The Diana model 50 underlever: Part 2
by Tom Gaylord
Writing as B.B. Pelletier
Diana model 50 underlever.
This report covers:
- Loading tap
- Rear sight
- Pre-test preparation
- Fastest way
- Velocity RWS Superpoints
- JSB Exact RS
- RWS Hobby
- Cocking effort
- Trigger pull
- Evaluation so far
Today we will learn the velocity of my new/old .177 caliber Diana model 50. But there are several things I need to clear up before we get to that. Let’s start with the loading tap.
The Diana model 50 is an underlever spring-piston air rifle. That means the barrel doesn’t open like a breakbarrel, so there has to be another way to load a pellet. On some underlevers the entire compression chamber slides back, exposing the breech, but others like the model 50 use a loading tap. A tap sits behind the barrel and rotates open to load the pellet and closed to align the pellet with the breech.
Drop a pellet into the open loading tap, nose-first.
Rotate the tap closed and the pellet aligns with the breech, ready to be blown through the barrel with the shot.
Push the loading tap lever forward (to the left) to open for loading.
I said in Part 1 that I thought the peep sight was dedicated and didn’t convert to a sporting open rear sight, but reader Mike Driskill showed me different. So, for this report I have removed the peep disk to show you.
The peep disc has been removed and the spring-loaded rear sight plate, with three different sighting notches and the large groove, is rotated to show the notches more clearly. Any notch can be selected.
This feature turns the model 50 into a sporting rifle. The entire rear sight is moved forward to a different sight base on the spring tube for the best eye relief.
I was also asked what the numbers on the left of the rear sight mean. They are elevation reference numbers that an index mark aligns with as the rear sight is elevated.
The index line on the bottom aligns with the elevation numbers as the sight goes up and down.
Pop quiz — the Diana 50 was made from 1952 to 1965. What is the piston seal most likely made of? Answer — leather.
What should we do to a leather piston seal before shooting it? Answer — oil it.
Extra credit — how do you oil the piston seal of a taploader?
There are two ways. The first takes the longest. Stand the rifle on its butt with the tap closed and drop 10 drops of oil down the muzzle. The oil will run down through the barrel, through the loading tap and into the compression chamber, where the piston seal will soak it up.
Here is the quicker way. Open the loading tap. Fill it with oil, close it and stand the rifle on its butt for an hour. The same thing happens without the barrel getting in the way.
More extra credit — what kind of oil should be used? That depends on the power of the gun which we don’t yet know. But we can guess pretty close. A spring-piston rifle made between 1952 and 1965 rifle will probably generate less than 12 foot-pounds at the muzzle. Velocities with be slow, so regular household oil can be used. I used Crosman Pellgunoil.
Velocity RWS Superpoints
Since this is a taploader I want to use pellets that will fill the tap when the gun fires. The RWS Superpoint is my favorite pellet for this, because it has such a thin skirt.
This string was both interesting and instructive. I will show you every shot from start to finish, because I want to discuss it. Remember — this was fired 60 minutes after oiling.
Wow! That string is a classic illustration of why a chronograph is so important to airgunners. The rifle was full of oil because I over-oiled it. So, all the extra oil shot out with the first several pellets. I’ve seen this before, so I just kept shooting and watching the numbers on the chronograph screen. When they stopped increasing, the rifle would be in its power band.
It’s arguable where they stopped but I am calling it at shot 12, which was 646 f.p.s. That one and the next 9 will be the string. So, for this pellet the low was 618 and the high was 685 f.p.s. That’s a large spread of 67 f.p.s. That much variation is probably due to the extra oil in the barrel. No doubt the velocity spread will tighten a little with more shooting.
The average for the string I selected is 660 f.p.s. I think that’s probably about where this pellet will be when the rifle settles down. At the average velocity an RWS Superpoint generates 7.93 foot pounds at the muzzle. That is about what I expected.
JSB Exact RS
Next I tested the JSB Exact RS dome. They are lighter than the 8.2-grain Superpoints. They weigh 7.33 grains. They averaged 648 f.p.s. in the model 50, with a spread from 572 to 708 f.p.s. That is 136 f.p.s., which is too much to expect any accuracy. Since the rifle had started to settle in, I think this shows that the RS pellet isn’t right for this rifle. I won’t test this pellet for accuracy. At the average velocity this pellet generated 6.84 foot-pounds.
The results of this pellet are another reason to have a chronograph. They clearly show when a rifle doesn’t like a pellet.
For the final pellet I chose the RWS Hobby. Since the rifle is a target rifle I wanted to try at least one target/wadcutter pellet, but I selected Hobbys for one additional reason. They have wide skirts that work well in loading taps.
Hobbys averaged 673 f.p.s. in the 50. The spread went from 640 to 685 f.p.s., so a difference of 45 f.p.s. See how much tighter that is? At the average velocity, the Hobby generated 7.04 foot-pounds at the muzzle.
The rifle cocks with 25 lbs. of effort. It feels like a little more than that to me, but I think that’s because I can feel the coils of the mainspring compressing as it is cocked. The rifle cocks with a crunch.
The trigger is set up to release with a single-stage pull that breaks at 1 lb. 8 oz. That’s on the light side, but it’s the single stage pull that throws me. It’s not breaking crisply like a good two-stage trigger would. Since this is a ball bearing trigger I thought I should be able to adjust it to a good two stages with a clean break on stage two.
The front screw locks the setting and the rear screw is the adjustment screw. The previous owner has either Locktited the locking screw to make a permanent bond or he glued it with Epoxy. It isn’t moving! If I want to adjust it I will need to put it in a press to hold tension on the screw as it is turned. Or I will have to drill it out and replace it. I don’t like it when I encounter things like this, but at least the trigger isn’t too bad where it is right now.
Evaluation so far
This model 50 is a fine classic airgun. It cocks easy, has a light trigger, decent power and intriguing sights. I can’t wait to test the accuracy!