Anatomy of Barrel Bushings
Hello,
We all know how important proper barrel bushings are. Unfortunately AirForce cut some corners with them and they are made from plastic. The problem with plastic bushings and current way to fix them is the co-centric alignment – especially if you accidentally knop the barrel.
Just yesterday I experienced this. I shot 30 shots from 25 meters with decent accuracy and then got little sloppy. I was loading my gun and accidentally hit the barrel against chair. Not hard. Just hold the gun in my hands and turned around little sloppy. POI shifted 10mm immediatelly.
Why did that happen? Well, the current plastic bushings are fixed with couple of allen screws. Longer from bottom all the way to barrel and another one from side. That hit to barrel caused about 0.04mm disalignment in bushings. Thats 10mm at 25meters.
I thought that now is the time for new and better bushings. Made quickly 3D-sketch from what I have seen with other people. This is not my design – just a copy of decent bushing thats been made many times. I wanted to share the pictures with you so that newbies can grasp the idea quickly.
Material is Aluminium. First bushing has venting-holes.
Idea is to lock and fix the bushing against barrel tightly with 3 allen screws from 120 angles. Top screw goes to hole in barrel. Bushing are with very thight fit to barrel. They will be turned little over-sized (26mm).
Assembled and screws tightened to barrel. THEN the barrel will be put in lathe and the bushings OD and O-ring slots are turned. This makes them perfectly cocentric with barrel. The OD has small gap between frame and the aligment is done with O-rings using 20-25% compression. This should avoid the misalignment even with slight hits to barrel which are unavoidable for me at least. Finally the assembly is insterted to frame and fixed with allen screws in frame. These screws will not go all the way to barrel as they used to do but just provide axial locking. There is no thread in bushing for screw – just a hole.
Hope this makes sense and please feel free to provide more tips and comments.
-mike
All Replies
I hear that, things better pick up out here on my end soon or I will be moving in with one of you guys. You guys like kids right?
😯 Tungsten!?!
Why stop there? Go directly to depleted uranium while you’re at it. I hear there are plenty to grab from certain desert 😆
Hmmm…. I wonder how well Uranium machines….. 😆
Yes, that would be a mild concern. I think the profiled area could be left slightly thicker if warranted, but it would only be a stress issue and not an accuracy issue as it’d obviously be before the bushing(s). Also, as we’d be moving the bushing rearward with all these efforts then the turning moment on it would be still reduced.
As with all things, real world data is the acid test. I had intended to see this to production but sadly finances are at a record low 🙁
😯 Tungsten!?!
Why stop there? Go directly to depleted uranium while you’re at it. I hear there are plenty to grab from certain desert 😆
Seriously. You guys keep impressing me again and again. There’s endless of things to improve and try. Well, that was one of the main reasons I choosed AF gun in the first place. Soon I’m gonna spend more time modding the gun than shooting it.
I assembled the bushings and so far looks good. I used pressfitting and heated the Al bushings with industrian hot air blower (whatever that is in english). Temp was about 100-200’C judging from the burn I got on my hand 😀 but the bushings are now REALLY tightly fixed on barrel. If I have used more tighter tolerance there would’ve been new choking on barrel. I bet they don’t need the three allen screws but I left them there to be sure. The OD O-rings are also pretty tight. I got little worried about disassembly when I worked them in. I think that the O-rings also helped to silence the report since there is nomore solid connection from barrel to frame.
And the new hammer with O-ring mod also seems to work so far. Especially it silenced the hammer slap. Got suprisingly more power too since the hammer is few mm longer now and weights 20% more. I was not looking for power since I had plenty of room there in standard setup.
It went from 40J to 60J only with that new hammer 😕 but I will tune it down back to 45-50J which is enaugh for me (now) and nice balance for shotcount. I have 1mm thick O-ring under tophat and tophat adjusted to apporx 2mm stoke.
If it ran on the breech/barrel instead of the frame wouldnt the hammer catch push up on the barrel. I know it wouldnt be much pressure but it would be some.
One piece bushing is exactly what I had in mind when I first got my rifle!
Think it’s already been done by CygnusX. When he first suggested he was going to do it I was happy someone with tools (unlike me 🙁 ) was going to attempt it. Don’t remember hearing anymore though.
I’m sure he’ll chime in soon.
Definitely lends itself to greater stability and could be made shorter to allow longer free floating section.
I’ve also been looking at new ways to move the rear bushing more rearward, eg no preload spacers, and a hammer that runs on the breech profiled area (this needs to be lengthened). This would allow the hammer to be shorter as it would naturally be heavier per unit length. I’d originally wanted Tungsten for this as a 100g hammer would only need to be 15mm in length 😯 but sadly Tungsten is WAY too expensive. I devised another way of reducing hammer length with lighter materials though if anyone’s interested in trying it.
Actually yes. This is also currently in thinktank 😛
One long bushning with solid connection to frame. That would be a free floating barrel setup. It might work or might not. Impossible to say before testing. It might need something extra to kill possible vibration. But me likes the idea (got some nice tips again from Finnish gunsmith for that. Thx Jussi 😉 )
Yesterday I also modified my frame-extender from barrel-alignment to frame-alignment. Not touching barrel anymore from any point. It’s a true frame-extender now with LDC-properties. I think I made a mistake in that design not fully understanding the problems with barrel-connection. I was just looking for perfect cocentric alignment with bore but it’s more important to stay away from barrel.
Furthermore changing barrels would be difficult.
You are correct Marc. P7 is propably overkill. I think I will go for just a light press. It’s difficult to dimension since the barrel OD is not constant all the way. Fixhole aligment is difficult but I have a feeling that since aluminiums conductivity is so much better than steel on barrel it will be possible to provide little heat to it even during assembly without disturbing the barrel.
I made 2 sets of new bushings so I have spare pair for (possible) extra barrel ready. Easier to change barrels with bushings already on the barrel.
And Thanks to all for positive feedback!
-Mike
Mike…
I’ve got nothing constructive to add to the post, just gotta say thanks for the informative drawings. I always liked the pictures in books better than the text anyway… 😆
Really… great stuff. Thanks for taking the time to post.
Shhh….that is one of my ideas 8) Well the cat is out of the bag now. But I thought one long bushing would free float the barrel per se and get rid of the power wheel. I think it would work.
Freefloating barrels are great if the scope mount is actually on the barrel or on the receiver which is solidly mounted to the barrel. I cant see that it would be a good thing to have your barrel be able to move separately from your scope mount. The way our hammer and spring are arranged it makes it almost impossible to do a true floating barrel anyway.
Has anyone given any thought to using one very long bushing instead of two small ones? This would stabilize it inside the frame and keep it from moving allot better than a barrel that is supported at two different points. It would be allot closer to a “floating” barrel arrangement. You could mill long slots down the sides to lighten it up or even do the slots around the circumference in a mock baffle setup. I think I would lean toward the lengthwise slots so you dont lose the rigidity of the one piece design.
I would advice against pressfitting bushings. For one positioning the holes where they belong would be difficult and from experience I can say that a the tolerence you suggest is pretty tight!
Furthermore changing barrels would be difficult. However if you found a setup you never want to change this would be a nice option!
You come up with great ideas!
Regards,
Marc
mm another good o0ne from u mate welldone 😀
THX1138 (Movie with Robert Duvall)
Didn’t PHX1441 (of the exact spelling of his name) do something similar to great effect? He was always making advancements that were importnat but frequently overlooked, such as free-floating barrels and hammer slap attenuators and such.
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I used to like kids, but have since been rehabilitated.