Pellet Probe Centering, Cocking Knob Position & Resistance Fixes
Here is how I checked and adjusted the centering of my pellet probe and barrel. I always noticed that the probe engaged a little off center then shifted as it entered the barrel. My probe is NOT bent, nor is my probe body. In my gun, the misalignment is simply due to the way the cocking rod aligned the probe body. There are about 3-4 mils of play in the probe body housing.
Some aluminum tape over the barrel lets you impression the barrel opening and pellet probe contact. Burnish the tape with your finger tip over the barrel until you can see a clear impression of the bore. Then bring the pellet probe forward to touch the tape. Now you have an impression clearly showing how well centered the probe is engaging barrel.
There are two major directions of motion you can make. Releasing tension of the cocking rod screws and positioning the probe body will let you make small changes in the direction indicated in red. Shimming the spacer lets you make changes in the direction indicated in green.
It is easier to make the adjustments if you remove the side bolt. That eliminates the trigger cocking tension. On my Vulcan, I needed to make changes in both directions of motion. It took two layers of aluminum tape to shim the rear of my probe body spacer. Then, the cocking rod was screwed on with the probe fully forward (hence centered) in the barrel. That set the alignment in the other plane.
Here are my probe impressions before and after adjustment. The top impression is post adjustment.
Now that the probe was centered with the barrel, I could check how it centers with the magazine and pellets. Impressioning a pellet was done by blocking the exit of the magazine with a piece of thin, sheet metal. Bringing the probe forward to touch the pellet makes the impression. Checking the magazine itself was done by disassembling the magazine and putting a piece of aluminum tape at its exit.
The hole of the magazine is well centered, but clearly the pellets are not centered within the magazine. The magazine over-rotates the pellets before stopping. If the magazine index as a couple mills thicker, the centering would be improved.
NB, when doing these adjustments, this is a good time to check that the shoulder of the pellet probe does NOT poke forward (out) of the breach during the cocking cycle. If it does, the forward end of the cocking cycle is much more difficult due to the probe shoulder hitting the magazine. You’ll need to adjust the cocking rod or the cocking bolt rotation to get the relative lengths and positions just right.
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Here are some picts to better illustrate adjustment of the cocking rod length / cocking knob position. If the cocking rod length is too short, forward motion of the cocking knob meets heavy resistance as the pellet probe body impinges against the magazine. If it is too long, locking the bolt back becomes very difficult or impossible.
Look at the u-slot and notice there is a bulge on each end of the central portion. You want to adjust the cocking knob position such that just as the SHAFT of the knob reaches the furthest forward portion of bulge, the probe body remains a tiny bit recessed into its guide. Impingement against the magazine makes the cocking cycle difficult and mechanically stresses the entire cocking mechanism.
Adjusting the knob position / cocking rod length is done by rotating the knob mount. You can select it in 1/4 rotation increments. If adjustment range is insufficient using just rotation of the knob mount, you’ll need to undo the rear end of the cocking rod. Then, thread the rod in/out to change its length.
When properly adjusted on my Vulcan, the forward motion of the cocking cycle meets light resistance even from the stout, stock spring.
A couple other things have also been done to make the cocking cycle smooth and quiet. I polished the u-slot edges, coated the cocking mechanism with moly dry lube, added two layers of electrical tape to quiet the clacks the cocking mechanism normally make as you move it forward and down. Result is smooth cocking cycle with little noise. My Vulcan’s cocking cycle is now so quiet that the two loudest noises are the click of the magazine indexing to next pellet and the sear engaging.
If you look carefully in the picts, you”ll see the electrical tape (2 layers thick) on the u-plate.
Also, my steel picatinny rail shim is between the rail and cylinder clamp assembly. That shim let me set rail elevation & windage so the scope doesn’t need many clicks.
Thank you, kris. Thinking about doing the same.
Guykuo,
Excellent findings, explanation, & post.
Many Thanks Sir…
I have replaced the spring and yes it improves it further. it was something from remote controlled car model/dumper spring, but you need to get the right thickness and size. mine is half the thickness and works much better, it is plastic coated, but cant find the supplier sorry.
if you get the lightconditions right, with no pellet loaded obviously.
if you look down the barrel from the muzzle end. you can see the pellet probe entering/alignement.
I used to put some paper and direct direct light. something like a LED torch or so, and can see what is in the barrel. the torch is on the breech side.
Well done Guykuo!!!.
Thanks for very very useful post.
Thank you for sharing, well illustrated and laid out and important info . I will certainly check my setup just to make sure.
Wondering if anyone replaced their forward cocking spring with a lighter one?
[Edited to clarify]
Hopefully you can narrow down what your problem is by going through the inspection & adjustment process.
Just keep in mind that there can be centering, o-ring, and also cocking knob/cocking bolt length & position issues.
The o-ring and centering issues appear when the probe’s narrow brass just reaches the barrel or o-ring, not the extreme ends of the cocking motion.
The cocking knob and cocking bolt issues will appear at the extreme forward and backward ends of the cocking motion.
Check that on most forward motion, the shoulder of the pellet probe body (the widest part) does NOT move forward and jam against the magazine.
On rearward motion, the trigger should cock before the bolt is all the way back. If either of these condition fail, try to adjust by changing which hole the cocking knob is in (rotation of the piece into which it screws changes the distance. If that doesn’t give you enough range, you can disconnect the cocking rod from the probe body and screw the rod in/out to change its length.
Another GREAT write-up and pics Guy! Probably explains why my .25 Vulcan is still not as smooth cocking (forward end cycle) as others even after 3000+ pellets thru it.
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