by B.B. Pelletier

Part 1
Part 2
Part 3
Part 4

This rifle is the .177 Diana model 27 I found at last October’s Roanoke airgun show. In the last report, I showed you how to make a leather breech seal, which I did because the one that was in the gun had deteriorated. I cut that seal flush with the breech face, and Vince took me to task for that. So I agreed to install an o-ring breech seal with a thin shim behind it and test it for you to see how it stacked up. Today is that test.

Installing the new seal
Before the new one could go in, the old one had to come out. This time the leather was fresh, so I got the seal out in one piece. I can reinstall it after this test, if I want to.

Vince sent me several o-rings and several thin steel shims to use as the new breech seal. He guessed that one shim was best, but told me what the critical dimensions were in case I wanted to check. I didn’t check because the arrangement he suggested looked so good after it was installed, which took all of 15 seconds. Easiest airgun job I ever did. Four times faster than taking out the leather seal.

The new seal stands proud of the breech face just a little.

Eley Wasp pellets
With the new leather seal, I got two distinct velocity ranges with Wasps. The faster range was from 588 f.p.s. to 620 f.p.s. The slower range was from 242 f.p.s. to 269 f.p.s.

With Vince’s seal, there was only one velocity range. The average was 598 f.p.s., and the range was from 588 f.p.s., to 612 f.p.s. Although the velocity remained about the same as it was with the new leather seal, the absence of the lower range means the breech was sealing perfectly all the time.

RWS Basic pellets
With the new leather seal, RWS Basics gave an average of 658 f.p.s., with a spread from 650 f.p.s. to 666 f.p.s.

With Vince’s seal the average was 643 f.p.s. f.p.s. with Basics, and the range was from 638 f.p.s. to 651 f.p.s. So, the average slipped just a little and the spread tightened up.

Crosman Premier 7.9-grain pellets
With the new leather seal, Crosman Premier 7.9-gain pellets averaged 588 f.p.s. The spread was from 577 f.p.s. to 595 f.p.s.

With Vince’s seal, the average was 605 f.p.s., and the range was from 602 f.p.s. to 614 f.p.s. So, the average velocity improved a little and the spread also tightened.

RWS Superdome pellets
The new leather seal averaged 588 f.p.s. with RWS Superdomes. The spread was from 582 f.p.s. to 601 f.p.s.

With Vince’s seal, the average was 586 f.p.s. and the range was from 577 f.p.s., to 596 f.p.s. So, the average was nearly the same and the spread tightened up.

What have I learned?
Vince’s synthetic seal works measurably better than my new leather seal on this Diana 27. There’s no huge jump in velocity, but the stability improves with every pellet I tried. And the performance with Eley Wasps was most dramatic.

You may remember that I’d promised to tune this gun for you so you can see the insides of a different springer. With Vince’s breech seal, I feel more confident that the results of that tuneup will be under better control than they would have been with the new leather breech seal.

I must also admit that Vince’s seal leaks less air than my leather one. That’s evident from the stability improvement. So, I’ll reverse my decision to put the leather seal back in the gun when this report is finished. In fact, I’m thinking that I should also replace the leather seal in my .22 caliber model 27.