I picked up one of these at Roanoke, what a wonderful gun! I've had my 27 for a little over a year now, and I thought IT was the ultimate Diana breakbarrel. Now, I'm not so sure. The 35 is more of a full-sized rifle, but it has the same well-balanced feel as the 27. I don't mean balanced in terms of how it shoulders, (although that's nearly perfect) but I mean balanced in terms of design, ergonomics, power-to-weight, trigger, firing behavior. These two guns just seem to have gotten it right.
Jim, I had much the same revelation...I love my 27's (I have 3, each from a different era), but the 35 is really a sweetie.
If you hold a modern 27 and 35 side-by-side, you will discover that the receivers are exactly the same length. The cocking geometry and stroke length are likewise the same, the 35's extra power comes solely from its larger-diameter piston.
........I'm going from memory here, but I'm pretty sure the bore on the 35 is 28 mm vice the 25mm of the 27. The stroke on the 27 is longer however, so that the swept volume on the two guns is the same.
I'd guess that the power difference you're observing may come from barrel lead in of the two guns as my 35 and 27 shoot exactly the same velocity. However, that is with JM springs in both guns, the 31 coil BSA Airsporter mercury in the 35 and the Global Tarantula XL small in the 27.
Well, since you bring it up--1958 Diana 35 vs. 1980 Diana 27:
Overall receiver length, front of breech jaws to back shoulder of rear cap: 35 is 352 mm, 27 is 342 mm due to diffent attachment detail for the cap.
Outer receiver diameter: 35 is 32 mm, 27 is 29 mm.
Center front stock screws, to center of main stock screw (front of trigger guard): 213 mm for both guns.
Center of barrel pivot, to center of front trigger/sear assembly retaining pin: 312 mm for both guns.
Center of barrel pivot, to center of cocking link pivot: 48 mm for both guns.
Horizontal displacement of cocking link pivot during cocking (from first contact with piston to "rebound" position after trigger engagement): 72mm for both guns.
Now these are only "eyeballed" measurements taken externally, and I defer to your much more detailed anatomical knowledge of their innards, but I stand by my initial statement.
My guns are different calibers, so I can't compare velocities directly. But speaking from previous experience, with factory springs a healthy .177 model 35 shoots roughly 40 to 50 fps harder than the same caliber 27 with most pellets.
To my mind the 27 is the more efficient of the two actions, and I think modern airgun design has demonstrated that more stroke is a better way to increase springer power than a fatter piston. But on older designs like this, the manufacturer often just "fattened up" a design to get more power out of it. Another classic example is the Weihrauch HW 50 and HW 35 series, which again share longitudinal dimensions but differ in diameter.
......the barrel on my 35 swings through about 30 degress of cocking arc before coming into contact with the piston and starting it's stroke. Does yours behave similarly?
It wouldn't be the first time someone subbed a piston in a gun. There's no question that the firing stroke in the 35 I have is about 10mm shorter than the 27 I have. Now ya got me wondering............
Just eyeballing things (we arkiteks love to do that), both guns drop the barrel about 15 degrees maybe before hitting the piston. Nowhere near 30 degrees. On both of 'em, you can see clearly when the link hits the piston, thanks to the view through the generous slot in the stock.
BTW the way I took these measurements, was to make two long perpendicular intersecting lines on a piece of cardboard. I then lay the gun down on the cardboard--horizontal line along the top of the receiver, and vertical line through the center of barrel pivot. The center of the link pivot was marked at first piston engagement, and after cocking; then the distance between the marks measured parallel to the receiver line.
Unless I was asleep that day in high school geometry, that distance should equate to the piston travel.
I also looked at one of my old Diana 50's while I was doing this little exercise, and noted that the piston travel seems to be about 5mm longer than the model 35? Since the 50 is notoriously less powerful than the 35, this surprised me. I suppose this says something about efficiency loss through the lengthy transfer port and tap, that soak up space between the 50's piston and pellet?