing, but today was a perfect example of why it’s better. Yes, this was a very specific situation that you wouldn’t encounter in a spring rifle, but those extra five shots give any gun every chance to express all of its bad traits. A 30-shot group would even be better, but except for extreme product testing or belaboring a point, it’s seldom done. Ten approximates 30 closely enough for most purposes.
Looking at the price of the rifle, it offers a hunter good value in a traditional-looking powerful rifle. The trigger is okay and nothing about the rifle is bothersome, except for the 232-bar fill pressure. The accuracy is stunning, as you have seen. It cracks like a .22 short, so be ready for that. In fact, after I finished the velocity testing, Edith said, “That was loud. What was it?”
I replied, “BSA Polaris.”
But that didn’t sound right, so I thought about it for a second until the name came to me.
“Scorpion!” I shouted a minute later.
“Where!” she answered, running into the room with her shoe off, looking for the arachnid invader.
So, the old girl still has life in her yet.
And, by the way, I can still shoot a rifle, in case anyone asks.