Marksman model 70 breakbarrel air rifle: Part 11
Marksman model 70 breakbarrel rifle.
This report covers:
- How to proceed
- What did it do?
- Cocking effort
- What have we learned?
Today we do the last experiment of increasing the preload in the Marksman model 70 air rifle. The next report will be with a new mainspring.
We have discovered that the most powerful state this rifle was in has been the state after I cut off the end portion of the mainspring that was bent. Then I removed almost as much mainspring again to bring the rifle down to very little preload.
After adding in a half-inch of preload by putting 8 washers inside the piston the power did increase, but not to its previous high point. Here is a reminder of where we were. All these shots were with RWS Hobby pellets.
10-shot average with extreme spread
Fac. Vel………..Cant spring cut……..More spring cut……..half-inch preload
715 (15 f.p.s.)….746 (12 f.p.s.)……….606 (22 f.p.s.)………689 (12 f.p.s.)
28 lbs……………..22 lbs…………….………20 lbs……………..…….26 lbs.
With most of the preload gone, this is where the threaded bushing is.
The 8 washers that I put in the rifle last time measured 0.456-inches in height. As you can see in the picture above, there was about 3/8-inches of preload on the mainspring at its shortest. The half-inch of the 8 stacked washers brought the spring out to what I am calling a half-inch of preload, but it’s clearly a little more that that.
With the half-inch of spacers, this is where the threaded bushing is.
How to proceed
Reader GunFun1 said I should use a lighter spacer to get more preload, because adding 8 more washers inside the piston would add weight that would change the performance characteristics. I had also thought about that and knew he was right. But my garage is not a machine shop. He offered to make a spacer for me, but I held off. I thought I could find something around here that would work. And I did.
I found a plastic part from which I was able to make a spacer. It isn’t perfect, but I was able to get it to go over the piston rod. This is not a tuneup. This is an experiment to answer the question of what happens when you increase the preload.
The plastic spacer is so bright that you almost can’t see the hole through its center. I drilled it out to fit the spacer over the piston rod.
How much more preload is there with the spacer and two washers? It’s a little over one inch. Let’s see.
I forgot to put the threaded bushing in this picture but it doesn’t take much of an imagination to see where it would be. Clearly more preload has been added.
What did it do?
This is the big question — the entire reason for this test. I would never leave an airgun set up this way. I just wanted to see what doubling the preload would do to the power.
The first three shots with Hobbys were 758, 740 and 728 f.p.s. Then I got a spread of 10 that went from a low of 731 to a high of 746 f.p.s. The average for the entire 13 shots was 740 f.p.s. The average for the last 10 shots was also 740 f.p.s. Only the spread of the 13 shots was higher (30 f.p.s.) than the spread for the last 10 shots (15 f.p.s.) I would say that 740 f.p.s. is a good average for the way the mainspring is preloaded.
The cocking effort is now 28 lbs. Let’s now look at what has happened over the course of all the work that has been done.
10-shot average with extreme spread
Fac. Vel………..Cant spring cut……..More spring cut……..half-inch preload…….one-inch preload
715 (15 f.p.s.)….746 (12 f.p.s.)……….606 (22 f.p.s.)………689 (12 f.p.s.)…………740 (15 f.p.s.)
28 lbs…………..…..22 lbs……………………20 lbs…………………….26 lbs……………………….28 lbs.
What have we learned?
From this simple series of tests we have learned that having the mainspring alone do the job of preloading gives the most power — by a slight amount. The best power with the spring alone was 746 f.p.s. with Hobbys.
The spring alone also gave us the lowest cocking effort, with the best power. The spring alone cocked with 22 lbs. of effort. The max preloaded spring (in this series of tests) cocked with 28 lbs. of effort — the same as the rifle from the factory.
What that tells me is that it’s best to use as much of the spring as you can, when trying for an optimum tune. You can get back most of the power when the spring is cut shorter by preloading, but at a cost of greater cocking effort. On the other hande, thin spacers can be added to nudge velocity up a bit. Keep them on the spring guide end and they don’t affect the weight of the piston.
This was just one simple test, and you see how much time and effort it took. Airgunsmiths learn these things the same way through their experience working on many similar airguns. They learn exactly what works best for certain models, which makes the time they spend to get what they want with one gun a lot less than the time you see here. But this kind of work is where they they got that experience.
I hope this series has been a learning experience for many of you. We still have things ahead of us with this Marksman model 70. The next step will be to find a good mainspring and tune this rifle nicely. We now have a chart of where it has been and what it is capable of, within reason, and I will use that as my guidline when I install the new spring.
Besides finding a good tune for the rifle I plan to install that other Perfekt trigger that came with the rifle. There is a whole story of why that trigger was in the box and I plan to share that with you in the future. Stay tuned.
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