Marksman model 70 breakbarrel air rifle: Part 8
Marksman model 70 breakbarrel rifle.
This report covers:
- The trigger
- Cut off the spring
- How much preload remains?
- One last thing
- Where are we now?
- Cocking effort
- No suction test
Today we remove the rest of the mainspring of the Marksman model 70 breakbarrel air rifle so no preload remains.
Because of the things I learned in Parts 6 and 7, the disassembly this time went fast and easy. As you recall, this rifle’s trigger unit is removed before releasing tension on the mainspring, which is held in place by a bushing.
The trigger is connected to the bushing by an anti-beartrap device. That is connected to the safety. It must be removed before the trigger will come out of the rifle. Let’s look.
Before the trigger can be removed, the anti-beartrap device must be removed. The trigger’s safety has a pin connecting it to the anti-beartrap (arrow).
When the circlip is removed the anti-beartrap slide and the spring above it can be lifted off the threaded bushing, freeing the trigger. See the three strips of galling on the beartrap slide? When I replaced the trigger I greased that spot with moly grease.
I vowed to measure how much preload was still on the mainspring from the first cut, so when I backed out the threaded bushing today, I measured where it stopped. There was way more preload than I told you in Part 7.
When I wrote Part 7 I thought there was still about an inch of preload remaining. But when I measured it today, it came to 1.826-inches. Now, my method of measurement is just a close approximation — from the center of the threaded hole in the bushing to the center of the hole the bolt passed through, and the base of the spring guide adds something. What I’m saying is don’t pay attention to the thousandths or even to the hundredths. But still — this is quite a bit more preload than I thought.
The mainspring is fully relaxed. This measurement is not precise, but there is easily more than an inch and three quarters remaining on the mainspring.
Cut off the spring
Out came the Dremel and I cut off most of the length I needed to lose, knowing that I would grind the end of the spring flat and crush the last coil. So a little more length would be removed.
A lot more of the mainspring was removed.
The first piece of spring I removed is on the left. This last piece that was removed in this report is on the right.
As you can see, almost as much spring was removed this time as last time. But there is still one more thing to do. Lube the spring lightly with Tune in a Tube then insert it and see how much preload remains, if any. I hope the spring is as close to right on but no less than that. I don’t want it loose when the gun is at rest.
This may look like something that is measured in the thousandths of an inch, but when you do it — cut the spring, grind the end flat and then crush the last coil — all the “precision” goes out the window. Yes, I believe someone who does this all the time may do a better job than me, but I doubt that anyone ever gets it right on without a little luck.
Used an angle grinder
I cut off the spring with a Dremel, but used an angle grinder with a cutoff wheel to flatten it. It’s just like a Dremel, but with a little more horsepower.
How much preload remains?
I got the mainspring down to about 3/8-inch of preload. That’s where we are right now.
This is the preload that remains. I’d say it’s about 3/8-inch.
One last thing
Before we test the rifle with the new shorter mainspring, let me show you one more thing. The threaded bushing that the mainspring and spring guide push against has two sides. The threaded hole is centered in the bushing but it doesn’t go all the way through, so there are only two ways the bushing can be installed. Let’s look.
Side 1 of the bushing.
Side 2 of the bushing. Which side do you think the spring guide pressed against?
Where are we now?
The rifle now fires dead calm. And the forward thrust has been reduced noticeably. It’s still there but the rifle is much smoother.
In the first velocity test with the rifle as it came from the factory and with the canted mainspring, RWS Hobby pellets averaged 715 f.p.s. with a 15 f.p.s. spread. After the first spring cut the velocity averaged 746 f.p.s. with a 12 f.p.s. spread.
Today, after the second spring cut, the velocity averages 606 f.p.s. The velocity spread went from a low of 595 to a high of 617 f.p.s., which is a difference of 22 f.p.s. So shortening the spring to almost no preload dropped the velocity 140 f.p.s. and boosted the spread by 10 f.p.s.
Now we have a baseline for this rifle. The couch commandoes will say that adding 1.75-inches of preload will bump her back up to 746 f.p.s. But there is about that much less spring in the airgun. We have lost almost 5 full coils of spring, and I think that will have something to do with the velocity, regardless of whether there is the same preload, or close to it. I guess we won’t know unless we try it.
From the factory this rifle cocked with 28 pounds of effort. After the first spring cut that dropped to 22 pounds. It now cocks with 20 pounds of effort, with a spike to 23 pounds when the trigger is cocked.
No suction test
GunFun1, I purposely did not do your piston seal suction test because a parachute seal acts best when it’s pushed forward and its rim is filled with air. I understand what you are doing with your test, but I just didn’t think it was one I wanted to do. It has been my plan from the beginning to replace the piston seal, once I find a good tune for the rifle. We aren’t there yet.
I never imagined in the beginning how much fun there would be in this old rifle. I am learning so much from our experiments. I hope you are, too.