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Accessories Diana 25 smoothbore pellet gun: Part 4

Diana 25 smoothbore pellet gun: Part 4

by Tom Gaylord, a.k.a. B.B. Pelletier

Part 1
Part 2
Part 3

Diana 25 smoothbore
This Diana 25 smoothbore was made in World War II.

One thing that I really like about this blog is the fact that it affords me the opportunity to test certain things thoroughly. In fact, it somewhat forces me to test them thoroughly; because as I test and write, I think about you readers and all the questions you’ll have for me. So, I test to be able to tell you as much as I can about our mutual interests.

This Diana 25 smoothbore airgun that I’m reporting on today is one such subject. I get to work with a vintage airgun that’s very enjoyable, plus I get to test how well diabolo pellets stabilize and how accurate they are when they don’t spin. In turn, that reflects on the test of how the rifling twist rate affects accuracy.

I tested this airgun at 25 yards — a serious distance at which any and all airguns will show their true colors. And I used 10-shot groups, another tool in our growing bag of diagnostic accuracy tricks. Just one group can reveal significant findings, instead of five 5-shot groups or, worse yet, I shoot a bunch of 5-shot groups and show only the best one.

I was on the rifle range last week with a young man who was shooting a .257 Weatherby Magnum and trying to get it to group. He obviously knew what accuracy is because he wanted groups that measured under .75 inches at 100 yards. But he was shooting only 3-shot groups! That isn’t enough shots to make more than a good guess about a rifle’s potential accuracy. When I called him on it, he pointed out that he was pasting his targets to a backer at the same place every time, so all his shots would overlap on the backer as he changed targets. That told me he’s afraid of shooting large groups in case he makes a mistake. I’ve been there and done that, too!

Today’s test frankly frightened me, as I wasn’t sure the gun was accurate enough to hit the pellet trap all the time. I decided to use the JSB Exact RS pellets that performed so well at 10 meters. I seated each pellet deep in the bore with the Air Venturi Pellet Pen and Pellet Seater because the 10-meter test showed that was the way the gun likes it best. Let’s look at the two targets from that test before I continue.

Diana 25 smoothbore Beeman Devastator flush-seated group
The flush-seated JSB Exact RS pellets made a 10-meter group that measures 1.158 inches between centers.

Diana 25 smoothbore JSB Exact RS deep-seated group
The same pellets seated deep made this 0.337-inch group at 10 meters. It looks significantly smaller!

The test
The first shot at 25 yards did hit the target paper, but it was high and outside the bull. I checked it with a spotting scope immediately after shooting it. I also checked after the second shot, just to make sure it was also on the paper. It was, so after that I settled down and put 8 more shots into the target. In the end, they were all high and formed a group that measures 3.879 inches between the centers of the two widest shots. So that’s what the gun seems to be capable of, but I wanted another 10-shot group, just to confirm it.

Diana 25 smoothbore JSB Exact RS deep-seated group1 25 yards
The first 25-yard group of deep-seated JSB Exact RS pellets measures 3.879 inches between centers.

I lowered the simple rear sight elevator for the second group and fired 10 more JSB Exact RS pellets. The first shot hit the target in the black, so I knew I was okay to complete the 10 shots without looking. At the end, I had 10 shots in a 3.168-inch group. As far as I was concerned, those two targets demonstrated the accuracy potential of this smooth bore air rifle at 25 yards with deep-seated JSB Exact RS pellets. But something nagged at me.

Diana 25 smoothbore JSB Exact RS deep-seated group2 25 yards
The second group of deep-seated JSB Exact RS pellets measures 3.168 inches between centers. It’s better than the first group, but it’s in the same general neighborhood.

How much worse would this gun shoot pellets that were only seated flush with the breech — in other words, loaded in the normal way? I had to test it. Once more, I shot 10 shots at 25 yards. This time, I was really scared because it looked from the 10-meter test that these pellets might not all hit the paper. Would this group be over twice as large as the other two — like the 10-meter group was? But the first shot went into the bull and the second one landed very close, so I calmed down and shot the other 8 shots without looking again. In the end, I had a 10-shot group that measures 2.421 inches between centers — the smallest group yet at 25 yards!

Diana 25 smoothbore JSB Exact RS flush-seated group2 25 yards
The first group of flush-seated pellets measures 2.421 inches between centers — the best group of the test!

What had happened? The gun was shooting more accurately at 25 yards with pellets seated flush, when it had clearly shot deep-seated pellets best at 10 meters? Not knowing what else to do, I shot a second group with the pellets seated flush. This time the group was larger, but at 2.957 inches it’s still the second-best group of the test.

Diana 25 smoothbore JSB Exact RS flush-seated group2 25 yards
The second group of flush-seated pellets measures 2.957 inches.

What have we learned?
This test demonstrates that diabolo pellets do stabilize from their high drag, alone. They do not require a spin to stabilize them because they all hit the target nose-first. But they’re not as accurate as they would be if shot from a rifled barrel. The spin introduced by rifling is important for accuracy, if not for stability.

A second lesson is this: Even though I shoot and record 10-shot groups, a single group may not be enough data. The difference in accuracy at 10 meters and 25 yards between deep-seated pellets and flush-seated pellets would seem to indicate that. Or it could just be that deep-seated pellets are more accurate at 10 meters, but flush-seated pellets are more accurate at 25 yards. If that’s the case (and I don’t know that it is), I have no idea of why it would be that way.

I think I need to test this gun once more and shoot 3 10-shot groups with each type of seating at each distance before we’ll know anything for sure.

49 thoughts on “Diana 25 smoothbore pellet gun: Part 4”

  1. Such interesting results from this delightful little airgun.I really don’t know what to say about the results of seated versus unseated except that it’s a great excuse to shoot this one some more.I can definitely tell that you won’t mind that a bit! I wonder what you would discover at some in between distance? Also I can’t help but be reminded of the little Belgian Hyscore 801s with their nifty onboard pellet seater.I need to dig mine out for a session.

  2. BB,
    I just purchased a Beeman Wolverine in .22, and even though with 4 different pellets I cannot get “perfect” groups with it yet, I love it, and I can tell it has potential. I remember some posts mentioning that spring guns need a “break-in” to get to the good results. What’s the best thing I can do with it at this point? I have no problem getting some JSB Exacts or other pellets, or even a cleaning kit to try (since I am planning to do so anyway at some point), but I want to know what you recommend, whether it be quick reply or reference to a couple posts you made that I cannot seem to track down. I have a few months ’til next hunting season, but once again I’ve got the itch that can only be scratched by putting pellets downrange in pursuit of bulls-eyes and small game to invite to dinner. Jp

    • Jp…

      It would help if you told us what kinds of pellets that you have tried, what your distance was, and what your group sizes were. Also, is it your first springer ?


    • Jp,

      Continue to do just what you are doing — shoot your gun and it will break in. And you might buy a cleaning rod, brass brush and JB Bore Paste, as well.

      Yes, get JSB pellets if you can. They work well in so many airguns.

      It will take at least 500 shots to break in your gun, and some Gamos I know have taken over 3,000 shots. So there is plenty of time to get to know your rifle.


  3. It’s the first springer I owned, but I’ve handled a few others infrequently. Range was 30 Ft, both scoped and iron sights, and the groups were similar size around 1 to 1-1/2 inches. Funny thing is, on a 50 yard range the scope shots didn’t increase spread dramatically (but I didn’t go measure them). Pellets I used were: Crosman Premier Domed, Crosman Premier Hollow-point, RWS Super H-point, and RWS Pointed. Iron sights showed Crosman Domed better groups, but scoped shows RWS H-point better than that. I wouldn’t say the groups are bad, but I’m thinking from the standpoint that it’s a new rifle that needs break-in and I haven’t found that perfect pellet yet. Jp

    • Jp…

      If you are using one of those “package deal” scopes, then the parallax is going to kill you at 30 feet. Sounds a little “loose” on target as is with open sights.
      You could have chosen worse pellets. Some guys try to start out with some of the worst there are.
      Watch out that the scope don’t get you because of poor quality or barrel droop. A bad scope or a scope adjusted out of it’s positive adjustment range will drive you nuts if you don’t suspect it.

      I suggest that you read up on how B.B. cleans a barrel the first time. It can make a big difference.
      I also suggest that you read up on what B.B. calls “the artillery hold”. That can make an even BIGGER difference.

      If it is still burning a lot of oil, you will have to give it time to burn off and stop smoking before you can expect much.
      If the breech is sloppy and flops around once cocked (but not closed and latched) it is also not going to shoot very well.


    • Jp..

      Too early in the day….yeah…

      Watch pellet fit. If the heads just drop into the breech and are only stopped from falling through the bore by their larger skirt, then they are too small. It should take at least some resistance to get the heads started.

      A lot of rifles do not like the lighter ones . Try the 15.9 grain if you do try JSB.

      A couple others that might be good are FTS and FTT.

      Look at the other things I mentioned before you buy a lot of pellets. Watch all of the screws too. They will loosen up on you.


  4. The breech seems solid. I’ll keep an eye on the scope, but it actually helped by providing a clear picture (the iron sights on this are going to take some getting used to). I think my hold is fine, but I’ll take note and perfect it as I put more rounds downrange. It hasn’t stopped smoking, but at least there’s no longer a bang and a basketball-size cloud of smoke: it quit that after the first 5 shots. Anyway, thanks for the tips. Jp

  5. MR. Gaylord,

    It never ceases to amaze me that you have such patience. I can see you at the range scratching you head and staying to yourself, “Now what in the name of air-gunning can that mean?”. Thank you for you for you time and effort. I look forward to reading these posts every weekday.

    Good day,


      • OK….. Time to set the record straight…..

        Bald heads are caused by excessive brain-power. Much the same way a nitro methane funny car can use several gallons of fuel in just a quarter mile, our brains suck all the available blood from other un-necessary functions such as hair growth, shopping activities, sports and the like to maintain the high speed processing that all great thinkers have. That’s why we are bald! (or maybe we just scratch our heads too much… 😉 )


          • I’m bald, too, but my dad (who was also bald) used to say, “Grass doesn’t grow on a busy street.”

            All of the advanced brainpower notwithstanding, men with male pattern baldness have more dihydrotestosterone (DHT) and Insulin-like growth factor 1 (IGF-1). Both DHT and IGF-1 are male sex characteristics hormones, components of Testosterone. At the face and chest DHT and IGF-1 stimulate hair growth, but at the front and top of the head they are a follicle inhibitor.

            Therefore, the truth is that bald men are more manly than the pretty-boy types with thick, flowing locks.

            Hey, Terry Bradshaw, Burt Reynolds, Rocky Marciano, Frank Sinatra, Michael Jordan, Y.A. Tittle, Ray Nitschke, Bruce Willis, Earnie Shavers, George Washington, Evander Holyfield, Jason Statham, Sean Connery, Telly Savalas, Yul Brynner, Marvelous Marvin Hagler (who claims not to be naturally bald, but look closely at the top of his shaved head, and, well . . .). Oh, I almost forgot, George “The Animal” Steele . . . ;^)


  6. B.B.

    Nothing like getting strange and unexpected results is there ????
    Must be some reason for it (at this point) . Something obscure. Just when you thought you would be able to move on to the next gun on your list.


    • As I’ve said before, I find it fascinating that changing “one” thing results in numerous ripple effects in the ballistics.

      I really enjoy BB’s blog because he tells us exactly what he did, and let’s us “join” in the experiments. Sort of like talking about it down at the coffee shop. I always seem to find something new in BB’s articles and the posts.

      • Herb….

        I have done a lot of things and observed some things that go against “the rules”. Some times I can half way figure out why these things happen. Some times I can’t.

        I try not to take anything I see as a hard and fast rule to apply to everything. You end up fooling yourself too often that way. It works the way it works for whatever reason. The gun decides its own rules. Learn ’em and live with ’em.


  7. This isn’t the first time deep seating pellets has made a difference in group size-obviously it can make a huge difference, but why? It seems to me that how the pellet exits the barrel would be more important than how it enters.

    • Btw, can I post a quick nit here about PA?
      Last night while browsing reviews that damn chat window kept popping up every 30 seconds or so. Is there anyway to disable that, or at least reduce the frequency?

    • Well I have a theory about deep seated vs flush seated at 10 yards. I think it has to do with deforming the pellets. A springer generates a massive pressure pulse.

      The test would be to float bubble wrap or a thin sheet of foam packing material on a 5 gallon bucket of water. In two tests shoot 3 pellets of one seating into the water. I think the flush seated will be noticeably more deformed.


      • Herb,
        I had one of those Bill Engval “Here’s Your sign” moments shooting into a bucket of water. I had a 15″ deep bucket filled with water. I got up onto a step ladder so the muzzle of my IZH-46M was four feet above the waterline because I knew it would splash a little. Ho Boy! I wish I had heard of the floating bubble wrap idea before that. You know, I don’t even remember why I did that either. I think it was something Tom said in one of his posts. I’ll bet my parents wouldn’t let me run around with him when we were youths.

        • Chuck

          I didn’t say that I figured out that it was necessary to float something on the water before I did my first test. It was either float something on the water or wear a raincoat. 😉


  8. BB,

    Interesting results. The groups are good sized, but the smoothbore Dianna 25 would still work well as a plinker at 25 yards. At least for soda cans which I seem to recall are about 3 inches in diameter.

    This makes me want to test that $20 Crosman 760 I got recently and see how it shoots. (It is new old stock, one of the new style Crosman 760s, but it came in a box not a blister pack hence the description.) Its going to have to wait for warmer weather though. (It was in the teens last night.)

    Out of curiousity, if I were to review a gun and submit it for the blog, could the review be posted here and on that my website with the other stuff I’ve written?


  9. I also think it’s tough to draw a conclusion from the small difference in group size between deep seated and flush seated JSB RS pellets.

    It appears there is consistently more lateral dispersion than vertical in all of those groups. Interesting.

    I’d be tempted to try some fat pellets, like wasps and superdomes, in that smoothbore at 25 yards.


  10. Wow, those groups are so…big. Remind me what is so interesting about smoothbores? 🙂 I’m starting to doubt the claim from my blackpowder seminar of the guy who claimed that he shot 1 inch groups offhand at 150 yards with his smoothbore.

    But hey, regardless, my PA order has shipped with JSB Exacts and I am back in business! I can’t wait to try out Victor’s killer shooting technique with my B30, my most accurate rifle. Results continue to be outstanding with my IZH 61 which is shooting as well as the B30 did before. I think we’ve found a live one here.

    Wulfraed, okay, that wasn’t an exact quote from the film. I have searched in vain to find it on Google, but the real one was just about as ridiculous. However, I do remember another line that is unique in film. Karen Blixen finally decides that she wants a divorce from what was a sham marriage anyway in order to marry her British boyfriend, Robert Redford. When confronted, the husband is very agreeable and says, “For the divorce papers, you can give any reason you want. I’ve probably done it.”


  11. BB,
    Crazy idea, but maybe try a wadcutter? Sorry if I missed where you did. Maybe the domes aren’t as completely drag-stabilized as we think… The ball would also be a good thing to try, also, as mentioned.

    • BG_Farmer,

      I have shot a lot of wadcutters in a lot of airguns out to 25 yards. A great group with wadcutters in my shooting was AT LEAST twice as large as those with domed pellets. The crosman premier hollowpoints have been a surprise a few times. To look at these pellets I would assume they would get squirrely and start spiraling wildly long before they reached a 25 yard target.

      What has your experience been with wadcutters at 25 yards?


      • Kevin,
        Though I haven’t shot wadcutters for groups in years, maybe never at 25 yards, I know they are pretty useless past 25 or 30 yards for me also (and only that far out of a much higher powered springer), but I thought with the smoothbore (only drag-stabilized) they might be better than the domes under good conditions (I think BB is testing indoors). I.e., what works in the rifled bore may not be the same as without rifling. Also, “better” might not in this case mean stunningly good groups, just better than domes? So, if he got the group size down to 1 or 1.5″ at 25 yards, it might still be a fun plinker with a little more “range”.

        It would be another datapoint also. My working hypothesis is that the wadcutters have more weight forward, thus are better stabilized by drag and potentially less amenable to spin stabilization. I’m assuming also that the domes have less weight forward and thus possibly are more dependent on spin. Just conjecture, but related to the test, I think.

        I’m almost through my first tin of CPHP’s. They seemed to be a good pick to break in the D34, but I don’t know how good or bad they are by any objective measure. I want to try the Destroyers, also — those are really funky looking. I’ve had pretty good luck with all the Crosman pellets, but they tend to have consistency issues from time to time, so that one tin or a dozen might be great and the next tin abysmal and be full of mismatches (i.e. peewees and oversized).

        • BG, On wadcutter pellets that show accuracy beyond 25 yards, try the JSB Predators in your 34. I have a 34 in .22 with the TO-5 trigger circa 2006 manufacture ,and that is it’s best pellet. I bought those pellets for use hunting squirrels in my .22 D-48 ,but the 48 likes JSB Jumbo’s and RWS Super H pellets better. As for accuracy, the RWS Super H pellets are as good as the domed JSB Jumbos in the 48 out to 40 yards or so.The Super H’s are a wadcutter type pellet also and they load easier than other wadcutter type pellets because they have a beveled edge in front. This can be a big deal with a MSP like my Benjamin LE, as the wadcutters with sharp edged noses hang up when seating them in the breech. As a point of interest, I mentioned firearms cast bullet lube coated RB for smooth bore airguns above. It works in my rifle guns where a bigger ball would be hard to load,or steel BB’s would be un acceptable for use in rifled bores. My Crosman MK 11 .177 CO2 pistol will allow a uncoated clean lead BB to roll right through the barrel with no resistance whatsoever . With the lube(I used alox ,and just rolled the BB in it to coat it) they don’t ,and accuracy was around 1″-1 1/4″ at 10 meters. In this pistol, a magnet would hold the a steel BB at the end of the breech not allowing it to fall out of the bore, but I would never shoot a steel BB in any of my rifled BB /pellet guns. Not outstanding accuracy but interesting to see what little it takes to make a ball stabilize.Thought you might appreciate this as you are a MLer shooter. Lube in airguns takes the place of the patch and even outs the friction in the bore. I have often experimented with a un-patched RB in MLer’s smooth bore and rifled,to see if I could use the tactic for a quick second shot, so that is where the idea came from.

          • Robert,
            I’ll give those recommendations a try, thanks.

            My old 36-2 loves the Copperhead Wadcutters and that is about all I shot for a while for plinking. They grouped as well as 0.15″ at 10 yards at one time, and I liked to shoot things off the nearest fence behind the house, which is about 30 yards from back door. I think what caused me to give them up was when I was trying to be Quigley by shooting a small tomato paste can at (ideally) 100 yards or more (I never got consistently good past 70 or so yards, unfortunately). I really pushed a bunch of pellets and found the trajectories quickly decayed beyond use. Superdomes were the most disappointing, as they were dropping like a rock about 40 yards or so out.

  12. Hello BB, I have a question. I have read about the Mendoza RM-10 in .22 caliber. Is it anything like or basically a Bronco in .22 caliber? Obviously the stock is different but are the works the same, except the bore size? Thanks! Toby

    • Toby,

      Your RM-10 is the rifle that the Bronco was built from. Of course the stock was changed, and here are the changes we made in the action. The muzzle brake was lengthened to twice its length and the hole for oil was eliminated from the spring tube. The fiberoptic sights were exchanged for plain sights. As I recall, those are the only changes we made, other than the markings on the spring tube (like the one that tells you to “Oil Here” around the oil hole.

      The RM-10 that I worked with had the Bronco name on it already and a silly stock that was dropped. We added the rearing horse to go with the name.


  13. BB,And there you are.This is one thing you were going to be looking for;nice round holes.Aren’t they beautiful?–And now I wonder if you aren’t limiting yourself like your brother-in-law who thinks neither you nor your gun is accurate if all the shots aren’t in the very center of the target.What I mean is this;all the shot groups seem to be more vertically challenged than anything.Could it be that without the right twist gyroscopic effect of placing downrange shots to the right,more and more as the distance,might they still hit more centered to the target?The group may enlarge more than if the barrel were rifled.However,for all the ranges between the near and far,no windage compensation would be needed.Isn’t it all in what one wants?Six of one instead of half a dozen of the other?The kill zone would need to be larger at the greater distance,but you would still certainly get it.–Phew…I think I lost some hair on that one!-Tin Can Man-

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