GTA
All Springer/NP/PCP Air Gun Discussion General => Machine Shop Talk & AG Parts Machining => Engineering- Research & Development => Topic started by: AmBraCol on June 15, 2013, 12:47:00 AM
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Reading on the forum the various builds that are being/have been done by different folks has been interesting. Two things keep popping up in build after build.
1. Transfer port - how big can you make it and not have your pellet fall into it or get messed up as it gets pushed over it.
2. Bolt probes - what shape gives best results, how long should it be, etc. So, what if a breech was built that would eliminate BOTH these issues?
This has been percolating in my mind and I figured you guys would be the ones to ask whether it would be remotely possible or worth pursuing. As I contemplate options for obtaining accuracy I was hit by the idea that I've been holding what may be the answer right in my hand - a Baikal IZH-46M. It uses no bolt probe and the transfer port is behind the barrel with a block that seals over it and to the rear of the barrel to swoosh the air from the valve right into the base of the pellet. So, there's the basic idea. It wont work with a magazine, at least not with one that depends on a probe to seat the pellet. I'm going to try and type this up in a reasonably coherent way.
- The idea is to make a "riser" breech - but that has the barrel ends in front of the transfer port, the distance perhaps to be determined by the caliber of the barrel.
- A block with a hole drilled in the bottom that matches up to the transfer port and an intersecting hole that matches to the rear or the barrel at a right angle to the first hole (this could be rounded inside, perhaps, to give a smoother transition for the direction of the air) would be fitted with seals for both surfaces
- A lever apparatus would be affixed to the rear of the block to hold it into place, thus assuring the sealing of the air as it passes from the valve to the rear of the pellet
- Perhaps this lever could be arranged in such a way as to also cock the hammer/striker (the bottom end would be Disco/22XX/Marauder in function) or perhaps the hammer would be cocked by another means.
My idea is that the pellet would be inserted into the rear of the barrel, just like in a Baikal IZH-46M and other target pistols. The moving block would then allow the air to be transferred from the valve to the rear of the pellet without the pellet having to pass over the transfer port and without a probe to interfere either. There would be a need for some kind of camming action to lock the block into place and perhaps a bolt could be affixed to the side to allow the block to be raised and lowered into place as well as to pull the block to the rear and cock the gun.
I can see most of it in my mind, but have not yet learned to draw things in CAD or much on paper for that matter. Does anything I wrote seem to make sense to those of you with more experience in design and building of air guns?
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1. If you are using a simple drilled (ie round) barrel port, you can use 75% of the bore size.... ie 0.162" on .22 cal, 0.187" on .25 cal.... The stock 0.134" on a .177 cal Crosman barrel is as big as you should go.... If you are very careful about deburring, and a bit lucky, you MAY be able to go to 80%.... To go bigger than that, I make my barrel ports oval, ie I keep them at those widths, but lengthen the front of the port on an angle.... By doing that, you can get the port to equal the bore area, but it requires care, particularly in how you blend the port into the transfer port within the thin barrel wall.... I would really suggest you stay to the 75% rule, and then match all the other ports to the same area, including the throat of the valve.... Since it has the stem in the way, you usually have to use a combination of slimming the stem and drilling the throat to achieve that....
2. If you are usng the 75% rule for the transfer port, the bolt probe can be up to 25% of the bore in diameter without causing a restriction to the flow.... eg. on a .22 cal, you can use a probe up to 0.109", and still have the area for flow greater than the barrel port.... Even though that's what the math says, that looks a bit fat to me, and I usually use 3/32" (0.094") on a .22 cal, that's plenty strong enough and the end is large enough to not damage the inside of the pellet skirt, especially if you round it.... I make the length long enough to seat all pellets just past the barrel port....
Check out how the USFT breech works.... it may give you some ideas.... The biggest problem you need to address is how to keep the wasted volume to a minimum.... All volume between the valve seat and the base of the pellet is a place for the air to expand and drop the pressure before the initial pulse starts to accelerate the pellet.... causing a loss in performance and efficiency....
Bob