Diana 25 smoothbore pellet gun: Part 2
by Tom Gaylord, a.k.a. B.B. Pelletier
This Diana 25 smoothbore was made during World War II.
Oh, the things we think we know — how they vanish when we test! Today, we’re going to look at the Diana model 25 smoothbore that Vince sent me. You may remember in the last report that I was pondering when this airgun might have been made. Well, Kevin told me to look on the bottom of the butt, as the date stamp used to be there. Indeed it was! This airgun was produced in June of 1940, during the first part of World War II.
The manufacture date of the gun is stamped in small numbers on the bottom of the wooden butt.
The curiosity of a smoothbore is the extent to which the rifled barrel affects performance of the gun. I should have two identical airguns to test — one rifled and the other a smoothbore, but even then there would be subtle differences in their individual performance. I think it’s safe enough just to say what I expect from such a gun and then see what I get.
I would think a Diana 25 in good condition would give a muzzle velocity of around 625-650 with lightweight lead pellets. Remember — this is a .177. The last model 25 I tested was a Winchester 425 that was a rifled .22-caliber gun. That one gave an average velocity of 440 f.p.s. with 11.7-grain Hobbys, which I thought was a little slow. I expected about 525-550 from it with that pellet.
Preparation: Oiling the leather piston seal
To prepare for this test, I oiled the leather piston seal with about 10 drops of 3-IN-ONE oil. I just stood the gun on its butt and dropped the oil down the muzzle. By leaving it standing that way for a couple weeks while I was at the SHOT Show, the oil ran down into the compression chamber and soaked into the leather piston seal. I also oiled the leather breech seal at the same time so it would be pliable for this test. And I note that the gun now smells of burnt oil when it shoots, so everything was successful. We can be sure that the gun is performing up to the limit of its capability.
You may remember that Vince tuned this gun before he sent it. The mainspring inside was one he cut down from another rifle, so it isn’t exactly what the Diana had in it from the factory. But he took the spring from the harmonica gun that we suspect used to be a Diana model 27, so the dimensions of the spring are probably pretty close to original. We can guess and conjecture all day long, but a better way is to just shoot the gun and see what it does.
For the first pellet, I chose the 7-grain RWS Hobby. It’s a lead pellet that’s both lightweight and also a bit large, so it fits a lot of airguns very well. Since the gun was so well oiled, I actually shot three strings of 10, rather than my usual single string. The reason for this will soon be obvious.
The first string ranged from a low of 593 to a high of 627 f.p.s. The gun started in the 620s and progressively dropped in velocity as more shots were fired. That tells me it’s burning off some lubricant; and from the smell, I knew that it was.
The average for the first string was 609 f.p.s., but I believe that is too high. I think the dieseling caused by the excess oil boosted the velocity a lot. Immediately after the first string, I shot a second one.
I expected the second string to be slower and less variable, and I was right on both accounts. The average velocity for string 2 was 598 f.p.s., and the velocity ranged from 593 to 613 f.p.s. At the average velocity, the Hobbys produced 5.56 foot-pounds of energy at the muzzle. I don’t think the gun has settled down completely at this point, and I expect to see the average drop a few more feet per second as the gun continues to shoot. But there was still one more thing I needed to test.
I’d been seating the pellets flush with just my finger to this point. What would happen if I seated them deep with the Air Venturi Pellet Pen and PelSet? This time the average dropped to 594 f.p.s. and the range went from 584 to 621 f.p.s. What I make of that is that the pellet pen and deep-seating has little to no effect on the velocity of this rifle with a Hobby pellet. I think breech seating will be good, but I’m not going to leave it at that. I’ll also try shooting a group with the most accurate pellet seated deep, to compare to flush-seating.
Hobbys fit the breech tight and just a little of the skirt stuck out of the barrel. I expected them to increase in velocity with deep seating, but I guess this gun needs the extra resistance to generate all the power. It’s right on the cusp because deep-seating produces almost the same velocity, but the variability is greater; so I don’t think deep-seating is worth the extra effort.
The second pellet I tried was the heavyweight Beeman Kodiak. At 10.65 grains, the Kodiak is way too heavy for this gun. But that’s why I wanted to try it. I expect I’ll also try it for accuracy because who knows what it’ll do in this smoothbore?
After a couple shots that obviously dieseled, the Kodiak settled down to shoot in the mid 400s. The average was 461 f.p.s., and the range went from 443 to 470 f.p.s. At the average velocity, the Kodiak produces 5.03 foot-pounds of energy at the muzzle.
Just for fun, I also tried deep-seating Kodiaks that fit the breech very loose. This time the result was more positive. The average velocity dropped to 448 f.p.s., but the range tightened to between 439 and 455 f.p.s. That’s just 16 f.p.s., compared to the 27 f.p.s. spread for flush-seated pellets. I guess I’ll also try deep-seating Kodiaks in the accuracy test.
JSB Exact RS
The 7.3-grain JSB Exact RS dome was the last pellet I tested in the gun. These fit the breech even looser than the Kodiaks, but they gave an average 517 f.p.s. velocity with the tightest spread of the test. The low was 512 and the high was 525 f.p.s., so only 13 f.p.s. between the top and bottom. At the average velocity, this pellet produces 4.33 foot-pounds of energy at the muzzle.
Naturally, I tried deep-seating the RS pellet, as well. And to my surprise, the consistency grew even tighter as the average velocity decreased. The average was 504 f.p.s., but the spread went from 500 to 511 f.p.s., for an 11 f.p.s. difference. I guess I’ll deep-seat all the pellets during the accuracy test, as well.
The Diana 25 cocks like many vintage breakbarrel springers. It begins easy, then stacks toward the end. The max effort required is 19 lbs., which makes this a youth airgun in my book.
The trigger is two-stage, and stage two is reasonably crisp. The first-stage pull is 1 lb., 8 oz., and stage two breaks at 5 lbs., 11 oz. It isn’t a target trigger in any respect, but it’s crisp enough that I know I can do good work with it.
Impressions so far
I’m finding that this smoothbore is, in fact, very similar to the rifled version of the Diana 25. The size, fit, trigger and feel of the gun give no indication that the bore is smooth. But this gun was made in 1940; and as such, has several differences from the Dianas of the 1970s that I’m used to. For starters, the sights are simpler, and there’s no rear base for a peep sight. Then, there’s the simpler trigger that cannot be adjusted.
I have to admit I’m very curious about how this gun is going to perform on target. I know it can’t be as accurate as a rifle, but I find myself hoping that it’s close. We shall see.
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