GTA
All Springer/NP/PCP Air Gun Discussion General => "Bob and Lloyds Workshop" => Topic started by: rkr on October 12, 2022, 03:39:39 AM
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Are there any rule of the thumb type guidelines for free floating airgun barrels? What I'd like to know is minimum diameter for certain length, need for CF tubing etc. I believe the caliber will also effect things? I have one practical example, BSA uses 15.5mm free floating barrels and they seems to work real well up to 45-50 fpe power with 18.5" length. When I went to 60 fpe I never got it working as well. I later tried a 24" 17mm barrel and that one was swinging wildly and needed barrel band to make it accurate. This of course may have been caused by breech flexing and not the barrel flexing. I'm now looking at 8mm and 10mm diameter liners in .22 and .172 and wondering what to do with them if I install them in one of my BSAs? Both liners are about 27" long and I have 12mm CF tube for the 8mm liner which seems rather stiff and light. I'm now wondering what to do with the 10mm liner, would 12mm CF tube be enough or should I go 14mm or would it be fine without a liner? With thinner barrels I could use c-clip to lock the barrel in place (not with 14mm liner) and thus be able to index it so I'm kind of looking at minimum viable diameter here.
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I don't know of any rule of thumb for diameter to length.... The stiffness goes up by nearly the 4th power of the diameter.... ie twice the diameter is 16 times as stiff.... :o
Some slim barrels are very accurate at one velocity and/or pellet weight and terrible when you change things....The stiffer the barrel, the less accuracy should change as you change things, until pellet stabilty becomes the issue.... On several of my builds I sleeve a 14mm barrel with a 20mm OD Carbon Fibre tube, which is about the same stiffness as a 3/4" OD steel barrel, at a fraction of the weight.... It increases the stiffness by a factor of 4....
(https://hosting.photobucket.com/albums/oo221/rsterne/6mm Sporter/.highres/Finshed Sleeves and Ports_zps38sbitkv.jpg?width=undefined&height=undefined&fit=bounds) (https://hosting.photobucket.com/albums/oo221/rsterne/6mm Sporter/.highres/Finshed Sleeves and Ports_zps38sbitkv.jpg?width=1920&height=1080&fit=bounds)
Whatever the OD you use, you should insert the sleeve into the receiver at least 2 diameters, and 3 is preferable.... This means that the width of the receiver is usually the governing factor for sleeve diameter....
Bob
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Thanks Bob! So starting with 10mm barrel, 12mm would be twice as stiff and 14mm would be 4 times as stiff. BSA's original 15.5mm would then be 5.7 times as stiff. Gives me something to consider. I could always start with 12 mm or even 10mm and use a barrel band if it doesn't work free floated.
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10mm is going to be pretty much a noodle, IMO.... Crosman barrels are 7/16" (~11mm) and they are on the thin side, which is why Crosman use barrel bands.... I think about 14mm is the lower end for free floating, 16mm is a lot better.... Here is a link to my thread on Metric CF tubing....
https://www.gatewaytoairguns.org/GTA/index.php?topic=164579.0 (https://www.gatewaytoairguns.org/GTA/index.php?topic=164579.0)
In the first post is a link to an eBay listing with an index of sizes....
https://www.ebay.com/itm/134143296862?var=433589556806 (https://www.ebay.com/itm/134143296862?var=433589556806)
You can get both 10 & 12 mm ID with a 14mm or 15mm OD.... and a 12mm ID with 16mm OD.... Figure out how big you can drill the receiver for the CF sleeve to slide into before you make your decision....
Bob
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CF pipes are actually a bit cheaper from aliexpress, at least delivered here to Europe https://www.aliexpress.com/item/1005004160356196.html?spm=a2g0o.productlist.0.0.3c587c091pRqD7&algo_pvid=c1b19185-d2bc-4ef6-9f0c-1635b988d724&algo_exp_id=c1b19185-d2bc-4ef6-9f0c-1635b988d724-6&pdp_ext_f=%7B%22sku_id%22%3A%2212000028234246464%22%7D&pdp_npi=2%40dis%21EUR%217.97%217.97%21%21%21%21%21%402101d91e16639263300068140e9721%2112000028234246464%21sea&curPageLogUid=dWV3VXXucsUv (https://www.aliexpress.com/item/1005004160356196.html?spm=a2g0o.productlist.0.0.3c587c091pRqD7&algo_pvid=c1b19185-d2bc-4ef6-9f0c-1635b988d724&algo_exp_id=c1b19185-d2bc-4ef6-9f0c-1635b988d724-6&pdp_ext_f=%7B%22sku_id%22%3A%2212000028234246464%22%7D&pdp_npi=2%40dis%21EUR%217.97%217.97%21%21%21%21%21%402101d91e16639263300068140e9721%2112000028234246464%21sea&curPageLogUid=dWV3VXXucsUv)
I'm not very keen on drilling out the breech as it is already a bit flexible and I prefer the possibility to use it with BSA barrels in the future as well. 10mm ID 14mm OD CF pipe with 15.5mm sleeve at the breech end sounds like the way forward. Getting the liner will take some more weeks and I need a .172 mold before I can proceed with this build so it will be slow going for now. Thanks for the advice Bob.
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A 14mm sleeve would be a good choice.... Note on that Index you can also get a 10mm ID x 15mm OD.... You can also order TJ's liners with a larger OD, I have a .172 cal with a 14mm OD, so I know they can do that.... In fact, they have a short sample of 14mm ID CF tubing I sent them to check the OD while they are hammer forging it.... The right hand barrel in the photo above is the .172 cal TJ's with a 14mm OD.... Before you order anything, talk to TJ's about barrel OD's available....
Sleeving to 15.5 mm so that a stock barrel will fit is a good idea, but of course if you had a 16mm OD sleeve, you could turn that down to fit the breech.... ;)
Bob
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A 14mm sleeve would be a good choice.... Note on that Index you can also get a 10mm ID x 15mm OD.... You can also order TJ's liners with a larger OD, I have a .172 cal with a 14mm OD, so I know they can do that.... In fact, they have a short sample of 14mm ID CF tubing I sent them to check the OD while they are hammer forging it.... The right hand barrel in the photo above is the .172 cal TJ's with a 14mm OD.... Before you order anything, talk to TJ's about barrel OD's available....
Sleeving to 15.5 mm so that a stock barrel will fit is a good idea, but of course if you had a 16mm OD sleeve, you could turn that down to fit the breech.... ;)
Bob
Sleeving 15mm barrel for 15.5mm breech would be very hard if not impossible so I'm looking at 14mm tube. Double tubing to 16mm is also worth consideration but with 30 eur shipping charges for a tube it gets costly.
Unfortunately TJ does not ship to Europe so I'll go for LW, https://www.lothar-walther.de/barrel-blank/rifle/rimfire/cylindrical/1213/barrel-blank.17rem/.17hmr-od-10-mm-l-715-mm-cr-moly-steel?c=34 (https://www.lothar-walther.de/barrel-blank/rifle/rimfire/cylindrical/1213/barrel-blank.17rem/.17hmr-od-10-mm-l-715-mm-cr-moly-steel?c=34)
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I had no idea LW even made a .17 cal blank with such a small (10mm) diameter.... It is also on the US website....
https://www.lothar-walther.com/gun-barrel-blanks/rifle-barrels/rimfire/cylindrical/406/barrel-blank-twist-9.17rem/.17hmr-od.39-l-28.15-cr-moly-steel?c=34 (https://www.lothar-walther.com/gun-barrel-blanks/rifle-barrels/rimfire/cylindrical/406/barrel-blank-twist-9.17rem/.17hmr-od.39-l-28.15-cr-moly-steel?c=34)
Also on the US website is this .22LR barrel with a 12mm OD....
https://www.lothar-walther.com/gun-barrel-blanks/rifle-barrels/rimfire/cylindrical/420/barrel-blank-twist-16.4.22l.r.-od.47-l-27.17-cr-moly-steel?c=34 (https://www.lothar-walther.com/gun-barrel-blanks/rifle-barrels/rimfire/cylindrical/420/barrel-blank-twist-16.4.22l.r.-od.47-l-27.17-cr-moly-steel?c=34)
Both are of interest for airguns and sleeving with CF tubing....
Bob