The Benjamin Discovery – Part 3 Velocity
by Tom Gaylord
Before we begin, I have some announcements.
This will probably miss those for whom it’s intended. For some reason, people have been emailing airgun questions to which they want immediate answers to [email protected] That address is for podcast questions, only. I never answer those messages. If you have a question, ask it here. Though there are now more than 750 blog entries, I get the comments from each of them all the time. I go to the segment where the question was asked and I try to answer it there. I do not send emails directly.
Right now, I’m getting 30-50 questions every day. It takes 3-4 hours a day just to keep up with the comments. I want to do it, but as the volume increases I can see the day coming when I can’t do it. What takes the most time is when someone asks me to choose a scope or a set of mounts for them. Another question that takes time is when a person wants me to compare four rifles for them and give reasons for my choices. I may have to stop answering those questions soon so I can answer the fundamental questions from new airgunners.
One of our own, .22 multi-shot, has his own blog. I went there and took a tour and so should you. I’m fascinated by the air-powered car that gets 800 km on 1.7 liters of fuel. As long as it’s not plutonium, that’s great mileage!
Another reader, John, just bought a Crosman M1 Carbine. It came without the BB holder, as so many do, so he wondered if a real M1 Carbine mag could be altered to fit. Go here to see how he did it.
Now, on to today’s topic and the Benjamin Discovery!
More development history
Remember those good-looking groups I showed you at 21 yards in Part 2? Well what I didn’t tell you was the gun was also very quiet. I was so thrilled that I emailed Crosman and told them they had made a quiet airgun. Then I thought about it some more and the reason for the quiet dawned on me. The valve was in partial lock and I was shooting slow. For those wanting a sneak peek at CO2 performance, that was it, but a lot slower. My point is, accuracy doesn’t suffer at close range when velocity drops.
Flirting with valve lock
Sure enough, the chronograph showed that the .177 was shooting Crosman 10.5-grain Premiers at about 575 and shooting .22 caliber Premiers at about 500! The valve wasn’t opening all the way. So, I continued to shoot and both guns came up on the power curve at around 1,700 psi. I had filled them to 2,000 psi on my pump’s gauge, which was pretty close to the built-in manometers on each rifle. Clearly, these guns didn’t like that. You may recall some time back I told you about the inaccuracy of a small pressure gauge. It had happened to me! By careful testing, I’d found the point at which each rifle wanted to be filled, which turned out to be above 1700 but below 1800 on the pump gauge I was using. Who cares what pressure it really was, because that gauge was all I had to go on.
No free lunch
Once I found the right fill pressure, the guns started barking with great authority. And there’s an answer to those who asked about the sound. These rifles are loud, just like any other PCPs in their power class. There has been some talk on the forums that because the rifle runs on 2000 psi, it should be quieter. Well, it isn’t. My Barnes .25 that ran on 800 psi and had a 33-inch barrel wasn’t quiet, either. Face it, guys, PCPs make noise.
Did Crosman think about a shrouded barrel? Yes, they did. They even considered putting one on the Discovery, but I strongly opposed it, because the cost of the rifle was climbing too fast. I wanted something everyone could afford to begin with, and we would worry about the issues of adjustable triggers and barrel shrouds on the next model. The Discovery is supposed to open the doors to precharged airguns, not to be the perfect PCP.
Back to my report…the initial velocity in .177 peaked at 953 f.p.s. with 7.9-grain Crosman Premiers and in .22, using Premiers, a peak of 834 f.p.s. The power curve was not flat for more than 20 shots, but if you could accept a wider velocity variation (about 45 f.p.s.), there were around 35 shots to each fill. Say what you want about the wide spread, those tight groups at 50 yards were shot with the gun in this initial state of tune.
At this time, the design engineer and I were discussing seals for the manometer, which was causing a minor problem. It sealed fine with plumber’s tape around the threads, but plumber’s tape is not a production sealant. Crosman needed something that was faster, required less labor and was positively reliable (no dependence on the skill of the worker). They settled on a flat seal, but then there were durometer questions to be answered (the hardness of the sealing material).
Another project he was working on was a filter to trap dirt before it enters the reservoir. Many, if not most PCPs do not have such a filter, but the Discovery does. It won’t make the gun any more accurate, but it’s evidence of the approach they took when building this gun.
After a couple months of playing with both rifles, Crosman changed the valve design and I was asked to return them both for an update. When I did, they also sent me the new breech cap that has a hole for the degassing tool. That’s how you dump the reservoir to change types of gasses, want to ship the gun or when you’ve overfilled the reservoir (which happens only with air).
When the rifles were returned to me, both would accept a fill to 2000 psi on my pump gauge. The manometer on the .177 showed exactly 2000 when filled, and the needle on the .22 manometer just kissed the low edge of 2000 when filled. Neither manometer appeared to have been changed. Velocities were definitely up with the new valve. In .177, the high was now 975 with 7.9-grain Premiers, so lighter pellets will have no difficulty breaking 1,000 f.p.s. Of course, you’ll actually shoot Crosman 10.5-grain Premiers and JSB 10.2-grain Exacts with velocities in the high 800s up to 900, because you want the most accuracy possible.
The .22 hit 860 f.p.s. with Premiers and averaged 856 for a string of 25. There are still 35 good accurate long-range shots in both rifles. If you shoot at 25 yards or less, you’ll get even more. Accuracy remains unchanged. Many months later, after much more shooting, a retest showed the average had settled back to 820, which is probably more like what you should expect to see in the long run. I would guess 820-840 will be where they end up. The number of total shots is still 35 if you accept a larger spread, but hunters who shoot at 40-60 yards will probably go with a string of 25. At 820 with Premiers, the gun has 21.36 foot-pounds. With Kodiaks, the velocity drops back to about 700 and the energy climbs to 22.85. Kodiaks are about the heaviest pellet I’d use with this rifle.
There’s the bulk of the report – the gun, accuracy and velocity. I’ve been asked to report the gun on CO2, which I’ll do next month. If you have any questions I failed to address, please make comments and I’ll look at them.