.308 barrel
What kind of barrel blank did you guys get that have done a .308 conversion ? Just a lothar walther barrel blank?
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Viewing 13 replies - 1 through 13 (of 13 total)
Hey Lasse, thanks for chiming in… Yep, been reading your emails, but I figured out the critical volume inside the breecdh needs to be kept as small as possible by studying the original barrel. I couldn’t figure out why they would design it so slender where the breech slides over it; bad design, asking for a bent barrel; then after watching how the breech works it became obvious that the thicker the barrel under the breech, the more ‘wasted space’ exists that must be pressurized for each shot, so it amounts to wasted air each shot. Making the breech shorter also has the same effect of increasing efficiency, but you have to juggle the distance your valve sticks out from the bottle to accomodate the shorter breech. I haven’t had time to work on mine, by the way; too busy. I just bought a new 6 jaw chuck which is huge, 8″ diameter, and I”M still trying to mount it to my lathe. Have to mill new mounting holes in the backing plate. And I have no mill!! DAMN. I’m welding up a fixture today so I can bolt up my backing plate to my tiny little milling attachmnet so I can use my lathe to mill those holes and slots.
One measure of how efficient your design is, will be to keep tabs on how thick your barrel walls are under the breech… The stock .22 barrel I have from Airhog has walls about .046″ thick… so normally I’d say that since the manufacturer uses this dimension, you should follow suit and not have your walls thinner than this; but it’s up to you. Thicker means safer, no chance of a blowup when you fire it; but with safer also comes more wasted air. I have mine at around .060″ thick, which to me means safety due to extra strength with acceptable decrease in efficiency.
As for 308; I like 308 because its easier to keep a 308 quiet than a 45; but having a subsonic 308 means you have to take headshots., which can get difficult if your pig happens to be running. That’ s where I’d love to have a 45 instead. Hey! Good thing I bought Cygnus’ .45 setup! Doh!
walt
seem like someone has been listening to what i said…hehe
you want to keep the breech part of the barrel as thin as possible, this will reduce resistance when the air has to enter barrel, and you will get less pressure loss and turbulence in the breech.
i made my breech slightly shorter than stock, about .25″ shorter…this allows for longer hammer travel, doesnt sound like much, but it does give a bit more power.
i belive i used 10mm orings in the breech, maybe 10.2mm bought 2 bags from mcmaster
definatly use aluminum for the breech, 1 its safer, 2 your gonna want to beef up the spring and that means the cocking handle has to be beefed up too including where it screws into the breech
the huntersupply bullets are hardcast…so they like to get stuck if you rifle doesnt have the power, try some purelead slugs instead….i found a guy that make them for DAQ’s in 83 and 119 grain, and these produce more power
hammer weight….i started out with a thickwalled brasstube…you dont need to cut it in a fancy shape like AF does, keep 2 riding bands one at each end and remove a little bit inbetween…i took off about .005″…the game is to keep the hammer as heavy, start out with 100grams, and see if you need more later on, my .457 hammer ended up 150 grams.
finally i understand the fascination with .308, better BC than 9mm, but consider .45 or .50
the larger bore is more efficient, giving greater power, with same amount of air as .308, they both go boom when you shoot them, the .45 just has more power, theres a shitload of different .45 ammo, in different sizes from .447-.459 your bound to find one that will work in your rifle, the roundballs weigh in at 141-143 grain, and i was able to push them to 1100 when the rifle was set for power…and about 10-15 shots at 900 fps…that will give you range out to 200 yards, and as most bigbore slugs they will carry most of the energy out there too
you can get a barrel liner from http://www.trackofthewolf.com/(S(ruf4jgbjaxmu4mmnb15f3u2r))/categories/tableList.aspx?catID=14&subID=142&styleID=483
they cut the liner as long as you want it…and they come in 5/8″ which is 15.9mm, allmost the same as stock .22 condor barrel,they are thick enough for airgun stuff…
I just want to clarify that I dont beleive I said I cut at Random and that wasnt the advice I gave. I said I didnt pick a measurement for the breech OD and then try and find an o-ring to fit that. I found an o-Ring and worked off that measurement. I obviously checked the o-ring grove depth agaisnt spec sheets and made a functional .308 because its there in the pics for all to see.
You can use any good quality 308 barrel. I used an old M95 Mauser that I had lying around. If you order one from midway, get the F34 contour, it’s less to cut down. You can’t design a gun by cutting at random, study the existing breech and barrel and design yours by imitating that and understanding what the various cutouts are for. O ring manuals exist online for free, that’ where I got my info from…. that give you dimensions to use for various O ring sizes when cutting grooves for them. As a rule, use the largest diameter O ring you can fit, it makes moving the breech on and off easier due to the larger rubber diameter having a slower taper. You will have to use a good silicon grease. You should see my bluprints for my 308 bbl… scribbled all over, changed alot of the dimensions on the fly due to wanting to make sure the O ring grooves didn’t bottom out against the front cutout or bolt hole. You want to make the part that the breech slides back over, as THIN AS POSSIBLE to reduce turbulence and increase efficiency; otherwise your high pressure air coming out from the valve will just be wasted in all that extra space in the breech.
Here’s a group I took –
Unfortunatly I dont know what distance I shot that at. My original post (all of them..) were deleted on TOG…. I never took it to a range, so its going to be somewhere between 10 and 40 yards….. I’d guess on the low end.
Hey YN! What kind of accuracy did you get with the Sabot rounds?
MIke
I am going for Big Bore, and .25 isn’t that. I think I am going to end up making my own valve. I was planning on something around 300 fpe, so a custom valve might be my only option.
No problem at all.
I originally got the F99, which is no contour and quite a large outer diameter. That would have taken me FOREVER with my little lathe so I sent it back and got the F34 which is tapered. That one was easier to reduce down to a suitable size for the Talon frame and not as much swapping ends needed.
Seems you have a mammoth lathe, I’d say go with the F99 because you lathe can handle it.
You’ll have to make a new breech because the caliber of the .308 round, is practically the same size as the hole in a stock breech – I made about 10 breech’s when I was making mine, I made about 7 out of delrin just because its quicker during testing – 1 of delrin failed so I’d recommend making it from Aluminum.
As to the size… Go to the hardware store and see what size O-Rings they have. Use one of those. I didn’t do any measuring. Just took a .308 round to the hardware store, found an O-Ring that was larger and would give a decent wall thickness to the barrel and machines my breech based on those dimensions… not the best method, but worked for me.
One thing to note is to make sure that you don’t pick an O-Ring that big that the cut out in the breech for the hammer will end up cutting into the O-ring grove.
You’ll also need to make a new tophate. Aluminum is fine for that.
So… Need a new barrel, barrel bushings, breech and tophat.
Everything can be done on the lathe except the cut out in the breech for the hammer. I didnt have access to a mill and just used a drill press with an end mill bit for mine – works fine because its not a sideways cut, just straight down to make the recess.
I dont have it anymore – I was using a stock hammer and condor valve. 104 fpe which isnt too shabby… but if you mod the valve (and I never did so I cant help) you can get 200 FPE and above….. Cygnusx might be able to help because his .308 got uber power.
Having said all this….. I ended up just getting a .25 barrel and think that’s the better option. You’d just need to get a $100 blank from LW and turn the breech down, put new bushings on and your ready to go.
Couple more questions. What contour should I get? F34, F54, of F99?
Do you machine the breach out of aluminum? What weight hammer are you using and what valve do you have in it?
Sorry for all the questions. Thanks!
I only ever used Sabot rounds in mine – Plastic .308 diameter sleeve which held a .223 full metal jacket round. Supposed to be used for reloading. OK results, but you get better results and more power with lead slugs.
Most people seem to get their .308 from the http://www.hunters-supply.com who are mentioned on DAQ’s website for use with his guns. I never got around to using them, but they come highly recommended if DAQ had them on his site and I think CygnusX bought from them with good results.
http://www.hunters-supply.com/shop/index.php?cPath=27&osCsid=b3462da987c10986c79d76c025b5caff
Thanks a lot for that link. What ammo do you use for it and where do you get it?
I used an Adams and Bennett center fire blank in mine. Cheap from midway USA.
Viewing 13 replies - 1 through 13 (of 13 total)
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Cyg, Walt,
I’ll wade in here. Back when i built my Bigbore I had a bottle dump.
The theory goes like this: The breech and tophat diameters are critical and must be minimized. When a firing pulse pressures the breech what you have are forces acting to hold open the valve. So if the area presented to the pressure is large to the point that the valve is held open after the bullet exits you get a bottle dump.
So in my opinion the diameter issue is more about force balance than it is about wasted volume. The volume is very few cc’s and would actually work in favor of making power. larger volume of higher pressure behind bullet.
just my 2-cents on the subject.
You fellas are getting me itching to cast some bullets :):)
Walter…