Marksman model 70 breakbarrel air rifle: Part 10
Marksman model 70 breakbarrel rifle.
This report covers:
- History of this report
- A word about the washers
- The lesson
- Shot cycle
Today we add shims to the Marksman model 70 mainspring, to see what that does.
History of this report
This Marksman model 70 breakbarrel air rifle was made in the 1980s by Weihrauch from both their parts and the BSF parts that they acquired when they purchased the company. In past reports we have seen many of the rifle’s strange design details.
We have also tested the power of the rifle as it came from the factory, then again, after the canted spring portion was cut away. Then once more after another large portion of spring was removed to get the rifle to almost no mainspring preload. Let’s look at the performance of those past reports now. The same RWS Hobby pellet that I have used in every previous test will be the pellet I use today.
10-shot average with extreme spread
Fac. Vel………..Cant spring cut……..More spring cut
715 (15 f.p.s.)….746 (12 f.p.s.)……….606 (22 f.p.s.)
28 lbs……………..22 lbs………………….20 lbs.
So that is where we were. To add preload to the mainspring, I shimmed it with common flat washers. I used 8 washers that together add about a half-inch of preload.
Eight washers were about a half-inch high.
A word about the washers
The washers I used were common flat washers with a 3/8 hole. The hole wasn’t big enough to go over the spring guide but it did go over the piston rod. So the washers went into the piston. That did add some weight to the piston.
Luckily the 3/8″ washers fit over the piston rod.
These washers are stainless steel. I had no choice in that. They are punched out of sheet steel, so one side is rounded and smooth and the other side is flat with tiny ridge marks. I put them into the piston with the flat side down (going into the piston first) and the rounded side toward the mainspring.
I lightly lubed each washer with thin grease, so they would slip and slide when the gun fired. I have no idea if that’s good or not. It just seemed like the thing to do.
When I last cut the mainspring the preload on the spring was still about 3/8-inch.
This is how much preload remained on the mainspring from the last spring cut.
And this is how much preload that half-inch of washers made.
Let’s look at what this amount of preload did to the rifle’s performance. Remember, a lot of mainspring was removed and those washers that added the space back in don’t add anything to the power of the spring, other than what the preload offers.
Ten RWS Hobby pellets averaged 689 f.p.s. The spread went from 682 to 694 f.p.s., so a difference of 12 f.p.s. The cocking effort is now 26 lbs. Let’s see how this compares to where we have been.
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.
The lesson today is that if we had remained with the spring after the cant was removed by the first spring cut, the rifle would now average around 746 f.p.s. and require 22 lbs. of cocking effort. Adding shims to the short mainspring brought back 83 f.p.s. of velocity that was lost when the spring was cut the final time, but the cocking effort increased by 6 lbs.
Looking at it from the other side, cutting the mainspring and adding the shim removes 57 f.p.s. of average velocity (from 746 to 689) and adds 4 pounds of cocking effort. From a performance standpoint, shimming the shorter spring hasn’t returned all the power that cutting the spring took away.
The rifle now shoots dead calm. It’s as nice as Michael’s Winchester 427, which is the smoothest spring rifle I have ever tuned. It just goes “Thuck” when it fires — not even “Thunk.” With Hobbys it’s nice and smooth but with H&N Field Target Trophy pellets there is a tiny buzz at the end of the shot. It’s almost unnoticeable.
Don’t worry about any of this because this mainspring wasn’t going to remain in the rifle anyway. It’s old and tired. A fresh new spring will probably do wonders for performance. But as long as we are here I think I will shim the spring one more time to see what that does.
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